Re: [Haskell-cafe] The maximum/minimum asymmetry
On 4 September 2011 21:44, Mario Blažević blama...@acanac.net wrote: I was recently surprised to discover that the maximum and maximumBy functions always return the *last* maximum, while minimum and minimumBy return the *first* minimum in the list. The following GHCi session demonstrates this: $ ghci GHCi, version 7.2.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude :module +Data.List Data.Ord Prelude Data.List Data.Ord let list = [(1, 'B'), (1, 'A')] Prelude Data.List Data.Ord maximumBy (comparing fst) list (1,'A') Prelude Data.List Data.Ord minimumBy (comparing fst) list (1,'B') I would normally consider this kind of gratuitous asymmetry a bug, but seeing that these functions' implementations have been specified in the Haskell 98 Library Report, I guess they are now a permanent feature of the language. Can anybody explain the reason for this behaviour? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe The asymmetry is a result of EQ and LT being treated as equivalent in both the minBy and maxBy helper functions in the report's definition of the two functions, which has opposite effects for minimumBy and maximumBy. Since the documentation doesn't specify a behavior for equal values, I could guess that it was just an oversight or that it was considered unimportant. But maybe someone more knowledgeable will correct me. Alexander Dunlap ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: taffybar: an alternative status bar for xmonad
On 14 August 2011 14:51, Tristan Ravitch travi...@cs.wisc.edu wrote: On Sun, Aug 14, 2011 at 02:45:24PM -0700, Alexander Dunlap wrote: I apologize if I'm missing something obvious here, but when I try to run taffybar I get Launching custom binary /home/alex/.cache/taffybar/taffybar-linux-i386 taffybar-linux-i386: ConnectionError connectSession: DBUS_SESSION_BUS_ADDRESS is missing or invalid. Is there some D-BUS configuration that needs to happen before the package is usable? Sorry, I assumed you would have dbus running already. If you add a line like: eval `dbus-launch --auto-syntax` early in your ~/.xsession (if logging in via some graphical login manager) or ~/.xinitrc (if starting X via startx). That should work for any normal -sh-style or -csh-style shell. That command starts DBus and sets the DBUS_SESSION* environment variables, and both xmonad and taffybar need to have the same settings for that variable, so make sure you execute that command before starting either of them. I'll add some notes in the documentation about this. Thanks, that worked! Also, is there a way to change the default color of text printed by the bar? (I've added span tags with colors to the different text widget formats, but it would be nice to be able to set a default - it shows up as a nearly-impossible-to-read dark grey for me.) Thanks a lot for the great program! Alexander ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: taffybar: an alternative status bar for xmonad
On 13 August 2011 08:56, Tristan Ravitch travi...@cs.wisc.edu wrote: I've wanted a slightly fancier status bar than xmobar for a while, so I finally made one. It uses gtk2hs and dbus extensively, so if you hate either of those things it probably isn't for you. Being written in gtk, though, it can have more graphical widgets. http://hackage.haskell.org/package/taffybar Current feature highlights: * It has a system tray * Generic graph widget for things like CPU/memory * XMonad log over DBus so it can be restarted independently of xmonad * Graphical battery widget There is still a lot that I want to add but I figured getting some feedback early would be handy. Documentation is currently at http://pages.cs.wisc.edu/~travitch/taffybar until I figure out how to appease Hackage (see the System.Taffybar module). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe I apologize if I'm missing something obvious here, but when I try to run taffybar I get Launching custom binary /home/alex/.cache/taffybar/taffybar-linux-i386 taffybar-linux-i386: ConnectionError connectSession: DBUS_SESSION_BUS_ADDRESS is missing or invalid. Is there some D-BUS configuration that needs to happen before the package is usable? Thanks, Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Small strings
On Sun, Dec 5, 2010 at 10:04 PM, Antoine Latter aslat...@gmail.com wrote: Hi Haskell, I've recently created the 'smallstring' package[1] and put it on Hackage. The goal was to create a string-like type with as little memory overhead as possible, and to have equality and comparison operations be competitive with the native String type. The idea is that this type would make ideal keys into data structures like Data.Map. I've taken pains to make the library portable to other compilers, but this isn't really tested, and most of my recent development has been on GHC 7. The library is young and likely has places it could be improved, so your feedback is welcome. The API provided is small - to/fromString along with Eq and Ord instances. This isn't meant to be a replacement for the text[2] or bytestring[3] packages - the reduced size comes at a cost. Both of these libraries offer substring manipulation functions which allow the sharing of the backing memory buffers. Also, I haven't implemented any sort of string-like operations on the type as it isn't the right thing for that. I've borrowed heavily from the previously mentioned text package for the underlying array implementation, so thanks goes to Bryan O'Sullivan for maintaining the package and to all of the other authors as well. Take care, Antoine [1] http://hackage.haskell.org/package/smallstring [2] http://hackage.haskell.org/package/text [3] http://hackage.haskell.org/package/bytestring ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe This looks really cool! Suggestion - would it be more efficient to provide direct text - smallstring or bytestring - smallstring functions rather than forcing clients to go through String? That might be useful e.g. if you were parsing (bytestring/text) data and building a Map in the process. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Crazy idea, building ghc with cabal?
On Mon, Nov 29, 2010 at 2:48 PM, Ivan Lazar Miljenovic ivan.miljeno...@gmail.com wrote: On 30 November 2010 03:46, Joris Putcuyps joris.putcu...@gmail.com wrote: Hello Could it be done and would it be interesting, building 'ghc' using cabal? * 'lhc' does it * Use 'cabal-rpm' and 'cabal2arch' to create packages for linux distributions * As ordinary user, install specific versions in your home directory. cabal install ghc = $HOME/.cabal/bin/ghc * Get base packages as a dependency. ... except we then have a chicken and egg problem: GHC is built with Cabal, which needs a Haskell compiler/implementation, etc. ;-) -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com Well, GHC is already written in Haskell, so you already need GHC to compile GHC. I think the reasons are more to do with the fact that Cabal doesn't have some features that are needed for such a complex build as GHC, maybe including the bootstrapping that needs to happen. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [web-devel] statically compiled css
On Sun, Aug 8, 2010 at 4:00 AM, Michael Snoyman mich...@snoyman.com wrote: On Sun, Aug 8, 2010 at 1:03 PM, Tim Matthews tim.matthe...@gmail.com wrote: On Sun, Aug 8, 2010 at 9:20 PM, Michael Snoyman mich...@snoyman.com wrote: Quick update: I'm including the Stylish code in the hamlet package now, and renaming it to Camlet (CSS-hamlet). I'm also including something called Jamlet, which doesn't do much besides variable interpolation. As you might guess, it's for Javascript. I mention it at the end of my most recent blog post[1]. Michael While It's just a name and not really important: hamlet was haml so I first imagined sasset, sasslet or another name from one of the works of Shakespeare but I then really liked stylish as I thought it would tell that something with solid foundations and theory could still appear, hip and pretty. What is important though is the code. This is absolutely great and success just keeps getting harder to avoid. This is by no means a final decision; I'm open to being convinced that other names are better. But I'll point out the main reason for the Camlet/Jamlet name choice: easy to remember and type. I found stylish to be much harder to get out than camlet; that might just be because I'm so used to hamlet already, but that's exactly my goal here: make these three templating systems work together nicely to make the developers life a little bit easier. Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe CaSSius and JSaesar? Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Maybe to Either -- is there a better way?
On Wed, Aug 4, 2010 at 9:21 AM, Henning Thielemann schlepp...@henning-thielemann.de wrote: Ivan Lazar Miljenovic schrieb: Yitzchak Gale g...@sefer.org writes: While useful, I think its ubiquity to simplicity ratio is not high enough to justify either depending on MissingH just for that, or adding it to a base library. Just like the swap :: (a,b) - (b,a) function a lot of people were discussing on librar...@? I have 'swap' in my utility-ht. In general, I agree. Problem in Haskell is, that it allows for a high degree of modularization such that most components become trivial. Does that mean that we should not define simple functions at all, which in the extreme case would mean that we inline all potential function calls? It's also nice for people reading code if common functions are functions from common libraries. This allows readers' vocabulary of common functions to increase, so they don't have to trawl through someone's personal utility library to figure out what each utility function does. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Handling absent maintainers
One issue that comes up is that when you fork a package, data can no longer be freely exchanged between libraries using the original package's datatypes and libraries using the forked package's datatypes. Something that might help here is the concept of extension or friend packages or modules: packages or modules that could break through to access concrete representations of abstract datatypes or classes. The advantage would be that in many cases, instead of forking a package, you could make the necessary changes through a tightly-coupled but separately-maintained friend package while still maintaining compatibility with the original package. This wouldn't help for the problem of maintaining a package that wouldn't compile. That might be helped by something like a provides or equivalent field, where Cabal could be informed that a new package should be treated as satisfying a dependency on a different package. Alex On Fri, Jul 23, 2010 at 11:36 PM, Mark Wotton mwot...@gmail.com wrote: That works fine for my own stuff, but I'd like it to work for people using my software that relies on those packages. mark On Sat, Jul 24, 2010 at 4:10 PM, Roman Beslik ber...@ukr.net wrote: I patch broken packages in my local repository. I increment a version so the local repository get a precedence over the Hackage. On 16.07.10 03:54, Mark Wotton wrote: 2. run my own hackage server and tell my users to use that instead. -- Best regards, Roman Beslik. -- A UNIX signature isn't a return address, it's the ASCII equivalent of a black velvet clown painting. It's a rectangle of carets surrounding a quote from a literary giant of weeniedom like Heinlein or Dr. Who. -- Chris Maeda ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell and the Software design process
On Sun, May 2, 2010 at 9:24 PM, Ivan Miljenovic ivan.miljeno...@gmail.com wrote: On 3 May 2010 14:17, aditya siram aditya.si...@gmail.com wrote: I'm a little confused about this too. I've seen many functions defined like: f x = (\s - ...) which is a partial function because it returns a function and is the same as: f x s = ... No, that's a partially applied function. A partial function is one such as: secondElement (_:x:_) = x Note that it is only defined for lists with at least two elements, for any other list (i.e. singleton or empty) it will throw an error; -fwarn-incomplete-patterns (which is included in -Wall) tells you about these. You can also argue that functions such as head are partial, as they explicitly throw an error if the input data isn't correct. Partial functions are bad because if you accidentally use one the wrong way, your entire program crashes in a flaming wreck. It's much better to do something like this: safeSecondElement (_:x:_) = Just x safeSecondElement _ = Nothing This will work with all possible input types. For more information, see http://en.wikipedia.org/wiki/Partial_function (from the mathematical perspective). -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Of course, there are situations where it is really awkward to not use partial functions, basically because you *know* that an invariant is satisfied and there is no sane course of action if it isn't. To take a contrived example: f ys = let xs = (1:ys) in last xs uses the partial function last. Rewriting it in the non-partial style gives f ys = case (1:ys) of [] - Nothing xs - Just (last xs) but what possible meaning could a caller of the function ascribe to a Nothing result? It just means that there is a bug in f, which is what an error would tell you anyway. Of course, this particular function could easily be rewritten in such a way to be total, but I believe there are more complex examples where it is awkward/impossible to do so. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Splitting list with predicate
On Sun, Mar 14, 2010 at 12:26 PM, michael rice nowg...@yahoo.com wrote: Is there a library function that will create two lists from one based on a predicate, one list for all elements that satisfy the predicate and one for all that do not? Don't want to reinvent the wheel. Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Data.List.filter will do the trick. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Prelude.undefined
On Tue, Mar 2, 2010 at 9:06 PM, Tom Hawkins tomahawk...@gmail.com wrote: How do I track down an reference to an undefined value? My program must not be using a library correctly because the program makes no direct use of 'undefined'. Running with +RTS -xc yields: GHC.Err.CAFTest: Prelude.undefined ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe While the debugger, etc., are very useful tools, I find that often the easiest way to track down this sort of bug is to test your code function-by-function. Make sure each function does what you want it to when run in isolation; this will quickly lead to tracking down the caller of undefined. (If your code is not structured in a way that allows this sort of testing, you might consider restructuring it to make it more modular.) Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: HPath-0.0.0
On Tue, Dec 29, 2009 at 11:52 PM, Jason Dusek jason.du...@gmail.com wrote: HPath is a command line utility to grab the Haskell source for a given identifier: :; dist/build/hpath/hpath HPath.Path.parse 2/dev/null parse :: String - Either ParseError Path parse s = Text.ParserCombinators.Parsec.parse (qualified []) s s This is an alpha release. There are many things HPath can't retrieve -- like constructors -- and there seem to be some complications with the underlying parser/pretty printer (some times you get far too many newlines). There will be another release when the newline issue is resolved. The hope is to release HPath in conjunction with LaTeX macros to allow easy inclusion of Haskell in elaborate documents. -- Jason Dusek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe The package doesn't build with haskell-src-exts version 1.5.1 (the latest version on Hackage). It would be good to either make it build with HSE 1.5.1 or else put a maximum version of HSE in your .cabal file so that Cabal can select a lower version of HSE if available. Looks like an interesting package; I'm looking forward to trying it out! Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Space Efficiency When Sorting a List of Many Lists
On Wed, Dec 30, 2009 at 8:39 PM, Peter Green kinch1...@me.com wrote: I'm a Haskell neophyte, so may be missing something obvious in the problem outlined below. I'm fairly proficient in Python + have some limited experience in OCaml and F#, so know just enough to be be dangerous, but not nearly enough to really know what I'm doing here. OK, I have text files containing 'compressed lists' Compressed lists look like this: 8+12,11,7+13,10 1+2+3,1+9,3+6,4 . . Sublists are comma-delimited, and sublist elements are separated by '+' character. I parse these to look like so: [[[8,12],[11],[7,13],[10]], [[1,2,3],[1,9],[3,6],[4]], . . ] I need to explode these and produce a lex-sorted list of exploded lists: [[1,1,3,4],[1,1,6,4],[1,9,3,4],[1,9,6,4],[2,1,3,4],[2,1,6,4],[2,9,3,4],[2,9,6,4],[3,1,3,4],[3,1,6,4],[3,9,3,4],[3,9,6,4], [8,11,7,10],[8,11,13,10],[12,11,7,10],[12,11,13,10]] I then output this data as comma-delimited lists: 1,1,3,4 1,1,6,4 . . 12,11,13,10 I can do all this in my (no doubt fairly naive) program shown below. In real life, one of my test data files contains compressed lists which all contain 7 sublists. This data (correctly) explodes to a list containing ~3,700,000 exploded lists. All good as far as correctly transforming input to output goes. However, sorting the data uses a lot of memory: Partial output from ./explode +RTS -sstderr: 540 MB total memory in use (4 MB lost due to fragmentation) If I do not do any sorting on the exploded lists, i.e. modify the main function to be main = interact (unlines . toCSV . explode . fromCSV . lines) I then see this partial output from ./explode +RTS --stderr: 2 MB total memory in use (0 MB lost due to fragmentation) I can guess that there might be be less laziness and more instantiation when sorting is introduced, but my questions are: (1) Am I doing anything terribly stupid/naive here? (2) If so, what can I do to improve space efficiency? TIA! import Data.List (sort) import Data.List.Split (splitOn) -- Cartesian Product over a List of Lists -- cp [[1,2],[3],[4,5,6]] -- [[1,3,4],[1,3,5],[1,3,6],[2,3,4],[2,3,5],[2,3,6]] cp :: [[a]] - [[a]] cp [] = [[]] cp (xs:xss) = [y:ys | y - xs, ys - cp xss] -- fromCSV [8+12,11,7+13,10, 1+2+3,1+9,3+6,4] -- -- [[[8,12],[11],[7,13],[10]],[[1,2,3],[1,9],[3,6],[4]]] fromCSV :: [String] - [[[Int]]] fromCSV = map parseOneLine where parseOneLine = map parseGroup . splitOn , where parseGroup = map read . splitOn + -- explode [[[1,2],[3],[4,5,6]], [[1, 2], [14,15], [16]]] -- [[1,3,4],[1,3,5], -- [1,3,6],[2,3,4],[2,3,5],[2,3,6],[1,14,16],[1,15,16],[2,14,16],[2,15,16]] explode :: [[[a]]] - [[a]] explode = concatMap cp -- toCSV [[8,11,7,10,12],[8,11,7,10,12],[8,11,7,10,12]] -- -- [8,11,7,10,12,8,11,7,10,12,8,11,7,10,12] toCSV :: (Show a) = [[a]] - [String] toCSV = map $ tail . init . show --toCSV = map (intercalate , . map show) main = interact (unlines . toCSV . sort . explode . fromCSV . lines) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe I'm not sure about your problem specifically but the external-sort package on Hackage may be of interest. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Use of the Try typeclass
On Wed, Dec 30, 2009 at 5:52 PM, Edward Z. Yang ezy...@mit.edu wrote: Hello all, I am currently playing with the new cadre of failure libraries, and I'm trying to figure out how to use the monadic version of Failure while also getting the Try typeclass, which appears to be the standardized mechanism for marshalling values from specific monads into the failure monad. Unfortunately, it only seems to be defined when I import Control.Failure, and this module is fairly incompatible with Control.Monad.Failure, as demonstrated by this GHCI transcript: ezy...@javelin:~/Documents/tmr$ ghci Prelude :m +Control.Failure Prelude Control.Failure :m +Control.Monad.Failure Prelude Control.Failure Control.Monad.Failure :t failure interactive:1:0: Ambiguous occurrence `failure' It could refer to either `Control.Failure.failure', imported from Control.Failure or `Control.Monad.Failure.failure', imported from Control.Monad.Failure I'm using MTL, which might be the reason why there's all sorts of unhappiness. :-) Cheers, Edward ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Why are you importing both Control.Failure and Control.Monad.Failure when the latter just re-exports the former? Are you using the latest versions of the two packages? Try importing just Control.Monad.Failure.MTL; that provides the MTL failure instances and also re-exports Control.Failure. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Use of the Try typeclass
On Wed, Dec 30, 2009 at 10:54 PM, Edward Z. Yang ezy...@mit.edu wrote: Excerpts from Michael Snoyman's message of Thu Dec 31 00:43:52 -0500 2009: What version of the packages are you using? Can you give the output of: ghc-pkg list|grep failure Sure thing: (control-monad-failure-0.4), (control-monad-failure-0.5.0), control-monad-failure-mtl-0.5.0, cpphs-1.8, cpphs-1.9, failure-0.0.0.1, happstack-0.3.2, happstack-data-0.3.3 Additionally: ezy...@javelin:~$ cabal upgrade failure Resolving dependencies... No packages to be installed. All the requested packages are already installed. If you want to reinstall anyway then use the --reinstall flag. Cheers, Edward Upgrade to control-monad-failure-mtl 0.6.0 and control-monad-failure 0.6.0. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Perhaps you haven't installed the dyn libraries
On Tue, Dec 29, 2009 at 11:34 AM, Gregory Propf gregorypr...@yahoo.com wrote: I'm trying out the dynamic linking in GHC 6.12 and getting this message a lot for different libraries. I assume I need to rebuild them with different ghc options in the cabal files and have tried -shared, -dynamic and -fPIC but with no luck. Is there something I'm missing. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Use the --enable-shared option to cabal install. If you want dynamic libraries everywhere, put shared: True in ~/.cabal/config. You'll need to recompile all of your libraries. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] New Hackage category: Error Handling
On Sat, Dec 5, 2009 at 3:00 PM, Michael Snoyman mich...@snoyman.com wrote: On Sun, Dec 6, 2009 at 12:55 AM, Henning Thielemann lemm...@henning-thielemann.de wrote: On Sun, 6 Dec 2009, Michael Snoyman wrote: I think there are plenty of examples like web servers. A text editor with plugins? I don't want to lose three hours worth of work just because some plugin wasn't written correctly. For many classes of programs, the distinction between error and exception is not only blurred, it's fully irrelevant. Harping on people every time they use error in the wrong sense seems unhelpful. Hope my commenting on this subject doesn't become my own form of *pedantry*. In an earlier thread I have explained that one can consider a software architecture as divided into levels. What is an error in one level (text editor plugin, web server thread, operating system process) is an exception in the next higher level (text editor, web server, shell respectively). This doesn't reduce the importance to distinguish between errors and exceptions within one level. All approaches so far that I have seen in Haskell just mix exceptions and errors in an arbitrary way. I think we can all appreciate why it would be a bad thing is we treat exceptions as errors. For example, I don't want my program to crash on a file not found. On the other hand, what's so bad about treating errors as exceptions? If instead of the program crashing on an array-out-of-bound or pattern-match it throws an exception which can be caught, so what? Michael I think the key is in the difference between the user/client and programmer/developer. As Henning has been saying, these roles change as you go through the different levels of the program, but I see the difference between an error and an exception as this: when a problem is relevant to the user/client, it's an exception; when it is irrelevant to the user/client, it's an error. Suppose you were using some sort of exception framework and you got an error from a piece of library code (not the head function) saying that head had failed somewhere. This is absolutely meaningless to a client. It just means there's a problem in the library code; it doesn't mean anything is amiss in the client's space. The client basically has to throw the function out, whether by gracefully aborting the program, disabling the plugin, etc. Contrast this with an exception, such as index not in map. This is entirely relevant to the client. All of the code knows exactly what is going on; it's just that the index isn't in the map. The client can recover from this by, say, substituting a default value, adding the index to the map, etc. Now, suppose the client knew a priori that the index was *supposed* to be in the map. Now this becomes an *error* to the *client* of the client, since there is a bug in the first client's code. I guess my point is that if I have a function, say, sort :: Ord a = [a] - [a], and sort calls head somewhere in it, there's no point having sort throw an exception for Prelude.head: [] since that means nothing to the client. It would make more sense to have an InternalError type that just says OK, sorry client, I screwed up, just clean things up as best you can. If really were something that the client could have responded to, sort should have caught the head error inside the function definition and rethrown a different exception; say, SortEmptyListError if your sort function didn't work on empty lists (contrived example, sorry). Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] New Hackage category: Error Handling
On Sun, Dec 6, 2009 at 10:40 PM, Alexander Dunlap alexander.dun...@gmail.com wrote: On Sat, Dec 5, 2009 at 3:00 PM, Michael Snoyman mich...@snoyman.com wrote: On Sun, Dec 6, 2009 at 12:55 AM, Henning Thielemann lemm...@henning-thielemann.de wrote: On Sun, 6 Dec 2009, Michael Snoyman wrote: I think there are plenty of examples like web servers. A text editor with plugins? I don't want to lose three hours worth of work just because some plugin wasn't written correctly. For many classes of programs, the distinction between error and exception is not only blurred, it's fully irrelevant. Harping on people every time they use error in the wrong sense seems unhelpful. Hope my commenting on this subject doesn't become my own form of *pedantry*. In an earlier thread I have explained that one can consider a software architecture as divided into levels. What is an error in one level (text editor plugin, web server thread, operating system process) is an exception in the next higher level (text editor, web server, shell respectively). This doesn't reduce the importance to distinguish between errors and exceptions within one level. All approaches so far that I have seen in Haskell just mix exceptions and errors in an arbitrary way. I think we can all appreciate why it would be a bad thing is we treat exceptions as errors. For example, I don't want my program to crash on a file not found. On the other hand, what's so bad about treating errors as exceptions? If instead of the program crashing on an array-out-of-bound or pattern-match it throws an exception which can be caught, so what? Michael I think the key is in the difference between the user/client and programmer/developer. As Henning has been saying, these roles change as you go through the different levels of the program, but I see the difference between an error and an exception as this: when a problem is relevant to the user/client, it's an exception; when it is irrelevant to the user/client, it's an error. Suppose you were using some sort of exception framework and you got an error from a piece of library code (not the head function) saying that head had failed somewhere. This is absolutely meaningless to a client. It just means there's a problem in the library code; it doesn't mean anything is amiss in the client's space. The client basically has to throw the function out, whether by gracefully aborting the program, disabling the plugin, etc. Contrast this with an exception, such as index not in map. This is entirely relevant to the client. All of the code knows exactly what is going on; it's just that the index isn't in the map. The client can recover from this by, say, substituting a default value, adding the index to the map, etc. Now, suppose the client knew a priori that the index was *supposed* to be in the map. Now this becomes an *error* to the *client* of the client, since there is a bug in the first client's code. I guess my point is that if I have a function, say, sort :: Ord a = [a] - [a], and sort calls head somewhere in it, there's no point having sort throw an exception for Prelude.head: [] since that means nothing to the client. It would make more sense to have an InternalError type that just says OK, sorry client, I screwed up, just clean things up as best you can. If really were something that the client could have responded to, sort should have caught the head error inside the function definition and rethrown a different exception; say, SortEmptyListError if your sort function didn't work on empty lists (contrived example, sorry). Alex (Sorry for replying to myself.) This is, of course, assuming that a non-empty list was not an invariant of the sort function, in which case it would be a programming error on the part of the client that called sort if that happened. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] instance Binary UTCTime (Was: Oprhan instances)
On Sun, Nov 29, 2009 at 5:37 AM, Duncan Coutts duncan.cou...@googlemail.com wrote: On Thu, 2009-11-26 at 16:40 -0500, David Menendez wrote: The problem with this solution is that it doesn't scale. If we have M packages providing types and N packages providing classes, then we need M*N additional packages for orphans. The best long-term solution is probably extending Cabal to handle this more transparently, perhaps by allowing packages to depend on flagged versions of other packages (e.g., foomonad = 4.0 4.1 HAS_MTL) Not going to happen. Such packages could not be translated into binary distro packages. Duncan Wouldn't the distro just choose one set of flags for each package and then other packages would either be satisfied or not satisfied based on which flags had been chosen? It seems to me that distros could even offer multiple options for the same package with different flags set. What wouldn't work? Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] instance Binary UTCTime (Was: Oprhan instances)
On Sun, Nov 29, 2009 at 4:41 PM, Duncan Coutts duncan.cou...@googlemail.com wrote: On Sun, 2009-11-29 at 09:55 -0800, Alexander Dunlap wrote: On Sun, Nov 29, 2009 at 5:37 AM, Duncan Coutts duncan.cou...@googlemail.com wrote: On Thu, 2009-11-26 at 16:40 -0500, David Menendez wrote: The problem with this solution is that it doesn't scale. If we have M packages providing types and N packages providing classes, then we need M*N additional packages for orphans. The best long-term solution is probably extending Cabal to handle this more transparently, perhaps by allowing packages to depend on flagged versions of other packages (e.g., foomonad = 4.0 4.1 HAS_MTL) Not going to happen. Such packages could not be translated into binary distro packages. Duncan Wouldn't the distro just choose one set of flags for each package and then other packages would either be satisfied or not satisfied based on which flags had been chosen? Here's the system I assumed you were talking about. You can tell me if I misunderstood. Instead of having N * M packages, you have a package that provides optional instances. For example package A defines a class and optionally provides instances for types defined in B. If you select to have it depend on B then the instances are provided, otherwise not. In a source based system this seems to work ok, you would provide optional instances for all the packages you already happen to have installed. Though if later you install another package that could have had optional instances provided then you have to go recompiling things. It's slightly worse for binary packages because the distro has to decide up front if they're going to provide the optional instances or not. Since someone might need them then you end up picking the maximal set of optional dependencies and you end up pulling in all sorts of apparently unrelated packages. Then the other bit you suggested foomonad = 4.0 4.1 HAS_MTL would be needed to be able to express that you want a package that has been built with a particular optional instance provided. This is the bit that cannot be translated into packages in most distros. Yes you could pick the flags up front, but you have to pick a single assignment that satisfies everyone. Well, that happens anyway with most packages since distros have to choose one set of flags that works. The proposal I was commenting on would just allow packages to depend on flags of other packages and so be explicit about this. It seems to me that distros could even offer multiple options for the same package with different flags set. Most distros cannot handle installing multiple instances of the same version of a package. Well, what I've seen is having different packages, i.e. foo-quickcheck, foo-no-quickcheck as separate packages. (Note that I can't take credit for suggesting the idea, I was just asking you about your objection.) Thanks! Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Newcomers question
The type of liftA2 :: Applicative f =(a - b - c) - f a - f b - f c. Thus, the type of liftA2 (==) :: (Eq b, Applicative f) = f b - f b - f Bool. In your case, f :: a - b, so liftA2 (==) :: (Eq b) = (a - b) - (a - b) - (a - Bool). (==) takes two arguments, so you're left with the type (liftA2 (==)) x y :: a - Bool. This contradicts the class definition of Eq, which says that the type of (==) after giving it two arguments must be Bool, not a - Bool. I hope that's clear enough. Busting out GHCi and using :t to find the types of a lot of these expressions can be really helpful. Alex On Sun, Nov 1, 2009 at 8:09 AM, b1g3ar5 nick.st...@gmail.com wrote: OK, I understand that now but I've got a supplimentary question. If I put: instance Eq b = Eq (a - b) where (==) = liftA2 (Prelude.==) to do the Eq part I get another error: Couldn't match expected type `Bool' against inferred type `a - Bool' In the expression: liftA2 (==) In the definition of `==': == = liftA2 (==) In the instance declaration for `Eq (a - b)' Now can someone please explain this? I'm hoping that when I've understood this stuff I'll have made a small step to understanding Haskell. Thanks. On 31 Oct, 23:38, Daniel Peebles pumpkin...@gmail.com wrote: For some reason, Show and Eq are superclasses of Num (despite Num not actually using any of their methods), meaning that the compiler forces you to write instances of Eq and Show before it even lets you write a Num instance. I don't think anybody likes this, but I think we're stuck with it for the foreseeable future. On Sat, Oct 31, 2009 at 7:31 PM, b1g3ar5 nick.st...@gmail.com wrote: I'm trying: instance Num b = Num (a - b) where fromInteger = pure . Prelude.fromInteger negate = fmap Prelude.negate (+) = liftA2 (Prelude.+) (*) = liftA2 (Prelude.*) abs = fmap Prelude.abs signum = fmap Prelude.signum but the compiler rejects it with: src\Main.hs:24:9: Could not deduce (Show (a - b), Eq (a - b)) from the context (Num b) arising from the superclasses of an instance declaration at src\Main.hs:24:9-29 Possible fix: add (Show (a - b), Eq (a - b)) to the context of the instance declaration or add an instance declaration for (Show (a - b), Eq (a - b)) In the instance declaration for `Num (a - b)' Could someone please explain this to me? I thought that it might be that it couldn't work out the functions necessary for (a-b) to be in the classes Show and Eq - so I tried adding definitions for == ans show, but it made no difference. Thanks ___ Haskell-Cafe mailing list haskell-c...@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list haskell-c...@haskell.orghttp://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] tips on installing System.Console.Readline
On Mon, Oct 26, 2009 at 12:52 AM, Michael Mossey m...@alumni.caltech.edu wrote: Before I ask my main question, incidentally has anyone noticed that the GHCI prompt, on Windows XP, now has auto-completion! (since 6.10) Awesome! I'm trying to install System.Console.Readline on Windows XP. I need to have GNU readline installed first, which I did (by multiple methods). But running 'cabal install readline' it reports that readline is not found. Obviously I need to set a path or environment variable... can anyone help? Thanks, Mike ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Try the --extra-include-dirs and --extra-lib-dirs options to Cabal. If there's a configure script, you might also need to modify the options to that. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Different semantics in identical do statement?
On Fri, Oct 9, 2009 at 4:25 PM, David Menendez d...@zednenem.com wrote:
On Fri, Oct 9, 2009 at 6:47 PM, staafmeister g.c.stave...@uu.nl wrote:
Daniel Peebles wrote:
I vaguely remember on IRC someone pointing out that the Parsec monad
broke one of the laws. I think return _|_ x === _|_ which could be
causing your problem. I may be wrong though.
Confirmed, working in the parsec monad
Prelude Text.Parsec runP (do {x - return undefined; return 10}) ()
*** Exception: Prelude.undefined
In the IO Monad
Prelude Text.Parsec do {x - return undefined; return 10}
10
Should be fixed.
It looks like the problem is a strict field in the definition of
GenParser (specifically in Reply). From what I can tell, Parsec 3.0.1
should not have this problem.
--
Dave Menendez d...@zednenem.com
http://www.eyrie.org/~zednenem/
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It works with Parsec 3.0.1 for me.
Alex
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Re: Re: [Haskell-cafe] Problem with result-type context restrictions in typeclasses.
I think
instance Bar (Ret c) = Foo c where
...
will do what you are asking.
Alex
On Tue, Sep 29, 2009 at 10:25 PM, DNM dnme...@gmail.com wrote:
Dan, thanks again for the response.
I changed my code to use type families to let each Cls instance (actually a
more complicated instance in my code) determine which Bar instance type it
will return, but this didn't seem to work. The problem is that the client
of the typeclass instance methds ('useThisStuff', which calls on 'toNum' and
'foo' in the contrived example) expects some guarantee that (Ret c) is going
to be an instance of Bar. The actual client code I'm using complains when
it sees that the associated type doesn't guarantee that an instance of the
appropriate class is instantiated. I don't see any way to guarantee this
without adding a context restriction in the class-level definition of Ret c,
something like:
class Cls c where
type Ret c :: (Bar *) = * -- or a better name
foo :: c - Ret c
which isn't legal Haskell. What I want to say is define Ret c however you
want, but make sure it is an instance of Bar in the *class-level definition
of Ret c*, so that any client of 'Cls' will know that Ret c will be
foo-able.
Maybe I'm missing some subtlety of type families...
Any suggestions?
--D.N.
Daniel Peebles wrote:
In your class, you have:
class Cls c where
foo :: (Bar b) = c - b
There's an implicit forall for b, meaning that the caller of the
method gets to choose what it wants for b (as long as it's an instance
of Bar). For you to be able to write such a method you'd need to write
functions that can return any instance of Bar. One solution to this is
to turn on the GHC extension -XTypeFamilies, and then modify your code
as follows:
class Cls c where
type Ret c :: * -- or a better name
foo :: c - Ret c
instance Cls G where
type Ret G = FU
foo = fuu
That should work (although I haven't tested it).
What type families do in this case is allow you to write not only
methods associated with typeclasses, but type functions associated
with them too. In this case you can think of Ret as a function that
takes a type (G in the instance above) and returns another type (FU).
Each instance can define new mappings for Ret.
Hope this helps!
Dan
On Tue, Sep 29, 2009 at 10:48 PM, DNM dnme...@gmail.com wrote:
Correction by the author:
It seems that ghc doesn't like the fact that I am saying 'foo' must
return a class 'b' of typeclass 'Bar', while providing a function that
returns a concrete data instance of 'Bar' (viz., FU or FI) later on
when I implement 'foo' in each type classes.
Should read:
It seems that ghc doesn't like the fact that I am saying 'foo' must
return something of TYPE 'b' implementing typeclass 'Bar', while
providing
a function that returns a concrete data instance of 'Bar' (viz., FU or
FI)
later on when I implement 'foo' in each type classes.
On Sep 29, 10:43 pm, DNM dnme...@gmail.com wrote:
N.B. I'm a newbie to Haskell, and this problem is a bit complex, so
bear with me.
I'm using typeclasses to implement a sort of common interface for all
things -- call them things of type 'Cls' -- that can be expected to
implement a set of functions -- an 'interface' in OOP-speak. (Yes,
yes, I'm aware that typeclasses are subtly different and far superior,
but my Haskell-ese is still a bit rudimentary.)
Essentially, I want to have a typeclass that expects its instances to
have an accessor function that results in something that is an
instance of another typeclass whose instances can perform some
operation. The ghc type-checker doesn't seem to like my code,
though, and I can't seem to figure out why.
To make it concrete, I've typed up some dummy typeclasses and a dummy
function that uses their instances to illustrate what I mean, as well
as the form of the ghc(i) error.
- BEGIN CODE --
class Cls c where
foo :: (Bar b) = c - b
class Bar b where
toNum :: b - Int
-- | One implementation of Cls
data D = D {fu :: FU}
data FU = FU {num :: Int}
instance Cls D where
foo = fu
instance Bar FU where
toNum f = (num f) + 47
-- | Another implementation of Cls
data E = E {fi :: FI}
data FI = FI {nuum :: Int}
instance Cls E where
foo = fi
instance Bar FI where
toNum f = (nuum f) + 100
-- | Yet another (this one re-uses FI)
data F = F {fii :: FI}
instance Cls F where
foo = fii
-- | And one last one, just to stress that
-- I really need to implement multiple
-- instances of Cls.
data G = G {fuu :: FU}
instance Cls G where
foo = fuu
-- | Good. Now, the function 'useThisStuff' need
-- not know anything about it's payload
-- other than that it its args are Cls's
-- (hence they are foo'able things that
-- can be used to construct an Int answer).
useThisStuff :: (Cls x, Cls y) = x - y - Int
useThisStuff x y =
(toNum $ foo x) + (toNum $ foo y)
Re: [Haskell-cafe] error on --++ bla bla bla
Comments are started with -- followed by a character that is not a symbol character. If it is followed by a symbol character (e.g. *) then the -- plus the symbol (e.g. --*) parses as an operator rather than a comment. p is not a symbol, so the -- starts a comment. For a precise description of Haskell syntax, see the Haskell98 Report[1]. It's quite readable, at least in comparison to other precise documents I've attempted to read. Hope that helps. Alex On Wed, Sep 30, 2009 at 7:48 PM, Hong Yang hyang...@gmail.com wrote: Nowhere any Haskell book mentioned line comments start with -- , not just --. It is just people usually put -- ahead of comments. I can successfully compile --print bla bla bla. On Wed, Sep 30, 2009 at 7:36 PM, Sebastian Sylvan sebastian.syl...@gmail.com wrote: On Thu, Oct 1, 2009 at 12:52 AM, Hong Yang hyang...@gmail.com wrote: Hi, I got an error if one of lines reads --++ bla bla bla where I tried to comment, but -- ++ bla bla bla (notice the space after --) is OK. Do you think this revealed a tiny bug in the GHC compiler (I am using Windows Haskell Platform 2009.2.0.2)? Line comments start with -- , not just --. -- Sebastian Sylvan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] error on --++ bla bla bla
Oops...forgot the footnote. The lexical syntax part of the report is at http://haskell.org/onlinereport/lexemes.html. Alex On Wed, Sep 30, 2009 at 7:56 PM, Alexander Dunlap alexander.dun...@gmail.com wrote: Comments are started with -- followed by a character that is not a symbol character. If it is followed by a symbol character (e.g. *) then the -- plus the symbol (e.g. --*) parses as an operator rather than a comment. p is not a symbol, so the -- starts a comment. For a precise description of Haskell syntax, see the Haskell98 Report[1]. It's quite readable, at least in comparison to other precise documents I've attempted to read. Hope that helps. Alex On Wed, Sep 30, 2009 at 7:48 PM, Hong Yang hyang...@gmail.com wrote: Nowhere any Haskell book mentioned line comments start with -- , not just --. It is just people usually put -- ahead of comments. I can successfully compile --print bla bla bla. On Wed, Sep 30, 2009 at 7:36 PM, Sebastian Sylvan sebastian.syl...@gmail.com wrote: On Thu, Oct 1, 2009 at 12:52 AM, Hong Yang hyang...@gmail.com wrote: Hi, I got an error if one of lines reads --++ bla bla bla where I tried to comment, but -- ++ bla bla bla (notice the space after --) is OK. Do you think this revealed a tiny bug in the GHC compiler (I am using Windows Haskell Platform 2009.2.0.2)? Line comments start with -- , not just --. -- Sebastian Sylvan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] unicode text libraries
On Mon, Sep 28, 2009 at 9:15 AM, Don Stewart d...@galois.com wrote: tittoassini: 2009/9/28 Don Stewart d...@galois.com: titto: Hi, I am looking for an unicode strings library, I found on hackage: http://hackage.haskell.org/package/compact-string http://hackage.haskell.org/package/text They both look solid and functionally complete so ... I don't know which one to use :-) As I am sure I am not the first one facing this choice, may I ask which one you preferred and why? Data.Text Thanks , but .. why? Sorry, was on the way out the door. Data.Text has growing use, is well designed, and builds on the pedigree of bytestring and the vector* series of fusion libraries. I trust that code. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe I just have a question out of curiosity - why was the decision made to have Data.Text, uvector, and ByteString all separate data structures, rather than defining the string types in terms of uvector? Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: HList and Type signatures / synonyms
2009/9/6 Günther Schmidt gue.schm...@web.de:
Hi Edward,
I suppose you're right, I could have made this a little more detailed.
I keep reading in and processing data in an accumulating way, ie. lets say I
have a DB table (don't take this literally please, just an example), into
in-memory records of type
data MyRecord = MyRecord {
name :: String,
birthDate :: LocalDate}
in a second step I read in Gender information so now I'd need an extended
Record
data MyRecord = MyRecord {
name :: String,
birthDate :: LocalDate,
gender :: Gender }
and so on for another 12 times.
In other words I need to extend the record.
I had been using tuples, ie:
(String, LocalDate) - Gender - (String, LocalDate, Gender)
but after 6 or so *extensions* things get a tad messy.
Anyway this is where HList really shines, but it is a bit akward when you
want to state the type of an HList record, ie.
type Entg =
Record
(HCons
(LVPair (Label HZero) DRG)
(HCons
(LVPair (Label (HSucc HZero)) BelegungsTyp)
(HCons
(LVPair (Label (HSucc (HSucc HZero))) EntgeltBetrag)
(HCons
(LVPair (Label (HSucc (HSucc (HSucc HZero Zuschläge)
(HCons
(LVPair (Label (HSucc (HSucc (HSucc (HSucc HZero) Abschläge)
HNil)
which happens to be my actual stage 1 data type (Entg), one that has 5
fields initial and in several distinct steps 8 more are *added*.
So I wonder if there is a more simple way to do this, ie to *extend* the
*type* signatures of extend records without such a lot of typing.
Günther
Am 07.09.2009, 02:44 Uhr, schrieb Edward Z. Yang ezy...@mit.edu:
Excerpts from Günther Schmidt's message of Sun Sep 06 19:40:05 -0400 2009:
I keep accumulating values and right now use plain tuples for that. I end
up with a 12 element tuple and things are a bit messy.
Hi, you may want to consider using the Writer monad. [1]
I'd like to use extensible Records from HList instead, thing is I'd like
to keep putting type signatures in my code. As it turns out that seems to
be where it gets messy with HList.
Perhaps more precisely explaining your problem domain would be useful.
Cheers,
Edward
[1] http://www.haskell.org/all_about_monads/html/writermonad.html
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Why don't you define your record to be the final set of all the
information that you will collect, and just leave some of the fields
undefined until you collect all the data? That would work well if you
are going to be collecting all of the data pretty much all at once.
You could also have a record of Maybe values that started out as all
Nothing and became Justs as the data was collected.
Maybe I'm misunderstanding your problem, but that's how I would do it.
Alex
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Re: [Haskell-cafe] How to preload the module of my own
On Fri, Sep 4, 2009 at 6:08 PM, zaxisz_a...@163.com wrote: I want to preload the module automatically when starting ghci. The module located in ~/work directory contains some functions i use everyday. Now i use an alias: alias ghci='ghci -i ~/money/Money.hs' which works fine. However i feel there maybe are more elegant way. thanks! If the module is part of a package you can put 'import NameOfModule' in your ~/.ghc/.ghci file. That file contains commands that are run when ghci starts. I'm not sure if you can load a module that isn't installed. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Time constrained computation
On Fri, Aug 28, 2009 at 6:01 AM, Mitarmmi...@gmail.com wrote: Hi! Ups, missed save button and pressed send. ;-) So I am not really sure if this is correct term for it but I am open to better (search) terms. I am wondering if it is possible to make a time constrained computation. For example if I have a computation of an approximation of a value which can take more or less time (and you get more or less precise value) or if I have an algorithm which is searching some search-space it can find better or worse solution depending how much time you allow. So I would like to say to Haskell to (lazily, if it really needs it) get me some value but not to spend more than so much time calculating it. One abstraction of this would be to have an infinity list of values and I would like to get the last element I can get in t milliseconds of computational time. One step further would be to be able to stop computation not at predefined time but with some other part of the program deciding it is enough. So I would have a system which would monitor computation and a pure computation I would be able to stop. Is this possible? Is it possible to have a pure computation interrupted and get whatever it has computed until then? How could I make this? Is there anything already done for it? Some library I have not found? Of course all this should be as performance wise as it is possible. So the best interface for me would be to be able to start a pure computation and put an upper bound on computation time but also be able to stop it before that upper bound. And all this should be as abstracted as it is possible. Mitar ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe In general, you need to do this either within the IO monad, to avoid breaking referential transparency, or using unsafePerformIO, if you know that your use of it is safe, which seems somewhat unlikely. You may want to look at System.Timeout. Another possibility would be the unamb package from Hackage. Hope that helps, Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] oauth in haskell - reviewers?
On Sun, Aug 23, 2009 at 9:25 AM, Diego Souzadso...@bitforest.org wrote: Hi Alex, - In the Token datatype, you can automatically create the accessor functions (oath_token, etc.) by using named fields: I though about that too and I was not sure about what to do. The reason I didn't use it is because I don't export the value constructors of Token type, that is why I created the access functions explicitly. I'm pretty sure you can export just the access functions and not the data constructors. They're just ordinary functions with a bit of syntactic sugar, so they can be exported even if the constructors aren't. - I think you can use join from Control.Monad and functions from Control.Applicative in your response function to make it quite a bit cleaner. To be honest I'm not familiar with Control.Applicative at all. I'll read about it and see if I can figure how to do this. A quick search pointed me to this: http://www.soi.city.ac.uk/~ross/papers/Applicative.html Is there any other resources you would suggest me to read? The rule of thumb I use is that you can replace foo = do x1 - action1 x2 - action2 x3 - action3 ... xn - actionn return (bar x1 x2 x3 ... xn) by foo = bar $ action1 * action2 * action3 * ... * actionn for any number of actions. ($) is a synonym for fmap and (*) is the same as Control.Monad.ap if the applicative functor is also a monad. (All monads can be made instances of Applicative, and most [all?] of the standard ones are.) Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] oauth in haskell - reviewers?
On Sat, Aug 22, 2009 at 4:36 PM, Diego Souzadso...@bitforest.org wrote:
Hi all,
I wrote a small library in haskell do deal with oauth authentication. It
turns out it is my first library in haskell as well. As I'm beginner in
haskell, I'm asking for a review of someone more experienced/proficient
before even daring to create a cabal pkg and dist it to hackage. :-)
Any help/comments on this will be highly welcome.
http://projects.bitforest.org/hoauth/
$ darcs get http://projects.bitforest.org/hoauth/
# should do the trick
Thanks in advance,
--
~dsouza
yahoo!im: paravinicius
gpg key fingerprint: 71B8 CE21 3A6E F894 5B1B 9ECE F88E 067F E891 651E
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Hi,
I'm not familiar at all with OAuth but I have some general suggestions
that might make your code in Consumer.hs a bit nicer. These
suggestions also probably apply elsewhere in your code.
- In the Token datatype, you can automatically create the accessor
functions (oath_token, etc.) by using named fields:
data Token
= Request { oath_token :: String, oath_token_secret :: String,
oath_extra :: R.Parameter }
| Access { oath_token :: String, oath_token_secret :: String,
oath_extra :: R.Parameter }
This will create your accessor functions for free, and you get update
syntax to boot. (Google for haskell named fields for details.)
- When you have multiple datatype constructors with similar arguments
(as with Token or Request), it may be better to use a Boolean-type
flag saying which one it is (e.g. HTTP or HTTPS) and then a single
datatype with all of the different arguments in it. This may help you
remove code duplication elsewhere.
- I think you can use join from Control.Monad and functions from
Control.Applicative in your response function to make it quite a bit
cleaner.
I know this email has been a bit terse; ask if you have any questions
about the above.
Hope that helps,
Alex
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Re: [Haskell-cafe] gbp sign showing as unknown character by GHC
On Wed, Aug 19, 2009 at 11:08 AM, Iain Barnettiainsp...@gmail.com wrote: 2009/8/19 David Leimbach leim...@gmail.com Interesting... GHCI bug? Didn't the readline dependency go away not too long ago? Could it be related? I just tried this Prelude putStrLn \£ ghc: panic! (the 'impossible' happened) (GHC version 6.10.4 for i386-unknown-linux): charType: '\163' Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug So perhaps I should put in a bug report, as that shouldn't happen (it doesn't with some other characters I tried), unless anyone has a different idea? I'm running Arch Linux with xmonad and using roxterm, so perhaps it's something to do with my setup? Iain ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe I can reproduce the panic on both urxvt and uxterm. On urxvt, GHCi works correctly with putStrLn £. On uxterm, it just prints a blank space. I can type the GBP sign into both terminals. Could it be a terminfo problem of some sort? It seems suspicious that there is a difference between terminals. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Thinking about what's missing in our library coverage
On Mon, Aug 3, 2009 at 4:44 PM, Don Stewartd...@galois.com wrote: Following Simon M's advice, I look over the typical batteries categories, using Python as input: http://docs.python.org/library/index.html The following things were missing from the current Platform. There are many. How would you identify the top, say, 5 libs to add? -- Don * String support o pcre regexes [pcre-light] [regex-pcre] — what’s our best regex lib? For something like this, it seems like all of the regex libs probably have pros and cons. It would be good if we could identify what these are and create a library that was the best of all worlds instead of just picking one and going with it. o unicode text [text] [text-icu] — packed, unicode text This is essential, although I don't know if it is stable enough for the platform (?). * text o pandoc — markdown, reStructuredText, HTML, LaTeX, ConTeXt, Docbook, OpenDocument, ODT, RTF, MediaWiki, groff No. Pandoc is too actively developed to go into the HP. It's also much more of an end-user application than a standard library - it's applications are not general enough to be included in the standard distribution. * math and numerics o blas — BLAS o cmath — C math functions o dimensional — physical dimensions o fftw o mersenne-random — fast randoms The interfaces here should be unified more to create a general Haskell numerical library, IMO. * compression o bzip2 o zip o tar Definitely. * file formats o config parser This would be quite useful. * crypto o hmac, md5, sha, hashing Yes. * systems o getopt o logging o termio o editline o mmap All of these, at some point. * Internet o network-bytestring o ssl o json o feed (rss, atom) o mime o base64 et al o uuencode o cgi o fastcgi o urls o ftp, http, imap, smtp clients o uuid o url parsing o http server o xml-rpc Interface unification would help. Especially network-bytestring seems to be too ad-hoc for me - it would probably be better to put ByteString support into the regular Network library * Internationalization o gettext o locale o i18n Yes, although again, maturity? * Development o hscolour Yes. My main concern is with libraries like attoparsec and network-bytestring - these libraries seem to be somewhat ad-hoc conversion of traditional libraries to use ByteStrings. We should come up with a cleaner, more elegant way of dealing with the String/Text/Strict ByteString/Lazy ByteSTring gap, as most libraries that apply to one of those types also apply to the other two, as the first two and last two especially and more generally all four essentially represent the same thing. We shouldn't bless libraries that don't have a good way of applying equally to these three. A possible solution could be a standardized package-common, package-string, package-text, package-bytestring, package-lazybytestring naming convention, or a Category.Module, Category.Module.String, Category.Module.ByteString.Strict, Category.Module.ByteString.Lazy, Category.Module.Text convention. We could also work something up with type classes. I just don't like how a lot of libraries implement support for various string types just based on need rather than creating a clean, comprehensive interface. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] TBC: Testing By Convention
On Sun, Jul 26, 2009 at 6:12 PM, Peter Gammiepete...@gmail.com wrote: Hello, Mark and I would like to announce our test harness, which has features complementary to existing harnesses. TBC provides two main features: - It attempts to compile and run all tests, even if some do not compile or run. - Aspiring to the write-it-once principle, tests following conventions require a lot less boilerplate. It is at an embryonic stage of development, and would greatly benefit from feedback and/or patches. :-) Hackage: http://hackage.haskell.org/package/TBC git: http://github.com/peteg/TBC/ Enjoy! cheers peter -- http://peteg.org/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Can it return an exit status based on whether or not all tests passed? If not, that would be a very useful feature that I have not seen in any other testing frameworks. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] following up on space leak
On Sun, Jul 5, 2009 at 7:46 PM, Uwe Hollerbachuhollerb...@gmail.com wrote: On 7/5/09, Paul L nine...@gmail.com wrote: Previously you had lastOrNil taking m [a] as input, presumably generated by mapM. So mapM is actually building an entire list before it returns the argument for you to call lastOrNil. This is where you had unexpected memory behavior. Now you are fusing lastOrNil and mapM together, and instead of building a list, you traverse it and perform monadic action along the way. This can happen in a constant memory if the original pure list is generated lazily. I think the real problem you had was a mis-understanding of mapM, and there was nothing wrong with your previous lastOrNil function. mapM will only return a list after all monadic actions are performed, and in doing so, it inevitably has to build the entire list along the way. -- Regards, Paul Liu Yale Haskell Group http://www.haskell.org/yale Hi, Paul, thanks for the comments. You're quite right that I am fusing the two functions together, but I think I wasn't mis-understanding mapM... I knew I was generating the entire list, and aside from the slight inefficiency of generating it only to tear it down an instant later, that would have been no problem. But I was expecting all of the memory associated with the list to be reclaimed after I had processed it, and that was what was not happening as far as I could tell. (This isn't one monolithic list, by the way; it's the small bodies of a couple of small scheme functions that get evaluated over and over. So the setup and teardown happens a lot.) I don't have very good intuition yet about what should get garbage-collected and what should get kept in such situations, and in fact I'm kind of in the same boat again: the test case now runs much better, but it still leaks memory, and I am again stumped as to why. Could I see something useful by examining ghc core? I haven't looked at that yet, no idea what to look for... Uwe ___ mapM_ might be useful to you. I know there are cases where mapM leaks memory but mapM_ doesn't, basically because mapM_ throws away all of the intermediate results immediately. You might want to condition on nullness of the list and then mapM_ your function over the init of the list and then just return the function on the last element of the list. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to pretty print code efficiently
On Fri, Jul 3, 2009 at 6:45 PM, John Kynewho...@gmail.com wrote:
Hi,
Currently I'm pretty printing code by building arrays of strings and calling
indent. For example:
instance JavaPrintableNamed AST.EnumeratedType where
javaLinesNamed parentName (AST.EnumeratedType memberDefinitions) =
[ public enum ++ asJavaId(parentName)
, {
] ++ memberCodeLines ++
[ }
,
]
where
memberCodeLines = indent $ javaLines memberDefinitions
The indent function takes a list of strings and adds an indent to the
beginning of every line.
I can imagine this to be very inefficient as it builds many strings and
concatenates them.
In Ruby, I might do the same thing like this:
class EnumeratedType JavaPrintableNamed
def writeTo(writer)
writer.print public enum
writer.puts self.asJavaId
writer.puts {
writer.indent do
self.memberDefinitions.writeTo(writer)
writer.puts
end
where above, the writer.indent takes care of the indent, and everything is
appended to a stream, which doesn't seem so bad in terms of efficiency.
I'm looking for a way to do something similar in Haskell.
Anyone can give me a hand?
Thanks
-John
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You may want to investigate the standard module
Text.PrettyPrint.HughesPJ, which contains a number of (I assume fairly
efficient) combinators for pretty printing.
Alex
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Re: [Haskell-cafe] Monoid wants a (++) equivalent
On Fri, Jul 3, 2009 at 10:17 PM, Jason Dusekjason.du...@gmail.com wrote: 2009/07/03 George Pollard por...@porg.es: This discussion points to a wider issue: at some stage we should look at pulling all the nice new stuff into Haskell prelude. I'm looking at you, Data.Foldable,Traversable. Also, throw out `map`. ;) What is the proper name for the operation on functions of a functor, anyway? The name `fmap` seems to driven by an analogy with `map`. -- Jason Dusek ___ I think map would be the right name. IMO, what would be really nice would be to rename fmap to map (and then fmap would become a deprecated synonym for map), etc., and get rid of many of the special cases for lists in the Prelude. The only backward compatibility problem that has been brought up is monomorphism restriction stuff, though. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monoid wants a (++) equivalent
On Wed, Jul 1, 2009 at 11:26 AM, Ross Patersonr...@soi.city.ac.uk wrote: On Wed, Jul 01, 2009 at 10:55:39AM -0700, Bryan O'Sullivan wrote: Okay, here's a tentative plan that will help to figure out the answer. I'll build a fiddled base package that rewires the Monoid class to have (++) be the binary operator, and mappend as a synonym for it. I'll import the Monoid (++) into the Prelude. I'll see how much breaks. If that much builds smoothly, I'll see how much of the rest of Hackage builds, both with and without this custom base package. I'll follow up here with the results, along with a suggestion of how acceptable I think the observed level of breakage is. Generalizing (++) will break some Haskell 98 code, e.g. append = (++) I think that's a show-stopper. ___ Could we use some default rules to keep H98 code working? I don't know much about defaulting, but times = (*) works fine and defaults to type Integer. Could we not do the same thing with monoids, having monoids default to type []? Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monoid wants a (++) equivalent
On Wed, Jul 1, 2009 at 10:11 PM, David Menendezd...@zednenem.com wrote: In Wed, Jul 1, 2009 at 3:38 PM, Thomas Schillingnomin...@googlemail.com wrote: 2009/7/1 David Leimbach leim...@gmail.com Just because the compiler can figure out what I mean because it has a great type system, I might not be able to figure out what I mean a year from now if I see ++ everywhere. Yep, had the same experience. On the one hand, using monoids lets you delay some design decisions for later and lets you reuse more library code. On the other hand, it sometimes makes it really hard to see what the code is actually doing--especially if you use more than one monoid. For this reason on of the first advanced features I implemented in the (yet unreleased) scion IDE library allows you to look up the instantiated type of an identifier. Unfortunately, jumping to the definition (or documentation) of the monoid instance is a bit more difficult. Haddock should allow documentation on instance declarations... I disagree. The solution is to not create instances when it isn't obvious what the instance does. That's why we have Sum and Prod in Data.Monoid instead of declaring instances directly for Int. With Monoid, I'd go further and say that you should not use mempty and mappend unless you are writing polymorphic code. If you are writing to a specific monoid instance, you should use a specific function. -- Dave Menendez d...@zednenem.com http://www.eyrie.org/~zednenem/ ___ I tend to disagree. I think that Haskell has seen a lot of syntax bloat in the interest of monomorphism. We have List.append, Map.union, Set.union, Sequence., etc., all with different notation, even though these all denote the same operation: taking two of (whatever) and combining them into one. With mappend, you know exactly what the function is supposed to do: combine two things together, and it doesn't matter what datatypes you're using, because that's always what it means. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] golf, predicate check function for MonadPlus (was Re: How to read safely?)
On Thu, Jul 2, 2009 at 3:36 AM, Jon Fairbairnjon.fairba...@cl.cam.ac.uk wrote: Dan Doel dan.d...@gmail.com writes: There was talk of adding a readMaybe a while ago, but apparently it never happened. As it is, you can use reads, read s becomes: case reads s of [(a, rest)] | all isSpace rest - code using a _ - error case which ensures that you have an unambiguous parse with only trailing whitespace. You can, of course, modify that if you don't care about ambiguity or trailing characters. I was wondering about a more algebraic way of writing that; here's a version (that doesn't care about ambiguity) readMaybe :: Read a = String - Maybe a readMaybe = join . fmap no_trailing_garbage . listToMaybe . reads where no_trailing_garbage = fmap fst . check (all isSpace . snd) check :: (MonadPlus m) = (a - Bool) - a - m a check p a | p a = return a | otherwise = mzero I tried Hoogling for a function like check, but couldn't find it. Surely there's one in a library somewhere? It looks useful to me. (I'm rather taken by way the check (all isSpace . snd) part reads) Monad.guard comes close but fails to get the cigar; in fact guard b == check (const b) () So check is more general. Also, I don't see a singletonListToMaybe that one could use in place of listToMaybe to require unambiguity. Could do isSingleton [a] = True isSingleton _ = False and then use listToMaybe . join . check isSingleton -- aha! Another use for check! Jón [Footnote: I thought of writing guard == flip (check . const) () but then realised it was pointless] -- Jón Fairbairn jon.fairba...@cl.cam.ac.uk You can use the Kleisli composition operator (=) to make it a little nicer. singletonListToMaybe :: [a] - Maybe a singletonListToMaybe [x] = Just x singletonListToMaybe _ = Nothing check :: MonadPlus m = (a - Bool) - a - m a check p a | p a = return a | otherwise = mzero readMaybe = fmap fst.check (all isSpace.snd) = singletonListToMaybe.reads Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to declare a Typeless Function
swap :: Array (Int, Int) a - [Int] - Array (Int, Int) a The lowercase a means that that type variable is polymorphic, i.e. it can be any type. Alex On Thu, Jul 2, 2009 at 8:05 PM, Fernan Bolandofernanbola...@mailc.net wrote: Hi I have a function that swaps rows of an array of double swap :: Array (Int,Int) Double - [Int] - Array (Int,Int) Double I then create a function that swaps rows of arrays of Complex Double swap :: Array (Int, Int) (Complex Double) - [Int] - Array (Int, Int) (Complex Double) In reality the function swap does not care whether its working on a double or a complex number. how do I declare swap so that it will work whether it's a complex or a double array. I tried googling but I wasn't sure what to google. fernan -- http://www.fernski.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Monoid wants a (++) equivalent
On Tue, Jun 30, 2009 at 11:24 PM, Ketil Maldeke...@malde.org wrote: You know, this might be the right time to start expanding our vocabulary beyond seven bits. Since we're likely to keep mappend around as an alias for some time, people would have a grace period to adjust. How about U+2295 (circle with plus inside it)? Or, if we would like to stick to the 8-bit subset to keep those 8859-1 users happy, how about ¤ (funny circle over an x, U+00A4) -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe The major disadvantage of that is that those symbols are not on my keyboard and thus are more of a pain to type, especially on the Linux console where compose key is not available... Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Obscure weirdness
On Sat, Jun 20, 2009 at 8:29 AM, Andrew Coppin andrewcop...@btinternet.com wrote: OK, so here's an interesting problem... I've been coding away all day, but now my program is doing something slightly weird. For a specific input, it summarily terminates. The registered exception handler does not fire. There is no output to stdout or stderr indicating what the problem is. It just *stops* half way through the printout. Weirder: If I run it in GHCi, then GHCi itself terminates. (I didn't think you could *do* that!) It's not as if my program is anything unusual. There are no unsafe functions. No FFI. Nothing. Just regular high-level Haskell. Is this a known bug in GHC 6.10.1? Will upgrading fix it? (Obviously, it's quite a lot of work to change GHC.) Suffice it to say that my program is quite big and complicated; it worked fine when it was still small and simple. ;-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe I think you'll need to provide a bit more detail about what you're doing in order for anyone to have anything to go off of. If you can link to the source, that would help, or even give a summary of what you're trying to do. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[2]: [Haskell-cafe] Re: Error message reform (was: Strange type error with associated type synonyms)
On Wed, May 27, 2009 at 3:24 PM, Bulat Ziganshin bulat.zigans...@gmail.com wrote: Hello Max, Thursday, May 28, 2009, 2:14:19 AM, you wrote: I absolutely agree about expected/inferred. I always forget which is which, because I can figure both could apply to each. That's actually true for me too. When you say it like that, I remember times when I've had the same confusion. it's why i asked beginners. it seems that we all go through times when ghc errmsgs looks cryptic but then we start to live with it and forget the first period actually, i don't have much problems with errrmsgs now, but trying to grok how i interpret them i've found that i mainly use *position* part of message, it's enough for me most times :) -- Best regards, Bulat mailto:bulat.zigans...@gmail.com Hi, I like the expected/inferred vocabulary. Maybe it comes from being a native English speaker, but to me, it says this is what we expected to get, but instead (through type inference), we got this type for this term. Of course, I've also been reading GHC error messages for a while, so I've gotten used to understanding what they mean. When I was new, I had more of a problem...but I'm not sure you can really eliminate that. Everything takes practice. :) Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Extensible Exceptions and IO
On Sat, May 23, 2009 at 9:09 AM, Ian Lynagh ig...@earth.li wrote: On Sat, Apr 25, 2009 at 11:18:43AM -0700, Alexander Dunlap wrote: In the extensible exceptions paper[1], which I believe is the guide behind the current Control.Exception in GHC 6.10, a SomeIOException type is discussed so that it would be possible to use the nice interface for IOExceptions. Is this implemented anywhere? In the GHC libraries, all I see is the old interface just plugged into a simple wrapper, so you have to use guard . isDoesNotExistError (for example) with catchJust to catch a certain type of exception. If not, are there any plans to implement it? I'm not aware of anyone currently working on putting the various core libs' exceptions into a sensible hierarchy, but it would be great if someone was to do so! I'd suggest that the best way to do this would be initially a combination of discussions on the mailing list (if anything is non-obvious) and building a design (and, if appropriate, rationale) on a wiki page, and ending with a library submissions proposal once you have a design worked out: http://www.haskell.org/haskellwiki/Library_submissions Thanks Ian I'm moving this from the GHC users' mailing list to the haskell-cafe mailing list because I think the discussion probably involves the entire Haskell community. Hi all, We have this new exception hierarchy in GHC base-4 and a lot of new machinery for dealing with it. Other libraries also seem to be springing up to provide alternative methods of dealing with these new exceptions. However, I have found that in most of my programming, the new exception functionality cannot be used to its fullest extent because the exceptions in the core libraries are not part of any sensible tree of exceptions, even though having this giant exception tree seems to be one of the premises of the new exception library. For example, there is an ArithException type in Control.Exception which records various arithmetic exceptions. However, there are two problems: (1) in order to fit in with the extensible exceptions theme, the type ought to be called SomeArithException and all of the types of ArithExceptions ought to be sub-exceptions of it, and (2) to my knowledge, this exception type doesn't seem to be thrown by anything in the base libraries. I think the biggest culprit is the IOException type, which also happens to be probably the most commonly-used exception type, since exceptions from IOException cannot really be anticipated and prevented. The current design of IOException is as follows. There is an abstract type called IOException. This can be queried with functions like isDoesNotExistError (which determines if the exception is because a file did not exist). Furthermore, to make an IOException, one has to use the mkIOError function, which takes a value of type IOErrorType, which is again an abstract type. I believe the reason for the abstract types is to allow implementations to extend the range of exceptions. Unfortunately, this has even more problems. For example, the error returned by using readFile on a directory cannot be determined using any Haskell98 function, meaning that code that wants to detect this error cannot be portable to compilers that don't have the new InappropriateType IO exception. The biggest problem with the current design of IOException, however, is that it doesn't play very nicely with the main hierarchy. Like with ArithExceptions, there should be a SomeIOException type. http://www.haskell.org/~simonmar/papers/ext-exceptions.pdf, which was the original paper on extensible exceptions, recommends having a class for IOExceptions that would allow us to get standard information from any specific IOException. I asked the GHC-users mailing list about this and Ian Lynagh recommended starting a discussion about how to put all of the core library exceptions into a sensible hierarchy. I think the goals here ought to be to make our new hierarchy as backward-compatible as possible. In particular, since many programs rely on the isWhateverError functionality, it would be nice to keep these in place. For IOExceptions, I think it would be nice to have a more multi-layered hierarchy than just SomeIOException and its subtypes. For example, I could see a hierarchy for IOExceptions looking something like this: SomeIOException - SomeCouldNotOpenException - AlreadyExists, NoSuchThing, PermissionDenied, InappropriateType - SomeResourceException - ResourceBusy, ResourceExhausted, ResourceVanished etc. I'm not all that familiar with the various exceptions and how they are used, so I'm not really able to propose a complete hierarchy for all the exceptions. I guess the purpose of this email was just to start off a discussion about what our exception hierarchy ought to look like. Thanks for reading this (admittedly long) message. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http
Re: Re: [Haskell-cafe] Re: A problem with par and modules boundaries...
On Sat, May 23, 2009 at 11:34 AM, Max Rabkin max.rab...@gmail.com wrote: On Sat, May 23, 2009 at 7:31 PM, Mario Blažević mblaze...@stilo.com wrote: Does anybody know of a pragma or another way to make a function *non-strict* even if it does always evaluate its argument? In other words, is there a way to selectively disable the strictness optimization? parallelize a b | False = (undefined, undefined) | otherwise = a `par` (b `pseq` (a, b)) might do, unless strictness analysis is smart enough to know that the False guard is always, well, False. --Max ___ GHC.Prim.lazy? Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Introducing Instances in GHC point releases
Since those types come out of the time library, and that library's version *has* been bumped (I assume), couldn't you use Cabal to condition on the version of the time library to determine whether or not to have CPP set a -DTYPEABLE_IN_TIME flag, and then #ifdef out your versions of the instances? I suppose a nice compiler extension would be recessive instances which would take effect only if they did not overlap with another instance. Probably a lot of corner cases involving multiple recessive instances to work out, though. Alex On Thu, May 21, 2009 at 2:53 PM, John Goerzen jgoer...@complete.org wrote: So this is annoying (CCing -cafe) I need NominalDiffTime and UTCTime to have Typeable instances. In 6.10.1, they didn't ship with them out of the box, so I added them. Apparently, in 6.10.3, they DO ship with those instances out of the box. Annoyingly, that means that my code breaks on 6.10.3. Even more annoyingly, __GLASGOW_HASKELL__ is still 610, so I can't even work around this via cpphs. There appears to be no way to make code that requires those Typeable instances work with both 6.10.1 and 6.10.3. Yet another reason to avoid API incompatibilities in point releases. Does anybody have an idea on the best way to handle this? -- John Don Stewart wrote: convertible appears broken with 6.10.3. Any thoughts? Writing new package config file... done. Downloading convertible-1.0.1... Configuring convertible-1.0.1... Preprocessing library convertible-1.0.1... Preprocessing executables for convertible-1.0.1... Building convertible-1.0.1... [1 of 8] Compiling Data.Convertible.Base ( Data/Convertible/Base.hs, dist/build/Data/Convertible/Base.o ) [2 of 8] Compiling Data.Convertible.Utils ( Data/Convertible/Utils.hs, dist/build/Data/Convertible/Utils.o ) [3 of 8] Compiling Data.Convertible.Instances.Map ( Data/Convertible/Instances/Map.hs, dist/build/Data/Convertible/Instances/Map.o ) [4 of 8] Compiling Data.Convertible.Instances.Num ( Data/Convertible/Instances/Num.hs, dist/build/Data/Convertible/Instances/Num.o ) [5 of 8] Compiling Data.Convertible.Instances.C ( Data/Convertible/Instances/C.hs, dist/build/Data/Convertible/Instances/C.o ) [6 of 8] Compiling Data.Convertible.Instances.Time ( Data/Convertible/Instances/Time.hs, dist/build/Data/Convertible/Instances/Time.o ) Data/Convertible/Instances/Time.hs:61:9: Duplicate instance declarations: instance Typeable NominalDiffTime -- Defined at Data/Convertible/Instances/Time.hs:61:9-32 instance Typeable NominalDiffTime -- Defined in time-1.1.3:Data.Time.Clock.UTC Data/Convertible/Instances/Time.hs:64:9: Duplicate instance declarations: instance Typeable UTCTime -- Defined at Data/Convertible/Instances/Time.hs:64:9-24 instance Typeable UTCTime -- Defined in time-1.1.3:Data.Time.Clock.UTC cabal: Error: some packages failed to install: HDBC-2.1.0 depends on convertible-1.0.1 which failed to install. convertible-1.0.1 failed during the building phase. The exception was: ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] HUnit
I believe you need to capitalize it correctly: HUnit. Alex On Thu, May 21, 2009 at 8:52 PM, Vasili I. Galchin vigalc...@gmail.com wrote: my bad ... what about: vigalc...@ubuntu:~/FTP/Haskell/Swish-0.2.1$ cabal configure Configuring swish-0.2.1... cabal: At least the following dependencies are missing: hunit -any On Thu, May 21, 2009 at 6:20 PM, Don Stewart d...@galois.com wrote: vigalchin: Hello, I have some code with several test cases that use HUnit. I added hunit as one of my cabal dependencies but cabal complained with: Setup: At least the following dependencies are missing: hutil -any ^ Typo. -- Don ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Building 'text-icu-0.1' on Windows
On Thu, May 14, 2009 at 10:25 AM, Bryan O'Sullivan b...@serpentine.com wrote: On Thu, May 14, 2009 at 10:18 AM, Bryan O'Sullivan b...@serpentine.com wrote: If one of you has the time to dig into this and send a patch that corrects the problem, I'd welcome the help. As I'm sure you can tell, I developed text-icu on Unix, and I don't have regular enough Windows access to make debugging this a quick and easy matter. By the way, if you run into problems building or using text or text-icu, please file a ticket here: http://trac.haskell.org/text/newticket Hi, I've attached a patch that fixes the build on Windows. It also doesn't seem to break the build on my Linux box. However, the patch basically consists of removing an #include statement and removing some library dependencies (which for some reason don't exist in the Windows distribution of ICU), so I don't know if it would break something else. The library compiles without warnings with the patch applied, though. Maybe Serge could test the patch on his machine? Thanks, Alex fix-windows-build.dpatch Description: Binary data ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data.FiniteMap deprecrated
You need to include the containers package in your cabal file. Alex On Sat, May 16, 2009 at 4:44 PM, Vasili I. Galchin vigalc...@gmail.com wrote: Yevgeni, I am specifying base as a Cabal dependency ... dumb question ... I have forgotten whether I need to import Data.Map also? Vasili On Sat, May 16, 2009 at 1:05 AM, Eugene Kirpichov ekirpic...@gmail.com wrote: It's Data.Map. It's in the base package. 2009/5/16 Vasili I. Galchin vigalc...@gmail.com: Hello, http://hackage.haskell.org/cgi-bin/hackage-scripts/package/FiniteMap ... since this is deprecated what is the orthodox way to implement finite map?? Thanks, Vasili ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Eugene Kirpichov Web IR developer, market.yandex.ru ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data.FiniteMap deprecrated
You need to import Data.Map in any file you use Map in. Which version of Parsec are you using? Versions before 3.0.0 do not have a module called Text.Parsec, the module is Text.ParserCombinators.Parsec (or something like that). Alex On Sat, May 16, 2009 at 5:03 PM, Vasili I. Galchin vigalc...@gmail.com wrote: yeah I do ghc can't find the Map constructor .. also in another module Parsec is being used ... I have parsec as a dependecy in my cabal file and I also have an import Text.Parsec but ghc could not find module Text.Parsec. ??? Thanks, Vasili On Sat, May 16, 2009 at 6:57 PM, Alexander Dunlap alexander.dun...@gmail.com wrote: You need to include the containers package in your cabal file. Alex On Sat, May 16, 2009 at 4:44 PM, Vasili I. Galchin vigalc...@gmail.com wrote: Yevgeni, I am specifying base as a Cabal dependency ... dumb question ... I have forgotten whether I need to import Data.Map also? Vasili On Sat, May 16, 2009 at 1:05 AM, Eugene Kirpichov ekirpic...@gmail.com wrote: It's Data.Map. It's in the base package. 2009/5/16 Vasili I. Galchin vigalc...@gmail.com: Hello, http://hackage.haskell.org/cgi-bin/hackage-scripts/package/FiniteMap ... since this is deprecated what is the orthodox way to implement finite map?? Thanks, Vasili ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Eugene Kirpichov Web IR developer, market.yandex.ru ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Building 'text-icu-0.1' on Windows
On Wed, May 13, 2009 at 12:54 AM, Serge LE HUITOUZE slehuito...@telisma.com wrote: Hi there, I fail to build text-icu-0.1 on Windows, and would appreciate some help... Thanks in advance! Let me describe what I've done so for, and what results I got: First, I got a version of ICU on my machine, specifically unzipping the file icu4c-4_0_1-Win32-msvc8.zip from http://icu-project.org/download/4.0.html. This made me having ICU stuff at location C:\HaskellStuff\icu\icu, as witnessed below: C:\HaskellStuff\icu\icudir 30/04/2009 16:08 REP . 30/04/2009 16:08 REP .. 30/04/2009 16:51 REP bin 30/04/2009 16:08 REP include 30/04/2009 16:36 REP lib 14/01/2009 22:48 2 032 license.html After downloading and untaring text-icu-0.1, I tried to configure, typing in: C:\HaskellStuff\pkg\text-icu-0.1runhaskell Setup configure Configuring text-icu-0.1... Setup: Missing dependencies on foreign libraries: * Missing C libraries: icui18n, icuuc, icudata This problem can usually be solved by installing the system packages that provide these libraries (you may need the -dev versions). If the libraries are already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where they are. OK, let's restart with appropriate values for said flags! C:\HaskellStuff\pkg\text-icu-0.1runhaskell Setup configure --extra-include-dirs=C:\HaskellStuff\icu\icu\include --extra-lib-dirs=C:\HaskellStuff\icu\icu\lib Configuring text-icu-0.1... And then proceed with the building per se: C:\HaskellStuff\pkg\text-icu-0.1runhaskell Setup build Preprocessing library text-icu-0.1... In file included from C:/HaskellStuff/icu/icu/include/unicode/umachine.h:47, from C:/HaskellStuff/icu/icu/include/unicode/utypes.h:36, from Data\Text\ICU\Error.hsc:148: C:/HaskellStuff/icu/icu/include/unicode/pwin32.h:120: error: redefinition of typedef 'int8_t' C:/ghc/ghc-6.10.1/include/mingw/stdint.h:27: error: previous declaration of 'int8_t' was here ... ... (a bunch of similar messages for other type aliases, e.g. 'uint8_t') ... compiling dist\build\Data\Text\ICU\Error_hsc_make.c failed command was: C:\ghc\ghc-6.10.1\gcc.exe -c -BC:\ghc\ghc-6.10.1\gcc-lib -IC:\ghc\ghc-6.10.1\include\mingw -D__GLASGOW_HASKELL__=610 -IC:\HaskellStuff\icu\icu\include -IC:\ghc\ghc-6.10.1\bytestring-0.9.1.4\include -IC:\ghc\ghc-6.10.1\base-4.0.0.0\include -IC:\ghc\ghc-6.10.1/include -IPAPI_INCLUDE_DIR dist\build\Data\Text\ICU\Error_hsc_make.c -o dist\build\Data\Text\ICU\Error_hsc_make.o C:\HaskellStuff\pkg\text-icu-0.1 The mingw/stdint.h file contains a series of lines of this style: typedef signed char int8_t; Whereas, the unicode/pwin32.h file contains contains stuff like: /* Define whether inttypes.h is available */ #ifndef U_HAVE_INTTYPES_H #define U_HAVE_INTTYPES_H 0 #endif ... /* Determines whether specific types are available */ #ifndef U_HAVE_INT8_T #define U_HAVE_INT8_T U_HAVE_INTTYPES_H #endif ... /* If your platform does not have the inttypes.h header, you may need to edit the typedefs below. */ #if U_HAVE_INTTYPES_H #include inttypes.h #else /* U_HAVE_INTTYPES_H */ #if ! U_HAVE_INT8_T typedef signed char int8_t; #endif ... #endif So, do I need to explicitely change the icu include file? And in which way? Any other solution/suggestion? Best regards. --Serge This isn't really helpful, but I received similar errors when trying to build icu-text on Windows. It seemed like the problem was that GHC's headers defined some of the symbols that were also defined in ICU...I don't know how to fix this though. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Main function error
On Tue, May 12, 2009 at 9:59 AM, applebiz89 applebi...@hotmail.com wrote: I have compiled each function independently and they have compiled the only problem is the main function.. I keep getting the error 'films not defined' and I am not sure why [code] type Title = String type Director = String type Year = Int type Fan = String data Film = Film Title Director Year [Fan] deriving Show -- List of films testDatabase :: [Film] testDatabase = [ (Film Casino Royale Martin Campbell 2006 [Garry, Dave, Zoe])] -- Function filmsInGivenYear :: Year - [Film] - [String] filmsInGivenYear year' films = [ title | (Film title director year fans) - films, year == year'] doFilmsInGivenYear :: [Film] - IO () doFilmsInGivenYear films = do putStrLn which year? text - getLine let year' = read text :: Int let answer = filmsInGivenYear year' films print answer main :: IO () main = do doFilmsInGivenYear films main [/code] if the other functions are compiling without this error im not sure as to why the main function does not compile because of the films...any light on this? Thanks When you say 'doFilmsInGivenYear films', where does the variable 'films' come from? It's not defined anywhere in your program. That's what the compiler is complaining about. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Just 3 = (1+)?
On Sat, May 9, 2009 at 12:31 PM, michael rice nowg...@yahoo.com wrote: Why doesn't this work? Michael data Maybe a = Nothing | Just a instance Monad Maybe where return = Just fail = Nothing Nothing = f = Nothing (Just x) = f = f x instance MonadPlus Maybe where mzero = Nothing Nothing `mplus` x = x x `mplus` _ = x [mich...@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude Just 3 = (1+) interactive:1:0: No instance for (Num (Maybe b)) arising from a use of `it' at interactive:1:0-14 Possible fix: add an instance declaration for (Num (Maybe b)) In the first argument of `print', namely `it' In a stmt of a 'do' expression: print it Prelude The type of (=) is (=) :: m a - (a - m b) - m b For the Maybe monad, that specializes to (=) :: Maybe a - (a - Maybe b) - Maybe b But when you say Just 3 = (+1) this desugars to (=) (Just 3) (\x - x + 1) but the second argument to (=) that you have given has the type (\x - x + 1) :: Num a = a - a, whereas it needs to return a type of Maybe a to fit the type signature. What you probably want is Just 3 = (Just . (+1)) so the second function returns a Maybe value. A nicer way of writing this is fmap (+1) (Just 3), which uses the Functor class. Intuitively, the fmap function applies a function to the inside of a container. All monads can be defined as Functors as well; all Monads in the standard libraries have their functor instances defined. Hope that helps you. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On Tue, Apr 21, 2009 at 8:34 PM, Richard O'Keefe o...@cs.otago.ac.nz wrote: On 21 Apr 2009, at 7:39 pm, Jason Dagit wrote: Not really. Obviously some programs use the feature, but let us restrict to interesting programs that have been shared with the world and have some potential to receive maintenance. Why? You are, in effect, saying that my code has no value at all. You are saying that code written by students has no value at all. Why do you think that only code that is shared with the world has some potential to receive maintenance? By the way, not all publicly available code is in Hackage. The hbc release that's on my SPARC -- and thankful I've been for it, the grief GHC has given me there -- has at least one use of an n+k pattern that I know of, and more in a specification comment. From these programs we can do a sampling. While I'm not a statistics expert, my understanding is the main problem with using hackage packages is a bit of selection bias. I can see no reason to assume that it's only a bit. Maybe it's a bit. Maybe it's a very great deal. It would be interesting to investigate this, but the only way you can investigate it is to examine a lot of code that _isn't_ there. I bet the selection bias isn't even that bad for this statistical test due to the nature of programming style diversity. Maybe someone with a stronger stats background could comment. I have a statistics degree. I don't know if that's strong enough for you. It's strong enough that I assume selection bias until I see evidence otherwise. I think that would give us an exhaustive collection of haskell code, but I assert we don't need that. Biologists don't need a DNA sample from every organism to draw conclusions about the genetics of a species. It depends on what _kind_ of conclusion they want to draw. If they want to show that some feature _is_ present, a sample will do. If they want to show that it's absent or rare, then they need a much bigger sample. Scientists work with incomplete data and draw sound conclusions in spite of that. The tools they use to do so are known as statistics. Yes, I know. That's why I get cross when people suggest silly things like trawling through Hackage to demonstrate that nobody is using n+k patterns. Where's the statistics in that? Where are the estimates of natural variation? Note: I do not assert that the use of n_k patterns is rare. Here's _all_ that I assert about n+k patterns: (1) they are part of Haskell 98 (2) I believe they make programs more readable (3) I use them (4) they are no worse than certain features proposed for addition to Haskell'. Okay, then prove n+k patterns are not rare in the publicly available sources. Why the X should I? I do not claim that they are common IN THE PUBLICLY AVAILABLE SOURCES, I have NEVER claimed that, and I don't even CARE whether they are rare in the publicly available sources or not. Because they make programs more readable, n+k patterns probably *should* be moderately common in the publicly available sources, but I have no idea whether they are or not. It *is* true that things that *are* used in the commonly available sources should continue to be supported in order to preserve the value of those commonly available sources. It is *not* true that things that are *not* used in the commonly available sources are therefore of no value and safely to be discarded. That's the challenge I was trying to make in my first email. It's a challenge to the irrelevant. Let's consider three versions of naive Fibonacci. fibA :: (Integral a, Integral b) = a - b fibA 0 = 1 fibA 1 = 1 fibA (n+2) = fibA (n+1) + fibA n Simple, readable, Haskell 98. pred2 :: (Integral a) = a - Maybe a pred2 n = if n = 2 then Just (n-2) else Nothing fibB :: (Integral a, Integral b) = a - b fibB 0 = 1 fibB 1 = 1 fibB x | Just n - pred2 x = fibB (n+1) + fibB n Uses a pattern guard, implemented in GHC. Pattern guards are neat. I like them a lot. They make sense. But it's impossible for me to see fibB as more readable than fibA. While pattern guards come _close_ to offering a replacement for n+k patterns, they don't quite. If I had f x (n+1) | p x = ... | q x = ... I would have to write the pattern guard twice as f x n' | Just n - pred1 n', p x = ... | Just n - pred1 n', q x = ... That doesn't seem like an advantage, somehow. Now for the third alternative. The view proposal in the Haskell' wiki and the views implemented in GHC are different. In fact the view proposal document goes to some trouble to show how views can replace n+k patterns, so I suppose I don't need to review that. Here's what it looks like using GHC syntax. (I can't make ghc 6.8.3 accept this; ghc --supported-languages does not list ViewPatterns. So this is UNTESTED CODE!) data Integral a = Nat2 a = Succ2 a | One2 | Zero2 nat2 :: Integral a = a - Nat2 a nat2 n |
Re: [Haskell-cafe] ANNOUNCE: Runge-Kutta library -- solve ODEs
It would also be nice if you could plug it into the hierarchical module system somewhere, perhaps renaming the module to Data.Algorithm.RungeKutta or Numeric.RungeKutta or Math.RungeKutta. This is pretty much the standard practice now, I think. Alex On Sun, Apr 19, 2009 at 4:04 PM, Uwe Hollerbach uhollerb...@gmail.com wrote: Damn, I never thought of that. Sorry! Guess I've only been haskell-hacking on my little old iMac... A new version is attached, with the version number bumped up by 0.0.1 and rungekutta.hs renamed to RungeKutta.hs. Hopefully it should work out-of-the-box now. Off to patch my repository... Uwe On 4/19/09, Alexey Khudyakov alexey.sklad...@gmail.com wrote: I have so far only tested it with ghc 6.8.3 on MacOS 10.3.9 (powerPC), but I know of no reason why it wouldn't work with other versions and OSs. It breaks on Linux (and all unices) because name of file in tarball is 'rungekutta.hs' not 'RungeKutta.hs' as required. In other words: god blame case-insensitive file systems. After renaming everything works just fine. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Infix tuple comma query (,)
On Mon, Apr 6, 2009 at 8:53 AM, Paul Keir pk...@dcs.gla.ac.uk wrote: module Main where data (:%^) a b = a :%^ b deriving (Show) main = do print $ 18 :%^ (Just 99) print $ (,) 9 10 print $ 9 , 10 The last line in the code above causes a compile error. Why does infix use of the comma (tuple constructor?) function fail without brackets? Thanks, Paul When I want a lighter syntax for pairs (when doing a long list of them, e.g.), I often define () :: a - b - (a,b) a b = (a,b) and then you can indeed write print $ 1 2 (assuming you get precedence right). Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Use unsafePerformIO to catch Exception?
On Thu, Mar 26, 2009 at 5:23 PM, wren ng thornton w...@freegeek.org wrote: Jules Bean wrote: wren ng thornton wrote: I have long been disappointed by a number of `error`s which shouldn't be. For example, the fact that `head` and `div` are not total strikes me as a (solvable) weakness of type checking, rather than things that should occur as programmer errors/exceptions at runtime. The use of `error` in these cases muddies the waters and leads to a laissez-faire attitude about when it's acceptable to throw one's hands up in despair and use `error` rather than writing a better function. head uses error in precisely the correct, intended fashion. head has a precondition (only call on non-empty lists) And that is *exactly* my complaint: the precondition is not verified by the compiler. Therefore it does not exist in the semantic system, which is why the error screws up semantic analysis. The type of head should not be [a] - a + Error, it should be (a:[a]) - a. With the latter type the compiler can ensure the precondition will be proved before calling head, thus eliminating erroneous calls. It's a static error, detectable statically, and yet it's deferred to the runtime. I'd much rather the compiler catch my errors than needing to create an extensive debugging suite and running it after compilation. Is this not the promise of purity? Ultimately, it's not detectable statically, is it? Consider import Control.Applicative main = do f - lines $ readFile foobar print (head (head f)) You can't know whether or not head will crash until runtime. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Data.Binary, Data.Text and errors
On Sun, Mar 15, 2009 at 9:13 PM, Bryan O'Sullivan b...@serpentine.com wrote: On Sun, Mar 15, 2009 at 8:40 PM, Alexander Dunlap alexander.dun...@gmail.com wrote: I have noticed that in both Data.Binary and Data.Text (which is still experimental, but still), the decode functions can be undefined (i.e. bottom) if they encounter malformed input. For decoding Unicode, it's typical to provide a flexible API that can do one of the following on a bad encoding: Substitute a character Skip it Return the partial decode Throw an exception I just haven't gotten there yet. Thanks! I guess we'll see how this comes together. Does anyone know about error handling options in Data.Binary? Are there plans to add an error handling mechanism? Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Editing Haskell wiki pages
On Fri, Mar 20, 2009 at 3:24 AM, Colin Paul Adams co...@colina.demon.co.uk wrote: Bulat == Bulat Ziganshin bulat.zigans...@gmail.com writes: Bulat Hello Colin, Bulat Friday, March 20, 2009, 9:18:59 AM, you wrote: How am I supposed to edit a page on the Haskell wiki? If I click on an Edit this page link, then Firefox prompts me to choose a tool to open an application/x-external-editor. When i just Bulat it should be a problem with your config, i just tried with Bulat ff 3.0.7/win32 - it edits fine. nothing was specially Bulat configured for this behaviour I haven't configured anything special either. ff 3.0.7/Linux x64 Go to your preferences page. Under the Editing tab, the field Use external editor by default is probably checked. Uncheck it, and you will be able to edit the pages directly in Firefox. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Editing Haskell wiki pages
On Fri, Mar 20, 2009 at 7:31 AM, Colin Paul Adams co...@colina.demon.co.uk wrote: Alexander == Alexander Dunlap alexander.dun...@gmail.com writes: Alexander On Fri, Mar 20, 2009 at 3:24 AM, Colin Paul Adams Alexander co...@colina.demon.co.uk wrote: Bulat == Bulat Ziganshin bulat.zigans...@gmail.com writes: Bulat Hello Colin, Bulat Friday, March 20, 2009, 9:18:59 AM, you wrote: How am I supposed to edit a page on the Haskell wiki? If I click on an Edit this page link, then Firefox prompts me to choose a tool to open an application/x-external-editor. When i just Bulat it should be a problem with your config, i just tried with Bulat ff 3.0.7/win32 - it edits fine. nothing was specially Bulat configured for this behaviour I haven't configured anything special either. ff 3.0.7/Linux x64 Go to your preferences page. Under the Editing tab, the field Use Alexander external editor by default is probably Alexander checked. Uncheck it, and you will be able to edit the Alexander pages directly in Firefox. What Editing tab? -- Colin Adams Preston Lancashire It's on the left-hand side of the page, at least on my screen. I guess it doesn't really look like a tab, but there's a link that says Editing on the Preferences page. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Editing Haskell wiki pages
On Fri, Mar 20, 2009 at 8:07 AM, Colin Paul Adams co...@colina.demon.co.uk wrote: Alexander == Alexander Dunlap alexander.dun...@gmail.com writes: What Editing tab? -- Colin Adams Preston Lancashire It's on the left-hand side of the page, at least on my screen. I guess Alexander it doesn't really look like a tab, but there's a link Alexander that says Editing on the Preferences page. Which preferences page - there are lots of them. But wherever I look, I can't see it. -- Colin Adams Preston Lancashire http://haskell.org/haskellwiki/Special:Preferences Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Data.Binary, Data.Text and errors
Hi all, I have noticed that in both Data.Binary and Data.Text (which is still experimental, but still), the decode functions can be undefined (i.e. bottom) if they encounter malformed input. What is the preferred way to use these functions in a safe way? For example, if one writes data to a disk using Data.Binary and wants to read it back in at a later date, how can one ensure that it is valid so that Data.Binary does not hit an error? Or do you just have to catch the exception in the IO Monad? Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Natural Numbers: Best implementation?
2009/3/12 Mark Spezzano mark.spezz...@chariot.net.au:
Hi,
I was wondering what the best way to implement Natural number would be. Is
there a package which already does this?
Here are some options:
1. Don’t bother. Just use Integer.
2. Use the type
data Natural = Zero | Succ !Natural
3. Use the following definition taken from the Gentle Introduction to
Haskell 98
newtype Natural = MakeNatural Integer
toNatural ::Integer- Integer
toNatural x | x 0 = error “Can’t create negative naturals!”
| otherwise = MakeNatural x
fromNatural :: Natural - Integer
fromNatural (MakeNatural i) = i
and then...
instance Num Natural where
fromInteger = toNAtural
x + y = toNatural (fromNatural x + fromNatural y)
x – y = etc..
x * y = etc...
Which method is best? So far, I’ve been picking option #1 – just leaving
things as they are and using Integer to keep things simple.
I’ve got that feeling that [2] would be fast and [3] would be slow. Comment
appreciated on the merits of each.
Cheers,
Mark Spezzano
I would tend to use option (1) unless there's a compelling reason not
to. Since naturals aren't closed under subtraction, you would in
practice probably have just as many non-total functions as you would
with the regular Int{,eger} type. Also, a lot of functions just take
Integers so it would be more of a pain to use.
In terms of speed, I think that [3] would be reasonably fast (unless
you do a ton of subtraction with bounds-checking) and [2] would
probably be quite slow, because you don't get the speed-boost from
doing computations right on the processor.
Alex
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Re: [Haskell-cafe] bytestring vs. uvector
On Mon, Mar 9, 2009 at 3:12 AM, Henning Thielemann lemm...@henning-thielemann.de wrote: On Mon, 9 Mar 2009, Claus Reinke wrote: Given the close relationship between uvector and vector, it would be very helpful if both package descriptions on hackage could point to a common haskell wiki page, starting out with the text and link above, plus a link to the stream fusion paper (I hadn't been aware that vector incorporates the recycling work, and had often wondered about the precise relationship between those two packages). Apart from saving others from similar confusion, that would also provide a place to record experience with those two alternatives. I have at least started a page which mentions the existing alternatives: http://www.haskell.org/haskellwiki/Storable_Vector ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Thanks for all of the responses! So let me see if my summary is accurate here: - ByteString is for just that: strings of bytes, generally read off of a disk. The Char8 version just interprets the Word8s as Chars but doesn't do anything special with that. - Data.Text/text library is a higher-level library that deals with text, abstracting over Unicode details and treating each element as a potentially-multibye character. - utf8-string is a wrapper over ByteString that interprets the bytes in the bytestring as potentially-multibye unicode characters. - uvector, storablevector and vector are all designed for dealing with arrays. They *can* be used for characters/word8s but are not specialized for that purpose, do not deal with Unicode at all, and are probably worse at it. They are better for dealing with things that you would generally use arrays for. If that seems accurate, I'll put it on the wiki. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: derive Drift - some help please
What problems are you encountering? Alex On Sun, Mar 8, 2009 at 11:58 AM, G?uenther Schmidt red...@fedoms.com wrote: Hi Sterling, sry, but I can't get it to work. Günther Sterling Clover schrieb: You shouldn't need any extra tools for this. A combination of the StandaloneDeriving and DeriveDataTypeable extensions in GHC should suffice. Cheers, S. On Sun, Mar 8, 2009 at 10:30 AM, Gü?nther Schmidt gue.schm...@web.de mailto:gue.schm...@web.de wrote: Hi, I need to derive Data, Typeable for Data.Time.LocalTime. I had hoped to use either derive or DrIFT to generate the instance declarations for it. I tried to understand the respective user manuals but it's still all Greek to me. Someone please help! Günther ___ Haskell-Cafe mailing list haskell-c...@haskell.org mailto:Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: derive Drift - some help please
Well you can do deriving instance Typeable LocalType etc. if you enable the DeriveDataTypeable and StandaloneDeriving extensions. You will get other error messages for missing instances; just keep adding deriving instance Typeable Day etc. Eventually, you will need to implement an instance by hand for Pico, because Pico's data constructors are not exported from Data.Fixed. I'm not very familiar with Data and Typeable so I can't help you there, but you can probably look at some existing instances for help. Hope that's useful for you. Alex On Sun, Mar 8, 2009 at 12:40 PM, Gü?nther Schmidt gue.schm...@web.de wrote: Hi Alexander, well I have this: newtype AdmissionDate = AdmDt LocalTime deriving (Data,Typeable ..) at least that's where I wonna get. I don't know how to create the proper instance declarations though for LocalTime. AdmissionDate is an element of a larger data structure, this data structure for instance also has a newtype DischargeDate = DchrgDt LocalTime deriving (Data,Typeable ...) With my haskell work so far I was blessedly free of the need for deriving. Until now ... Günther Alexander Dunlap schrieb: What problems are you encountering? Alex On Sun, Mar 8, 2009 at 11:58 AM, G?uenther Schmidt red...@fedoms.com wrote: Hi Sterling, sry, but I can't get it to work. Günther Sterling Clover schrieb: You shouldn't need any extra tools for this. A combination of the StandaloneDeriving and DeriveDataTypeable extensions in GHC should suffice. Cheers, S. On Sun, Mar 8, 2009 at 10:30 AM, Gü?nther Schmidt gue.schm...@web.de mailto:gue.schm...@web.de wrote: Hi, I need to derive Data, Typeable for Data.Time.LocalTime. I had hoped to use either derive or DrIFT to generate the instance declarations for it. I tried to understand the respective user manuals but it's still all Greek to me. Someone please help! Günther ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org mailto:Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] bytestring vs. uvector
Hi all, For a while now, we have had Data.ByteString[.Lazy][.Char8] for our fast strings. Now we also have Data.Text, which does the same for Unicode. These seem to be the standard for dealing with lists of bytes and characters. Now we also have the storablevector, uvector, and vector packages. These seem to be also useful for unpacked data, *including* Char and Word8 values. What is the difference between bytestring and these new fast array libraries? Are the latter just generalizations of the former? Thanks for any insight anyone can give on this. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] environment variables for runghc?
I doubt there's an env variable, but you could do $ alias ghc='ghc -Wall' Alex On Tue, Mar 3, 2009 at 7:07 PM, Erik de Castro Lopo mle...@mega-nerd.com wrote: Hi all, Is there some environment variable I can set so that runghc can be told to always use -Wall? Cheers, Erik -- - Erik de Castro Lopo - ... a discussion of C++'s strengths and flaws always sounds like an argument about whether one should face north or east when one is sacrificing one's goat to the rain god. -- Thant Tessman ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Practise of hiding contsructors and serialization
On Sun, Feb 22, 2009 at 5:23 PM, Marc Weber marco-owe...@gmx.de wrote: I usually use Data.Binary for serialization. Sure. But you can't derive Data.Binary easily for NominalDiffTime. That's the point. You can only do so by using toReal or by adding data MyOwnPico = MyOwnPico only to create serialization instances. I don't like this kind of code duplication.. Marc One advantage of the current system is that is preserves data abstraction. For example, right now NominalDiffTime is declared newtype NominalDiffTime = MkNominalDiffTime Pico so it would be possible for GHC or another automated tool to derive an instance of Read, Show, Binary, whatever for it. However, what if, in the future, the maintainer of the Time library wanted to change this to newtype NominalDiffTime = MkNominalDiffTime Double ? Now your derived instance wouldn't be the same, so your serialized data would not be compatible even between the same version of your library even if the version of your library stayed the same because the time library version had same. Even worse, what if the Time library maintainer changed it to newtype NominalDiffTime = MkNominalDiffTime (Int - Pico) ? Now you can't make a derived instance at all! There's no really good reason why they would make these changes; it seems reasonable that the Time library will stay in much the same form for the forseeable future. However, the whole point of abstract datatypes is that the maintainer can change the internals without disrupting dependent packages, as long as the accessor functions has the same semantics. In order for this to work, you can't have any external code - even derived instances - relying on the internal structure of abstract datatypes. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: A typeclass for Data.Map etc?
On Fri, Feb 20, 2009 at 6:40 AM, Achim Schneider bars...@web.de wrote: wren ng thornton w...@freegeek.org wrote: (b) allows instances to have a fixed type for keys (like Data.Trie and Data.IntMap have), Can't we do some type magic to automagically select Data.Trie if the key is a (strict) bytestring? I believe this is a key feature of Associated Types: you can create self-optimizing libraries that select a different container structure depending on the type. For example, class XMapK k where type XMap k :: * - * insert :: k - XMap k v - XMap k v and then instance XMapK Int where type XMap Int = Data.IntMap.IntMap insert = Data.IntMap.insert etc. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A typeclass for Data.Map etc?
On Thu, Feb 19, 2009 at 10:18 AM, Jamie Brandon jamii...@googlemail.com wrote: Maybe this is of interest: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/gmap The edison api is much more stable. The gmap api was already in place when I started working on it but I would prefer to at some point make it a superset of the edison api. No sense in having more than one map interface lying around. Jamie What would really be nice is a general interface to a lot of different types of containers that was both standardized by the community AND used as the basis for lots of different libraries. For instance, the new split package is a well-crafted library for dealing with splitting lists, but it only works on lists. The exact same logic that's already there could be used for splitting ByteStrings, Lazy ByteStrings, Data.Sequences, some of the sequence types in Edison, fixed-length vectors, etc., but currently, the way the package is made forces you to use only Haskell's lazy lists. Now if we look at the zlib package, this package decompresses data into the lazy ByteString format. So you can't directly split data that comes out of zlib - you have to convert it to String and then back again, which is annoying and also imposes a performance cost. If we could figure out a way to have a general interface to sequence-like or map-like or set-like objects, we could make our libraries a lot more general and also more useful in combination with each other. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Open unqualified imports
On Wed, Feb 4, 2009 at 4:22 PM, Henning Thielemann lemm...@henning-thielemann.de wrote: On Wed, 4 Feb 2009, Simon Marlow wrote: Ian Lynagh wrote: On Fri, Jan 16, 2009 at 06:42:46AM -0800, eyal.lo...@gmail.com wrote: Closed-unqualified import: import Data.Map(Map, lookup) One problem with this style is that you can get lots of conflicts from your VCS if you have multiple people working on the same module. Right; in GHC we actively discourage the use of explict import lists (or closed-unqualified import to use the terminology of this thread). http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Imports I find that the reasons not to use explicit import lists outweigh the reasons to use them, at least for code that I'm working on (rather than just reading). Ideally the import lists would be maintained automatically by an IDE, and would be intelligently handled by the VCS. This will work only if you exclusively import from modules of the same project, where you can choose identifiers to never clash, and external library modules where no functions are added anymore. Certainly for GHC this works since the compiler as basis tool must not depend on many libraries. However for me the situation is that I use many libraries, each maintained by only a few people. Within this kind of development name clashes are more likely than versioning conflicts. The only style that avoids both kinds of conflicts is the qualified import, which I try to use whereever possible. Another solution would be an automated tool to add explicit import lists as part of the release process. Then you don't have to worry about maintaining them but once you do the final release, you know that it won't break if new functions are added to dependencies. Perhaps I will create such a tool sometime... Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Quite confused by simple transformations on this code not working
Instead of declaring (/\) :: Eq a = Sentence a - Sentence a - Sentence a, you could say (/\) :: Eq a - [a] - [a] - [a]. Then it would work in both places. ([a] - [a] - [a] is a more general type than [[Term a]] - [[Term a]] - [[Term a]], so functions with the former type can be used in place of functions of the latter type but not vice versa.) Alex 2009/1/20 Andrew Wagner wagner.and...@gmail.com: So...there's just no good way to avoid the duplication? On Tue, Jan 20, 2009 at 11:10 PM, wren ng thornton w...@freegeek.org wrote: Andrew Wagner wrote: Strange little bit of code: http://moonpatio.com:8080/fastcgi/hpaste.fcgi/view?id=829#a829 If I do any of the following, all of which seem natural to me, it fails to typecheck: 1. move f out of the 'where' clause (with or without a type signature) 2. put the same type signature on f as is on (/\) 3. replace f with (/\) completely What's going on here? :t (nub .) . (++) (nub .) . (++) :: (Eq a) = [a] - [a] - [a] :t foldr (map . (nub .) . (++)) foldr (map . (nub .) . (++)) :: (Eq a) = [[a]] - [[a]] - [[a]] The type you give to (/\) is more restrictive than the type of the expression, and f uses the generality of the expression. -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell-beginners] pattern matching on date type
On Thu, Jan 1, 2009 at 12:36 AM, Max.cs max.cs.2...@googlemail.com wrote: thanks! suppose we have data Tree a = Leaf a | Branch (Tree a) (Tree a) deriving Show and how I could define a function foo :: a - Tree a that foo a = Leaf a where a is not a type of Tree foo b = b where b is one of the type of Tree (Leaf or Branch) ? The following code seems not working.. foo (Leaf a) = a foo a = Leaf a saying 'Couldn't match expected type `a' against inferred type `Btree a'' any idea? Thanks, Max You can't define such a function. foo :: a - Tree a, but the definition foo b = b has the type a - a, which is why the compiler says it can't match type a against Tree a. In general, Haskell functions can't look at the type of their arguments: they are either monomorphic or are polymorphic, in which case you can only use polymorphic functions that match the polymorphic type. For the problem you are trying to solve, you probably need to encode this logic higher up in the overall function (e.g. have one function to deal with as and another to deal with Tree as). Hope that helps, Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Time for a new logo?
2008/12/17 George Pollard por...@porg.es: Might be interesting to try angling the ends of the stems to look something more like the guillemot in [1]. I might try this in Gimp but I'm no designer :P Here is what I got; I think I chose the wrong yellow :) Based it on the font Diavlo, which is free. I love this one. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Multi-parameter type class woes
2008/12/15 Mario Blazevic mblaze...@stilo.com: Alexander Dunlap wrote: On Sun, Dec 14, 2008 at 8:10 PM, Mario Blažević mblaze...@stilo.com wrote: I'll take a swing at this one: instance Container (Maybe x) [x] where wrapper = isNothing . . . That isn't a sensible definition of 'wrapper', but I believe without trying to compile it is completely legal. Which wrapper do you use? You /don't/ have a different matching Container instance, but without the functional dependency you /might/, and ghc barfs. But liftWrap doesn't require any particular instance, it's a generic function accepting any pair of types for which there is an instance of Container. Instance selection (as I understand it) shouldn't come into play until one applies liftWrap to a particular type, and indeed it does cause problems there: note the type annotations on the last line. That part I understand and accept, or at least have learned to live with. The problem is that y is not mentioned in the signature of wrapper. When you call wrapper x, there could be many different instances of Container x y with the same x, so GHC doesn't know which version to call. I guess I see it now. However, if the explicit 'Container x y =' context couldn't fix the y to use for instantiation of Container x y, I don't see any way to fix it. And if there is no way to call wrapper in any context, the class declaration itself is illegal and GHC should have reported the error much sooner. Should I create a ticket? You can fix this problem either by adding a functional dependency or by splitting wrapper out into its own class (Wrapper x, e.g.) so all of the type variables in the class head are mentioned in its type and the instance can be determined by the call. Thanks for asking this question, by the way. I had known about this issue but had never really realized why it happened. Now that I have thought about it, I understand it too. :) Hope that helps, Alex I think that http://www.haskell.org/pipermail/haskell-cafe/2008-April/041461.html may be relevant. It's a design decision. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re: [Haskell-cafe] Multi-parameter type class woes
On Sun, Dec 14, 2008 at 8:10 PM, Mario Blažević mblaze...@stilo.com wrote: I'll take a swing at this one: instance Container (Maybe x) [x] where wrapper = isNothing . . . That isn't a sensible definition of 'wrapper', but I believe without trying to compile it is completely legal. Which wrapper do you use? You /don't/ have a different matching Container instance, but without the functional dependency you /might/, and ghc barfs. But liftWrap doesn't require any particular instance, it's a generic function accepting any pair of types for which there is an instance of Container. Instance selection (as I understand it) shouldn't come into play until one applies liftWrap to a particular type, and indeed it does cause problems there: note the type annotations on the last line. That part I understand and accept, or at least have learned to live with. The problem is that y is not mentioned in the signature of wrapper. When you call wrapper x, there could be many different instances of Container x y with the same x, so GHC doesn't know which version to call. You can fix this problem either by adding a functional dependency or by splitting wrapper out into its own class (Wrapper x, e.g.) so all of the type variables in the class head are mentioned in its type and the instance can be determined by the call. Thanks for asking this question, by the way. I had known about this issue but had never really realized why it happened. Now that I have thought about it, I understand it too. :) Hope that helps, Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Multi-parameter Type Class
2008/12/11 Thomas DuBuisson thomas.dubuis...@gmail.com:
I see Lennart answered your question. For more fun you could also do this
with TypeFamilies, which are the new hot thing in Haskell type level logic.
Since you are just getting into MPTC, FunDeps etc I figured you'd be
interested.
-- START CODE --
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies,
TypeSynonymInstances, FlexibleInstances, TypeFamilies #-}
data Foo = Foo Barderiving(Show)
data Bar = Bar String deriving(Show)
-- A family of types will evaluate from one type to another.
-- Here, I chose the word 'Eval', which you could make more meaningful.
-- It is basically a function over types.
type family Eval b
-- This is three definitions for the type function 'Eval'
type instance Eval Foo = Integer
type instance Eval Bar = String
type instance Eval [x] = [Eval x]
-- And instead of a functional dependency
-- you have a type level function (Eval) that operates on the type 'a'.
class ZOT a where
zot :: a - Eval a
instance ZOT Foo where
zot x = 17
instance ZOT Bar where
zot x = Eighteen
-- And don't forget that x must be an instance of ZOT to apply zot.
instance (ZOT x) = ZOT [x] where
zot xs = map zot xs
main = do print $ zot $ Foo $ Bar Blah
print $ zot $ Bar Blah
print $ zot $ [Bar Blah, Bar Blah] -- No map here please
I don't mean to hijack the original question, but I have a question
about this code. Is this the same as saying
class ZOT a where
type Eval a
zot :: a - Eval a
and then appropriate instance declarations? Is there any reason to
have the type function inside or outside of the class?
Thanks,
Alex
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Re: [Haskell-cafe] Data.Map.fromListWith
On Sat, Dec 6, 2008 at 12:22 PM, Paul Johnson [EMAIL PROTECTED] wrote: I've just been looking at the Data.Map function fromListWith. According to the docs, it has the type: * fromListWith* :: Ord http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Ord.html#t%3AOrd k = (a - a - a) - [(k, a)] - Map http://www.haskell.org/ghc/docs/latest/html/libraries/containers/Data-Map.html#t%3AMap k a I'd have thought that a better type would be * fromListWith* :: Ord http://www.haskell.org/ghc/docs/latest/html/libraries/base/Data-Ord.html#t%3AOrd k = (a - b - b) - [(k, a)] - Map http://www.haskell.org/ghc/docs/latest/html/libraries/containers/Data-Map.html#t%3AMap k b This wouldn't break any existing code, but would allow things like fromListWith (:) to do the Right Thing. Would this be a sensible change (along with the other with functions in the module). Paul. Hi, I don't think that type makes sense. fromListWith takes a list of [(key,value)] and a combining function to combine the values when there are multiple pairs with the same key. Thus, a type of (a - b - b) for the combining function doesn't make sense because the values are all going to have the same type (i.e. they are all as). We might consider (a - a - b), but this doesn't make sense either because then you have some values with type a (the ones that didn't need to be combined) and some with type b (the ones that were combined). (a - a - a) is really the only type that works. I don't think fromListWith (:) makes sense either. (:) :: a - [a] - [a], so you would end up with some values of the map as individual items and others as lists of items. All of the values need to have the same type. Regards, Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Moving the messages for compiler to another file?
On Tue, Dec 2, 2008 at 2:56 AM, nml [EMAIL PROTECTED] wrote: How about moving the messages for compiler to an additional file? My motivation is that we often face a trade-off between aesthetical(elegant code) and practical(efficient code). Like pragmas and strictness annotations, I often feel they make the source code ugly. They don't affect the semantics of our programs, do they? Some people would say this beautifying should be accomplished by an editor, like hiding/showing option for those information. But such separation has additional benefits. For instance, making the source code more compiler-independent. (yeah, this is not the case with language-extensions) And we avoid dispersing those information among our lines. (or even files) In some cases, it would be convenient. I'm not sure if this idea is reasonable, reachable or just naive. Suggestions? Best regards -nml ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe One problem is that in many cases, the things that you're talking about DO affect semantics. Consider strictness annotations: f (!x) = Just x g x = Just x h (Just _) = True h Nothing = False Now h (f _|_) == _|_, but h (g _|_) == True, even though a strictness annotation was the only difference between them. (For those not already familiar with it, _|_ (bottom) is a semantically undefined value, equivalent to an infinite loop.) I agree that it's good to reduce semantically-irrelevant code, but I'm not sure how feasible the proposal would be. I think GHC's rewrite rules may be of interest in this problem (the debate between elegant and fast code). Don Stewart has also written about this issue and has a couple of really good posts on his blog (http://cgi.cse.unsw.edu.au/~dons/blog/2008/05/16#fast). Warm regards, Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Could not find module ... which is hidden problem
On Sat, Nov 29, 2008 at 11:22 PM, Belka [EMAIL PROTECTED] wrote: Hello! START-- $ sudo runghc Setup configure --user Configuring HCL-1.2... $ sudo runghc Setup build Preprocessing library HCL-1.2... Preprocessing executables for HCL-1.2... Building HCL-1.2... HCL.hs:302:7: Could not find module `System.Random': it is a member of package random-1.0.0.1, which is hidden $ sudo ghc-pkg expose random Saving old package config file... done. Writing new package config file... done. $ sudo runghc Setup configure --user Configuring HCL-1.2... $ sudo runghc Setup build Preprocessing library HCL-1.2... Preprocessing executables for HCL-1.2... Building HCL-1.2... HCL.hs:302:7: Could not find module `System.Random': it is a member of package random-1.0.0.1, which is hidden -THE-END Perhaps, i'm 1000th, who asks how to solve this. Sorry that didn't search better, but it's really tiring for a newby. :-( [ http://www.haskell.org/haskellwiki/Upgrading_packages/Updating_to_GHC_6.8 Here ] found something, which maybe called a gentle explaination of what's going on. I have ghc ver: 6.8.2, base ver: 3.0.1.0. So there are 2 questions of mine: (1) Is there any centralized and well organized Haskell Knowledge Base, where it would be much easier to find solutions for popular problems? Anybody, please, your recommendations. (2) As for the topic: the only option for me would be obtaining older GHC (base?) - is that correct? So that require me reinstalling every Haskell package I have installed so far? Thanks in advance. -- View this message in context: http://www.nabble.com/%22Could-not-find-module-...-which-is-hidden%22-problem-tp20742582p20742582.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Is the random package in the build-depends of your cabal file? If not, that's why it's hidden. Cabal hides all of the packages that aren't listed in build-depends so that you can't accidentally depend on something that you don't list. Hope that helps. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] performance difference for binary-0.4.3.1 with ghc-6.8.3 and ghc-6.10
On Sun, Oct 26, 2008 at 9:36 AM, John Lato [EMAIL PROTECTED] wrote: Hello, I was experimenting with using ghc-6.10.0.20081007 on a project, and it seems that binary-0.4.3.1 has markedly worse performance in certain cases. With the following simple test: import qualified Data.ByteString.Lazy as L import Data.Binary import Data.Binary.Get import Control.Monad main :: IO () main = do b - L.readFile some_binary_file putStrLn $ show $ runGet getter b getter :: Get [Word16] getter = replicateM 100 getWord16le running this program compiled with ghc-6.10 takes about 4 times as long (and consumes much more memory) as when compiled with ghc-6.8.3. The extra time appears to be proportional to the number of elements processed in the Get. Running the programs with -hT shows a clear memory difference, which I think is the source of the problem. I've placed pdfs of that output at https://webspace.utexas.edu/latojw/data/ The difference seems to manifest itself only when the elements are actually processed; changing show $ runGet to show $ length $ runGet is slightly faster in 6.10. I was working on an Intel Mac with OS 10.4, binary-0.4.3.1, and bytestring-0.9.1.4. Can anyone confirm this, or suggest what might be the difference? Thank you, John Lato ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe With GHC 6.8.2: test: too few bytes. Failed reading at byte position 1613914 real0m27.573s user0m12.917s sys 0m0.087s With GHC 6.11.20081003: test: too few bytes. Failed reading at byte position 1613914 real0m21.528s user0m14.759s sys 0m0.135s I'm not using the exact same versions as you are, but I seem to be getting different results. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Linking and unsafePerformIO
On Tue, Oct 14, 2008 at 2:14 PM, Jules Bean [EMAIL PROTECTED] wrote: David Roundy wrote: On Tue, Oct 14, 2008 at 05:20:35PM +0100, Jules Bean wrote: Running a program on a different interpreter or compiler had better not change its denotation, otherwise it [the denotation] is not much use as a basis for reasoning. But you're saying above that we can't change programs, right? You probably won't be surprised to hear that different compilers are different programs. And different packages are also different programs. Are you the only one who's allowed to fix bugs? No. I think we must be at cross purposes. I'm saying that we can change programs, and that changes their denotation, and that's fine, and anyone can do that. But the denotation of a program is supposed to be something independent of a particular compiler or OS or MAC address or RAM size or any of the millions of other things which probably don't change during the single run of a program. Putting these things into the IO monad is not an abuse of the IO monad. It is simply an acknowledgement that they are runtime things, and not denotational constructs. Jules ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe But they aren't runtime constructs, they are compile-time constructs, just like how (+) has a different meaning when you are on Windows than it does when you are on Linux: they have different ways of allocating memory, adding two numbers, putting the result somewhere, etc. Really, they still denote the same thing (add two numbers), they just abstract over system-dependent details. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] MPFR / FFI - Nefarious (Simple?) bug
On Sun, Oct 5, 2008 at 5:38 PM, Chris Mears [EMAIL PROTECTED] wrote: Jared Updike [EMAIL PROTECTED] writes: Can someone else verify if this is a Mac/BSD only problem by compiling and running my code? (Does the C executableworks work? Does the Haskell executable noworks not work?) Can anyone on Linux and Windows attempt to compile/run and see if you can get the same results? I can't help you fix the problem, but can confirm that it behaves as you describe on my Linux system. The program works produces correct output, but noworks gives: == [...] 290 1.0 291 1.00315821571076304370386829039486164636536120569099678825575157477354990710405658927062619547595122867857873765133026050519075772712996650488675128428778696175272153632855303857710094714398932056909218681833163883482512398576534211955878126971368731378321605715030861892325510831739618679352919703855747262870175016933079782350322237289103929997053812694251557912158924211585973481445271933347978517620 292 [...] 384 1.00315821571076304370386829039486164636536120569099678825575157477354990710405658927062619547595122867857873765133026050519075772712996650488675128428778696175272153632855303857710094714398932056909218681833163883482512398576534211955878126971368731378321605715030861892325510831739618679352919703855747262870175016933079782350322237289103929997053812694251557912158924211585973481445271933347978517620 385 1.00315821571076304370386829039486164636536120569099678825575157477354990710405658927062619547595122867857873765133026050519075772712996650488675128428778696175272153632855303857710094714398932056909218681833163883482512398576534211955878126971368731378321605715030861892281684860507747503219323326708248193093246881500522519352192987169395899758472958160241015525770811984166890239749344848735134541218 386 ../get_str.c:149: assertion failed: size_s1 = m Aborted == All the values from 291-384 are the same, but 385 is slightly different. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe My results (from Linux i686) are slightly different. I am getting [...] 287 1.0 288 get_str.c:149: assertion failed: size_s1 = m Aborted i.e. I never get the random numbers; they are all zeroes but it aborts after 287 successful repetitions. Changing the default heap size (with +RTS -Hsize) changes how far it runs (running with -H32M let it go past 8000 repetitions), so this might be a garbage-collection issue. Hope that is some help to you. Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell Propeganda
On Thu, 28 Aug 2008, Brandon S. Allbery KF8NH wrote: On 2008 Aug 28, at 13:21, Tim Newsham wrote: GNU ld supports pragmas which cause the use of certain functions to output warnings at link time (try compiling a C program that uses gets()). It occurs to me that this, either in compiler or linker, would be a nice thing for ghc to do when using fromJust or other partial functions. would you include all partial functions in this, such as head? I'd like to, but IMO head is a little more acceptable because lists aren't binary like Maybe. fromJust really is the Haskell equivalent of dereferencing a pointer without checking if it's NULL, aside from being more reliably detectable. -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] [EMAIL PROTECTED] system administrator [openafs,heimdal,too many hats] [EMAIL PROTECTED] electrical and computer engineering, carnegie mellon universityKF8NH Tools like Neil Mitchell's Catch can do more sophisticated checking, as long as your program can be compiled by YHC. Sometimes fromJust can be quite useful, though, especially in tandem with isJust. For example, prop_foobar :: SomeType - Property prop_foobar x = isJust (someTypeToMaybe x) == fromJust x == expectedResult Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type family fun
On Sat, Aug 23, 2008 at 7:55 AM, Chris Eidhof [EMAIL PROTECTED] wrote: Hey all, I was playing around with type families, and I have a strange problem. Suppose we have an alternative to an Either datatype: data (:|:) a b = Inl a | Inr b and a class Ix: class Ix i where type IxMap i :: * - * empty :: IxMap i [Int] Now I want to give an instance for (a :|: b): instance (Ix l, Ix r) = Ix (l :|: r) where type IxMap (l :|: r) = BiApp (IxMap l) (IxMap r) empty = BiApp empty empty BiApp is defined as following: data BiApp a b c = BiApp (a c) (b c) However, it looks like the recursive calls to empty can't be unified, I get the following error message: Couldn't match expected type `IxMap l' against inferred type `IxMap i' Expected type: IxMap (l :|: r) [Int] Inferred type: BiApp (IxMap i) (IxMap i1) [Int] In the expression: BiApp empty empty In the definition of `empty': empty = BiApp empty empty In the inferred type, there should be IxMap l instead of IxMap i, does anybody know what I'm doing wrong? Thanks, -chris ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Hi, I'm not very familiar with type families, but shouldn't BiApp be defined as data BiApp a b c = BiApp (a b) (a c) since you're applying it as BiApp (IxMap l) (IxMap r)? Alex ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Reader monad, implicit parameters, or something else altogether?
On Wed, Aug 20, 2008 at 1:49 AM, Henning Thielemann
[EMAIL PROTECTED] wrote:
On Mon, 18 Aug 2008, [EMAIL PROTECTED] wrote:
G'day all.
Quoting Bjorn Buckwalter [EMAIL PROTECTED]:
I'd store the constants in a data structure along the lines of:
data AstroData a = AstroData
{ mu_Earth:: GravitationalParameter a
, leapSeconds :: LeapSecondTable
}
I would like to know if there is any consensus on what is the best way
to make such a data structure accessible in pure functions. Passing it
explicitly would be a mess.
In this situation, there isn't necessarily any shame in using a
top-level unsafePerformIO as long as it's well-hidden:
module AstroData (AstroData(..), globalAstroData) where
data AstroData = AstroData Int
But here my argument about playing around with the Planck constant becomes
relevant.
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You can always change how you unsafePerformIO the data, though. If you
want to set Planck's constant to 42 (or whatever), just change the
unsafePerformIO $ whatever to unsafePerformIO $ return 42.
Alex
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Re: [Haskell-cafe] how can this code be less?
I think you're looking for Data.List.isInfixOf. Alex On Thu, Apr 24, 2008 at 7:40 PM, Dan Weston [EMAIL PROTECTED] wrote: cetin tozkoparan wrote: I wrote this code and Can it be less? [2,4,5] list is sub list of [3,7,*2,4,5*,9] list and return True but not of [3,7,*4,2,5*,9] list ; return False sublist :: Eq a = [a] - [a] - Bool sublist [] _ = True sublist (_:_) [] = False sublist (x:xs) (y:ys) | x == y = if isEqual (x:xs) (y:ys) == False then sublist (x:xs) ys else True | otherwise = sublist (x:xs) ys isEqual :: Eq a = [a] - [a] - Bool isEqual [] _ = True isEqual (_:_) [] = False isEqual (x:xs) (y:ys) | x==y= isEqual xs ys | otherwise= False One way is to use existing library functions as Henning suggested (but maybe you missed it because he mischievously changed the subject!) Henning Thielemann wrote: try 'List.tails' and 'List.isPrefixOf' You should be able to define sublist using only some combination of the following (and one pair of parentheses) in one line of code! import List(isPrefixOf,tails) (.):: (b - c) - (a - b) - a - c any:: (a - Bool) - [a] - Bool tails :: [a] - [[a]] isPrefixOf :: (Eq a) = [a] - [a] - Bool ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
