[Haskell-cafe] Re: [Haskell] Re: Trying to install binary-0.4
Hi I agree. = 1.0 isn't viable in the long term. Rather, a specific list, or bounded range of tested versions seems likely to be more robust. In general, if it compiles and type checks, it will work. It is rare that an interface stays sufficiently similar that the thing compiles, but then crashes at runtime. Given that, shouldn't the tested versions be something a machine figures out - rather than something each library author has to tend to with every new release of every other library in hackage? Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] On the verge of ... giving up!
Hi Then, I set out to learn Monads + Category Theory from a Math perspective. This is where you went wrong. I know none of this stuff and am perfectly happy with IO in Haskell. Read http://www.haskell.org/haskellwiki/Monads_as_Containers and then read lots of other Monad tutorials for Haskell. So, I requested my institute to buy Dr. Graham Hutton's book. I would be getting hold of that quite soon, and am willing to start from the beginning. I'm not sure this covers IO in any great detail - it will be useful for general Haskell though. 1. An online judge system (Like http://spoj.pl). I had already done one for contests that were held during our Technical festival here, using php. The ideas are laid out. All I had to focus on was learning Haskell. 2. A solver for the Peg-Solitaire. This was more of an academic interest. I have seen Richard Bird's presentation on 'How to write a functional pearl, with an example' and was quite impressed by it. But the actual modelling might be slightly tricky here, and I am yet to start off with it. I would try number 2 first. IO in Haskell can be tricky, especially while you are learning all the other bits of the language at the same time. Network stuff is also not as well developed in terms of libraries as something like Python - but something like HappS should be able to do a spoj clone easily enough. A better choice for an initial IO application might be something like du, then moving to an online judge system later on. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: On the verge of ... giving up!
Hi main n = print . sum . map read . take n . reverse . lines = getContents Could someone describe succinctly how to compute the space complexity of this program, if there are m lines of input and m n? Many thanks. --PR The space complexity is the size of the file - i.e. of size m. reverse will buffer up all the lines before it gives any back, which is a well known property of reverse. You could rewrite (take n . reverse) as a function that only requires n lines of buffer. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell] Fingerprints and hashing
Hi Simon, We are all familiar with the idea of an MD5 checksum, which provides a reliable fingerprint for a file, usually 128 bits or so. If the file changes, the fingerprint is (almost) certain to do so too. There are lots of techniques: CRC, shar?, MD5, etc. I believe the basic operations are all in the Crypto library: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Crypto-3.0.3 - see Data.Digest.SHA1 and Data.Digest.MD5 - digest is simply another word for fingerprint in this sense. However, your fingerprint stuff sounds a lot more like a request for a Hash function - rather than something that operates over streams of bytes. To get both, I'd recommend something like taking the digest of the data after calling show, or after serialising it to a ByteString with the binary library. Doing it this way means you have no additional need for a FingerPrint class but can reuse the existing Show/Binary class. Thanks Neil ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell-cafe] Adding GLUT package to WinHugs
Hi The file you have requested (http://www.cs.york.ac.uk/fp/cpphs-1.5-win32.zip) could not be found on this server. The slightly older version works: http://www.cs.york.ac.uk/fp/cpphs-1.2-win32.zip I'm unable to get SSH from this machine, so can't tell where that file has gone to. Malcolm should be able to check if the file is present, and if not I'll create a new one shortly. If you have GHC installed, building cpphs is not that hard. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell] Abstract syntax representation
Hi In future questions like this are usually best directed to haskell-cafe, with haskell being left for annoucements. The standard full haskell representation is in Template Haskell (http://haskell.org/ghc/docs/latest/html/libraries/template-haskell/Language-Haskell-TH.html). If you want to work on a reduced language then I recommend Yhc.Core (http://www-users.cs.york.ac.uk/~ndm/yhc/ - there is a link to the Yhc.Core paper on that page). Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Adding GLUT package to WinHugs
Hi Peter, Also typing runhugs Setup.hs configure fails with runhugs: Error occurred ERROR c:\program files\winhugs\packages\base\Text\ParserCombinators\ReadP.hs:156 - Syntax error in type expression (unexpected `.') This is because Cabal gets it wrong. You need to type runhugs -98 Setup configure. There is an easy fix, which is for the Cabal developers to always use the bundled ReadP rather than the library one. I talked to Duncan about this in Germany, and he said he'd fix it :-) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Hugs, dotnet, C#...
Hi Peter, There is a Dotnet tree in the Hugs source code files, which I believe is what supports dotnet. As far as I am aware, it probably won't work. A windows developer may be able to build it, but I've never tried. You might also be interested to know that Yhc supports --dotnet to generate a dotnet binary. I think it can FFI call the dotnet framework, but I'm not sure. IDE's are hard, and only Microsoft does them right - perhaps one day if Haskell becomes popular enough we'll get something. Thanks Neil On 10/3/07, Peter Verswyvelen [EMAIL PROTECTED] wrote: In the (Win)Hugs documentation, I found Only the ccall, stdcall and dotnet calling conventions are supported. All others are flagged as errors. However, I fail to find any more information on how to invoke dotnet methods. This might be really handy for me, as I'm very familiar with the dotnet framework. For example, yesterday I rewrote and extended a program that I wanted to develop in Haskell in just 3 hours using dotnet, while I spend weeks trying do this in Haskell. Of course, I'm a Haskell newbie and a dotnet expert, so this is not a fair comparison. However, I got a strange feeling, which I want to share with you :) First of all, it was a *horrible* experience to program C# again; I needed to type at least 3 times the amount of code, much of which was boilerplate code, and the code is not elegant. Haskell really changed my point of view on this; before I knew Haskell, I found C# (I'm talking C# 3.0 here) a really neat and nice language. On the other hand, the great Visual Studio IDE and Resharper addin made it at least 3 times faster to type, navigate, refactor, and debug the code... Somehow, I get things done really really really fast in C#, albeit in an ugly way. Once again, I just wish Haskell had such an IDE... And yes, I know of the existance of Visual Haskell, EclipseFP, Haskell Mode for Emacs (which I'm using), VIM, YI, but still, these do not compare with the experience I have when using Visual Studio/Resharper (or Eclipse or IntelliJ/IDEA for Java). But that might just be me of course... A slightly frustrated Peter ;-) BTW: I don't want to bring up the IDE discussion again, no really ;-) ___ 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] Extract source code from literate Haskell (LHS) files
Hi Peter, This is of course very easy to do manually, but does a command line tool exist for extracting source code from literate Haskell files? Cpphs is the perfect tool to do this. Thanks Neil On 9/30/07, Peter Verswyvelen [EMAIL PROTECTED] wrote: Thanks, Peter ___ 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] GLFW for WinHugs
Hi Peter, It sounds like the problem is that you need to convert it to Hugs, rather than WinHugs specific (which might put a few people off looking to see if they can help). Perhaps an email to the author of the library might help you find if they would be interested in doing a port to Hugs. From my very limited knowledge, RTS.h defines various constants and links into the GHC runtime, but I don't think Hugs has any equivalent. Depending on what they are using RTS.h for, they might be able to use alternatives on Hugs. However, my knowledge of this side is even less than my knowledge of football, so if you can find a second opinion, take it! Thanks Neil On 10/3/07, Peter Verswyvelen [EMAIL PROTECTED] wrote: The latest version of SOE comes with a wrapper for a nice GLFW library. This library comes with a demo of a 3D bouncing Amiga ball so it must be the best library in the world ;-) ;-) Since I'm letting my students play with WinHugs, I would prefer to have a WinHugs compatible version of that library. I tried to convert it, but I got stuck when ffihugs complained about not finding RTS.h, which seems to be a GHC-only include file. Would it be possible to convert this library to WinHugs? I guess similar work has been done for other libraries, so any hints are welcome. Thanks, Peter ___ 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] Very crazy
Hi show_system = unlines . zipWith (\l ms - Eq ++ show l ++ : ++ (concat $ intersperse + $ zipWith (\n x - x ++ x ++ show n) [1..] (init ms)) ++ = ++ last ms ) [1..] . map (map (take 8 . show)) And people complain that *Perl* is bad? This function is quite obviously absurd. I mean, it works, but can *you* figure out what it does without running it? No. Can you say what the intention of this code is? Maybe a few examples? The type signature? That way I think people will be more able to give you hints. Generally, I find list comprehensions to be a good way out of general listy mess. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Very crazy
Hi complex. The input is quite simple (it's a bunch of numbers), the output is quite simple (it's a neatly formatted string), but the process in the middle is... a mess. I'd like to find a more readable way of doing stuff like this. It's not just this specific function; any general hints would be good. ;-) A nice auxiliary would help: showEqn :: Int - [Double] - String showEqn i vs = ... where (add,ans) = (init vs, last vs) Then you can half the complexity. There are probably a few useful functions that aren't in the standard libraries (consperse, joinWith etc) that you could make use of. You seem to be doing take 8 on the double - what if the double prints out more information than this as the result of show? 100 could end up a bit smaller. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell] Re: [Haskell-cafe] PROPOSAL: Rename haskell@ to haskell-announce@
Hi Forgive me, but I would much prefer a newsgroup to a mailing list. True, I could unsubscribe now and just browse the mailman archives - but for posting, I'd have to temporarily re-subscribe, which is awkward. (Indeed that's the only reason I'm not doing it.) I believe you can post from the gmane archives, and that they also offer a newsgroup feed of this list. I think the general consensus (and my personal view) is that a mailing list is the preferred mechanism for most people. Thanks Neil We already have comp.lang.haskell. - Best regards, -- -- Johannes Waldmann -- Tel/Fax (0341) 3076 6479/80 -- http://www.imn.htwk-leipzig.de/~waldmann/ --- ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell ___ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
Re: [Haskell] Re: [Haskell-cafe] PROPOSAL: Rename haskell@ to haskell-announce@
Hi Forgive me, but I would much prefer a newsgroup to a mailing list. True, I could unsubscribe now and just browse the mailman archives - but for posting, I'd have to temporarily re-subscribe, which is awkward. (Indeed that's the only reason I'm not doing it.) I believe you can post from the gmane archives, and that they also offer a newsgroup feed of this list. I think the general consensus (and my personal view) is that a mailing list is the preferred mechanism for most people. Thanks Neil We already have comp.lang.haskell. - Best regards, -- -- Johannes Waldmann -- Tel/Fax (0341) 3076 6479/80 -- http://www.imn.htwk-leipzig.de/~waldmann/ --- ___ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Shouldnt this be lazy too?
Hi Vimal, I was surprised to find out that the following piece of code: length [1..] 10 isnt lazily evaluated! The problem is that Int and Integer are both eager. It is possible to write a lazy peano number based data type, but I'm not aware of anyone who has - I have half the code (in joint work with Matt Naylor and Greg Manning) but none of us have taken the time to finish it off properly. For details about why we should have lazy naturals read http://citeseer.ist.psu.edu/45669.html - Colin Runciman, What About the Natural Numbers (1989) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Shouldnt this be lazy too?
Hi In this world, use length (take 11 [1..]) 10... not (null (drop 10 [1..])) is surely faster (not tested...) Faster? There might be a few microseconds in it. Clearer? Possibly... ;-) lengthNat [1..] 10 Couldn't be clearer, and can be made to work perfectly. If anyone does want to pick up the lazy naturals work, I can send over the code (or write it yourself - its not hard!) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Shouldnt this be lazy too?
Hi lengthNat [1..] 10 Couldn't be clearer, and can be made to work perfectly. If anyone does want to pick up the lazy naturals work, I can send over the code (or write it yourself - its not hard!) Um... isn't a lazy natural just a list with no data, where the list length encodes a number? Pretty much, yes. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Shouldnt this be lazy too?
Hi Pretty much, yes. So I just need to write newtype LazyNatural = LazyNatural [()] or data Nat = Zero | Succ Nat it's your choice really. and then add some suitable instances. ;-) Yes. Lots of them. Lots of instances and lots of methods. Hey, the length function would then just be ln_length :: [x] - LazyNatural ln_length = LazyNatural . map (const ()) Ooo, that's hard. Nope, its really easy. Its just quite a bit of work filling in all the instances. I bet you can't do it and upload the results to hackage within 24 hours :-) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Shouldnt this be lazy too?
Hi I'm guessing there's going to be fairly minimal performance difference. (Or maybe there is. My way uses a few additional pointers. But it also allows me to elegantly recycle existing Prelude list functions, so...) I think we can safely assume that people using peano numbers aren't actually overly interested in performance... *ALL* the instances? No. A small handful of them? Sure. How about this... How about creating a darcs repo, a cabal package and uploading it to hackage? Two design decisions you made could have been done differently (not sure which is best) If you read the lazy naturals paper it says that 4 - 8 should be 0. This follows the whole drop 8 [] = [], not _|_ Your show function is strict. Not sure whether this is a good idea or not. You could also include the constant infinity in your library: infinity = LN (repeat ()) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] what is f=f (not) doing ?
Hi I'm not sure, but since it would require the detection of an evaluation that does not terminate, it comes down to the halting problem, which is not generally solvable. Maybe the experts can confirm my intuition? I think your intuition is off. This isn't the problem of detecting that a computation might not halt, its a question of detecting after the fact a very restricted case of non-termination has occurred. I think it should be possible to assign threads etc to these things, but may make the code run slower in the common case. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell] Math behind Haskell
Hi The haskell-cafe@ mailing list is more appropriate for messages such as this. haskell@ is just for announcements (it should be called haskell-annouce@ !) * Lambda calculus - the basis of functional languages * Category theory - where all these mysterious things like monads, arrows, and functors come from. I'd add: * Discrete Maths - booleans, relations, functions etc. * Type theory * Logic programming (Prolog) * Semantics Most of these are computer science courses, rather than maths ones. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] PROPOSAL: Rename haskell@ to haskell-announce@
Hi I've just replied to another first poster with wrong list. Its entirely not their fault, but its also probably a bit off-putting that your very first post gets a (very polite) you got it wrong message. To steal the reasons and explanations from Ian: - pretty much what would be haskell-announce@ anywhere else, and haskell-cafe@ what would be haskell@ elsewhere. A haskell-cafe@ list elsewhere would probably be where the haskell@ people discuss things which aren't actually related to Haskell (like e.g. the demon.local newsgroup). I think the number of posts in the wrong place would be lower if these were more conventionally named (although there aren't a lot of them anyway). I think it would make sense to: * Rename haskell@ to haskell-announce@, and redirect mails from haskell@ to haskell-announce@ for some period. (more, but can be done step by step in the future) - I suggest that following somewhat the http://www.haskell.org/haskellwiki/Library_submissions we allow 4 weeks for discussion, and depending on the result we then move the mailing list. Bug to track: http://hackage.haskell.org/trac/ghc/ticket/1732 Deadline: 23 October 2007 Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] PROPOSAL: Rename haskell@ to haskell-announce@
Hi I agree. Unless... do some people subscribe to haskell@ (not haskell-cafe@) and like the existing stuff that's sent there (not all announcements... I'm not sure if I'd call e.g. Oleg's occasional demonstrations announcements even)? There are four things sent to the haskell list@ 1) Calls for papers 2) Annoucements 3) Oleg's stuff (which are really announcements of a library or technique) 4) Off topic stuff I'm initially only proposing to mop up category 4, which I am pretty sure the haskell@ people don't want to see. I did wonder whether this discussion should take place on the haskell@ list or the haskell-cafe@ one - the great ambiguity of the lists. (more, but can be done step by step in the future) Maybe, but I'm doubtful... (it might seem different to me in a year though, i.e. some amount of time after a --haskell-announce@ change has been made.) This proposal has nothing to do with any future steps. They may happen, they may not, but I think this proposal is worthwhile on its own. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] what is f=f (not) doing ?
Hi f = f and then try to evaluate 'f' in GHCi, as one would expect, the interpreter never returns an answer. The funny thing is that, while it is stuck in an infinite loop, GHCi doesn't seem to use any CPU time at all. It's called a black hole. The runtime can detect that f directly depends on f, so just gives up early. Essentially it marks f as black hole once it starts evaluating it, and then when it comes back to f, it knows its already doing evaluation on f, so just fails. It would be useful if there was a page on the wiki about black holes, but I can't find one... Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC 6.7 on Windows / containers-0.1 package?
All dependencies etc. have changed when going to 6.7/6.8 - you are probably better off using 6.6.1 for now. That's a petty. I really would like to experiment with the debugger :-) Me too! A proper release of GHC 6.8 is very nearby, so you should get your wish then. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] help me ! who has vty ?
Hi who has vty for haskell? I can't find the right one so who can mail one to me ? http://hackage.haskell.org/cgi-bin/hackage-scripts/package/vty-3.0.0 hackage has a lot of packages, and is always a good place to start a search. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Why isn't pattern matching lazy by default?
Hi Now why isn't pattern matching lazy by default? This seems odd for a newbie since everything else is lazy by default. f ~(x:xs) = rhs f ~[] = rhs' Now guess what f [] does... If you use a where binding then pattern matching is lazy. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (win)hipe for Haskell?
Hi Peter, During a googling session, I can across (Win)HIPE, a visualization program for the functional language HOPE. See http://dalila.sip.ucm.es/~cpareja/winhipe IMHO a similar tool would be a nice for learning/teaching Haskell; does that exist, or something else that comes close? Yes, Hat does this http://www.haskell.org/hat/ (if you can get it to work, I typically have little success) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (win)hipe for Haskell?
Hi Yes, Hat does this http://www.haskell.org/hat/ (if you can get it to work, I typically have little success) Thanks. WinHIPE uses graphics and animation. If briefly encountered Hat before, but I had the impression it did not visualize the graphs using graphics, only text. Is this correct? Yes, mainly. There is a graphical user interface for Hat, but its not very well developed. http://www-users.cs.york.ac.uk/~ndm/hat/ Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC 6.7 on Windows / containers-0.1 package?
Hi Peter, So I grabbed ghc-6.7.20070824 (=the latest one for Windows I could find) and the extra-libs, compiled and installed the GLUT package (which I needed), but when I compile my library, I get Could not find module `Data.Map': it is a member of package containers-0.1, which is hidden All dependencies etc. have changed when going to 6.7/6.8 - you are probably better off using 6.6.1 for now. I also don't think that the debugger will help you track down infinite loop style errors. You might be better off posting the code and asking for help. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Library Process (was Building production stable software in Haskell)
Hi What is the process for the inclusion of modules / packages in ghc, hugs and other compilers interpreters? Propose to have the packaged added. There is a very low chance of this being accepted. The only packages to have recently been added were FilePath and ByteString, both of which were obvious deficiencies in the libraries. I'm now not aware of any hole that is likely to get plugged by bundling an additional library. Should something experimental be a base package? And shouldn't all modules that are base packages declare their status? No, they should all be reasonably stable. Things that are unstable are likely to move out of the standard libraries and just become hackage packages. Perhaps these are questions for the libraries mailing list but I thought I'd start here. Now there is a question I can't answer - I have no idea what should go down the libraries list and what should go down the haskell-cafe list. Perhaps someone could put up a guide of if your question/comment is like this, send it on this list Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Library Process (was Building production stable software in Haskell)
Hi I think there is a niche for a subset of the hackage libraries providing an officially sanctioned standard library collection. Currently, hackage includes, well, everything. As such, it is a useful resource, but it would be useful to have a partitioning into two levels, where the SLC would only include libraries that meet specific criteria. Maybe: - considered stable - is portable - relies only on other standard libraries - avoids needless duplication of functionality - with a responsive, named maintainer (not libraries@) - with acceptable documentation and unit tests - required by at least one separate application I think there is a niche for this, but I don't think it should be an officially sanctioned collection - since otherwise everyone is just going to be debating how to add their library to this collection - and we are going to descend into voting and politics. Instead, I think several people should make their own personal list of libraries they would vouch for - which meet the criteria above AND they have personal positive experiences of. Off the top of my head my list would include gtk2hs, and that's about it. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] unique id for data types
Hi Barney, This may be of interest, since all types already have an Int associated with them: http://haskell.org/ghc/docs/latest/html/libraries/base/Data-Typeable.html#v%3AtypeRepKey Thanks Neil On 9/18/07, Barney Hilken [EMAIL PROTECTED] wrote: In order to make my records system practically useable, I need a type family type family NameCmp n m which totally orders datatypes. More precisely, it should return one of the following types: data NameLT = NameLT data NameEQ = NameEQ data NameGT = NameGT for each pair of datatypes n m, according to whether n m, n = m, or n m in some global ordering. This ordering needs to be independent of the context, so it can't be affected by whatever imports there are in the current module. What I want to know is: does GHC give datatypes any global id I could use to generate such an ordering? Would fully qualified names work? Secondly (assuming it's possible) how easy would it be for me to write a patch to add NameCmp to GHC? Where in the source should I start looking? Thanks, Barney. ___ 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 production stable software in Haskell
Hi okay, but this fails in some cases. i wrote a package to obtain financial quotes. yahoo changed the webservice url on me. i rolled out a change within a day. in your model, people suffer a broken service for two weeks. I don't think Yahoo will change the syntax or semantics of filepaths anytime soon :-) If a new operating system (say Vista) had changed the semantics of filepaths then that would have been a fix, not a change, and provided it doesn't alter the behaviour for others then that would have been a bug fix that went in immediately. The code review is for interfaces - internal details are a bit more free, and clear bug fixes are not voted upon. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: STG to JavaScript translation
Hi Are you aware that Dimitry is still working on ycr2js - and has made great progress. There are details available in The Monad Reader, issue 7 (http://www.haskell.org/haskellwiki/The_Monad.Reader) Thanks Neil On 9/17/07, Victor Nazarov [EMAIL PROTECTED] wrote: Hello. I'm working on the translation of GHC's STG language to JavaScript. I've started my implementation, but I've got stuck with the STG case statements. The problem is the binder in case expression. StgCase expr livevars liverhsvars bndr srt alttype alts Operationally, I need to save continuation and evaluate expr expression, but I have no idea what to do with the bndr. It seems to me that I need to build a closure binded by bndr with the body of expr evaluate it, update it, and use it in RHSs of alternatives. But It seems that this behavior isn't intended by GHC. Can you explain briefly how GHC implements this binder and what this binder points to. Here are some notes about Dmitries last year activity and my activity: http://haskell.org/haskellwiki/STG_in_Javascript -- vir http://vir.comtv.ru/ ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: STG to JavaScript translation
Hi
case e of b { pati - rhsi }
* evaluates 'e',
* binds the resulting value to 'b',
* performs case analysis on the result to find which alternative to choose
* binds the variables of the pattern to the components of the value
The Yhc.Core translator converts this to:
let b = e in case b of { pati - rhsi }
I'm not sure if that would be a clearer form for you to work with, as
it is closer to standard Haskell.
Thanks
Neil
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Re: [Haskell-cafe] Building production stable software in Haskell
Hi Would you care to explain why you have this aversion to libs that aren't bundled with ghc? They are less stable and have less quality control. It is also an additional burden for a user to install the library to get the program working. cabal-install should fix the second. Some useful community feedback on hackage could fix the first. By removing most bundled libraries from GHC, we can get to the point where people _have_ to use non bundled libraries, then everyone will be on a more equal footing. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Building production stable software in Haskell
Hi What's bad about stagnation is that nobody will bother to produce anything better (at least not as a fully polished publicly available open source project), precisely because they have little chance of achieving a user base exceeding 1 (at least not if the attitude of David and Neil is typical of the community culture). I merely gave the reasons I suspected that David (and others) held their beliefs - I don't share them particularly. As a library author I have produced quite a few libraries (at least 5), and have received useful feedback from users which has helped me improve the libraries further. I think the largest barrier to having many libraries used in projects is the lack of a working cabal-install. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Building production stable software in Haskell
Hi They are less stable and have less quality control. Surely you jest? I see no evidence of this, rather the contrary in fact. No, dead serious. The libraries have a library submission process. Compare me changing my tagsoup library, to me changing my filepath library which comes bundled with GHC. I can do anything I want to the tagsoup library, but I need to wait at least 2 weeks and get general consensus before changing filepath. Also some libraries on hackage are 0.1 etc - even the author doesn't particularly think they are stable! Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell] question about a failure to generalize
Hi Monomorphism restriction? Replacing fold with foldRegsUsed would work because there's a type signature for foldRegsUsed. That looks like it. Another solution would be: fold = foldRegsUsed becomes: fold x = foldRegsUsed x Now the monomorphism restriction doesn't kick in because fold has an explicit argument. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Is take behaving correctly?
Hi Although I appluad the semantics of the safe package, I'm not delighted with the idea of replacing our concise elegant standard library names with uglyAndRatherLongCamelCaseNamesThatCouldBePerlOrEvenJava though. Conciseness of expression is a virtue. They aren't that long - merely an extra 4 characters over the standard one to indicate what the specific semantics are. If you can think of better names, then I'm happy to make use of them. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Is take behaving correctly?
Hi Similarly, I expect foo and foo' to be equivalent, except for strictness properties, but perhaps an underscore could be used for slightly different behaviors (interpretations, as it were)? tail_ or zip_, anyone? There are 4 variants of tail: tail :: [a] - [a] -- normal tailDef :: [a] - [a] - [a] -- returns the first argument on [] tailMay :: [a] - Maybe [a] -- returns a Nothing tailNote :: String - [a] - [a] -- crashes, but with a helpful message tailSafe :: [a] - [a] -- returns [] on [] tail_ would not be a good name! Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Is take behaving correctly?
Hi Is there a reason for not having tailM :: Monad m = [a] - m [a] which, at least for me, is much more useful? No, that probably is a much more sensible choice. Patches welcome :) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Is take behaving correctly?
Hi From the logical point of view tailMay is the right one. It pushes the error handling to the caller programm. tail = fromJust . tailMay The error messages suffer: tail [] = error: fromJust Nothing That's why I supplied tailNote, where tailNote foo broke its invariant! [] gives the message error: tail [], foo broke its invariant! Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Is take behaving correctly?
Hi A more serious point is that in some cases we might want take to underapproximate, or zip to truncate (or tail [] = [] ?). I don't think there's always a clear library choice here. I have a zipWithEq function I often use, which crashes if the zip'd lists aren't equal. I also have tailSafe which does the tailSafe [] = [] behaviour. I created a hackage package safe for the tailSafe function and others, http://www-users.cs.york.ac.uk/~ndm/safe/ . If anyone wants to extend that with deliberately unsafe functions, such as zipWithUnsafe, zipUnsafe, takeUnsafe etc, I'd be happy to accept a patch. If not, I'll probably do it myself at some point in the (potentially distant) future. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Is take behaving correctly?
Hi The same should apply to head and tail. head or tail of [] should be []. What does the list think? Disagree, strongly. Its not even possible for head, since [a] - a. Wadler's theorems for free states that if head is given an empty list the _only_ thing it can do is crash. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Building production stable software in Haskell
Hi Peter, The way I see it as a newcomer, Haskell shifts the typical imperical programming bugs like null pointers and buffer overruns towards space/time leaks, causing programs that either take exponentially long to complete, stack overflow, or fill up the swap file on disc because they consume gigabytes of memory. Time bugs are quite rare - usually a simple profiling will fix them up, and they are exactly the same sorts of bugs that exist in an imperative programming language. Usually its a case of picking a better algorithm, or thinking clever thoughts. Space leaks are much more tricky - there are profiling tools, but I've never got enough experience using them to say anything more than that. Other bugs I found are incomplete pattern matches at runtime, but I already got an email of how to fix this using an external tool Did the email suggest using Catch? http://www-users.cs.york.ac.uk/~ndm/catch/ If you care enough about pattern matching, you can eliminate them all statically. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Building production stable software in Haskell
Hi
Well, I actually meant more something like the imperative equivalences
of code coverage tools and unit testing tools,
hpc and HUnit cover these two things pretty perfectly. hpc will be in
GHC 6.8, and its really cool :-)
because I've read
rumors that in Haskell, unit testing is more difficult because lazy
evaluation will cause the units that got tested to be evaluated
completely different depending on how they are used. In strict
languages, this is not the case.
That's a really weird statement, and one that goes completely opposite
to my view of things. Do you have sources for these rumours? In a pure
language, if you evaluate some code it will do exactly the same thing
every time - there is no different behaviour. If you test the code,
then run it again later, you'll get the same result. Compare that to
something like C where:
int breakRandomly = 0;
int return42()
{
breakRandomly = !breakRandomly;
return (breakRandomly ? 666 : 42);
}
In C you can test this, and it will work, then you can test it twice
with identical values, and it will break.
Lazy evaluation can hide bugs if you only demand some small portion of
the output, but if your property/test is constructed properly, you
shouldn't have any problem.
Thanks
Neil
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Re: [Haskell-cafe] haskell and reflection
Hi there is no runtime representation of type available for programmatic representation Data.Typeable.typeOf :: Typeable a = a - TypeRep there is no runtime representation of the type-inferencing or checking machinery Pretty much, no. The GHC API may provide some. there is no runtime representation of the evaluation machinery Yhc provides some representation with the Yhc API. there is no runtime representation of the lexical or parsing machinery lex provides some of this. There are various Haskell parsers out there in packages for us. I wouldn't have considered these things reflection - certainly the Java/C# use of the word reflection is quite different. Data.Generics does provide many of the reflection capabilities of Java. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] WinHugs shortcut keys
Hi Peter, It does not seem to have shortcut keys for reload, edit, etc... When do you use reload? WinHugs automatically reloads modified files, so I've never felt the need to reload things. Similarly for edit, typing just edit on its own isn't that useful unless you give it a module. However, I'm not averse to including more shortcuts - its relatively easy and if it is of benefit to you, I'm happy to do so. These would be really handy. It is possible to customize the shortcuts assigned to the commands? If not, can it be build from source? It can be built from source, but the libraries changed slightly, so a new build from source won't work with the existing WinHugs packages libraries. Building the libraries is much more tricky and requires mingw. You can report bugs from http://hackage.haskell.org/trac/hugs - so if you do want some shortcuts, say what you want a particular key to do, and I'll include it for the next release. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] WinHugs shortcut keys
Hi Peter, Automatic reloading is of course THE best solution, and that's already in :-) Maybe it could help new users to display a simple message into the WinHugs statusbar like module XXX reloaded after external modification at HH:MM:SS) or something? But that's really minor. That would be neat. There has also been a suggestion to display the message file modified but not reloaded if auto-reloading is turned off. These things are in the same area, so when I take a look at it I'll see what I can do. The code does seem rather crufty though... Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Tiny documentation request
Hi I've never really understood what the benefit of this is... I mean, Google make the Google toolbar, but what's the point? Why not just click on the Google bookmark and type in your search? What benefits does installing a special addon provide? You can setup firefox so in the location bar (Alt+D) typing h query (without the quotes) will perform a hoogle query. Simply right click in the search box and click add keyword search with h as the keyword. You can also do the same for Google, which I've been using for years (Opera 5 had this feature!) re: Hoogle not always getting it exactly right, there are a few known bugs floating around which I'm working on. Hoogle can permute the order of arguments though, so that shouldn't be a problem. It doesn't really like Monads, but I wrote Hoogle before I was aware of higher-kinded type classes, so its understandable... Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Tiny documentation request
Hi I have the following page bookmarked: http://haskell.org/ghc/docs/latest/html/libraries/ Just bookmark: http://haskell.org/hoogle It's not perfect, but it probably solves lots of your problems. A tip is to use Firefox's search as you type feature if you know the module name. This will be better supported in Hoogle 4 - but unfortunately degrees etc. are coming in the way of Hoogle development... A more aesthetical note: We should really get rid of the ugly table/CSS layout mixture, the lower part of the page renders a bit ugly and varies between browsers. Switching to pure CSS should be safe in 2007, I guess. Replicating actual tables with CSS is a nightmare - you shouldn't use table's for lots of things, but there are sometimes when it really is the best option. Fixing up the CSS and still keeping tables is a perfectly valid option. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Speed of character reading in Haskell
Hi (Some list operations are too expensive with ByteString but for most string processing it's perfectly fine and much faster than String). I'm sure it's true, but it's quite irrelevant to my question, which is why is using getChar so much slower than using getContents? Buffering, blocks and locks. Buffering: getChar demands to get a character now, which pretty much means you can't buffer. Blocks: getContents reads blocks at a time from the underlying library, whereas getChar has to do one character at a time. Locks: getChar has to acquire locks, as does getContents. However, because getContents can operate on blocks, this requires many fewer locks. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Consistency of reserved operators and bang patterns
Hi Re ! as an operator: This caused a number of complexities in the parsing of stuff, including shift-reduce conflicts. Someone would need to look into this, and determine that the rules are completely unambiguous. Backwards compatibility requires that it be implicitly imported from Prelude even in a module that does import Prelude ( ) (although Hugs is already broken in this regard). In particular, Haskell-98 bans import Prelude ( (:) ) Yhc does not meet this restriction either. Perhaps this is a change that is breaking but minor enough to be permitted for H', since everyone already does the reverse. Thanks Neil ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: [Haskell-cafe] Hackage and GHC 6.8
Hi Neil, Given that GHC 6.8 is just around the corner and, given how it has re-organised the libraries so that the dependencies in many (most/all) the packages in the hackage DB are now not correct. Is there a plan of how to get hackage DB up to speed with GHC 6.8 ? I think whatever we go with will be deeply painful. Especially given the switch to Cabal configurations comes at the same time, rather than before. However, remember this is a one of set of pain, which will hopefully never be repeated. In the absence of anyone else doing anything, I'll probably give it a month or two, then upgrade all my packages on hackage. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Elevator pitch for Haskell.
Hi * Create sophisticated GUIs. Gtk2hs. Could do with a nice wrapper on that, but Conal is doing some interesting stuff, and I've got PropLang on the back burner. People are thinking the right thoughts, it just needs time. * Read and write standard binary file formats. (Images, compressed files, etc.) Data.Binary is the low level frameworks, now people can pick up the rest. * Talk to a database. Databases are dead, long live flat files :-) It's possible, but I don't have a clue how. * Use various network protocols (possibly custom, possibly standardised). The great thing about Data.Binary is that its for binary stuff, whether it be networks or files. * Access the Windoze registry and play with COM stuff. H/Direct, System.Win32. My only question is why you want to build non-portable software when Haskell is so beautifully portable. * Get system-specific file information (protection bits, modification times, security information, etc.) THe Unix stuff does all this, and the win32 stuff does it on Windows. * Query the OS. (How many CPUs? How much RAM? What is my IP address?) Easy enough, if anyone took the time to FFI to a library. I don't know how to do any of that in Haskell. Some of it can be done, just not very easily. Other items are, AFAIK, impossible. All of it can be done. The problem is that it hasn't been done, since Haskell libraries tend to be written lazily, usually after the need is present. And then there's just random stuff like the Prelude numeric classes being broken, the fact that the array API is virtually skeletal compared to what you can do with lists, the lack of a clean, standardised character encoding system [that handles more than 2 encodings], etc. All these things are minor. The Prelude numeric classes aren't broken - they don't quite match what a mathematician might have picked, but its certainly easy enough to do numeric operations! The array API is a bit poor, but should never be used - lists are more functional. I'd like to think that Haskell will soon be ready for prime-time. But let's face it, the language is 20 years old already... Most of your problems are lack of libraries. We've had Cabal in mainstream for maybe a year, hackage is even newer. People know where the problems are, and they are being fixed. Haskell as a language is pretty much done, people just like tweaking the corners. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Elevator pitch for Haskell.
Hi Data.Binary is the low level frameworks, now people can pick up the rest. Last time I checked, there's about half a dozen binary packages. All incompatible. All with different design. Seriously not obvious which one to use... Data.Binary is the answer, http://www.cse.unsw.edu.au/~dons/binary.html The Unix stuff does all this, and the win32 stuff does it on Windows. Mmm... looks lovely. Any danger of documentation? msdn.microsoft.com - if you know the C API, you can figure out the Haskell one. No one should ever have to learn the Win32 API though, its horrible. Still, everybody keeps muttering that functional is the next big thing, and Erlang is taking over the world, so many Haskell's day will come. I quite like the fact that I'm massively more productive than most other programmers - at the end of the day that's what will make me worth more money and happier than everyone else. Haskell might never be popular, that's fine by me - I like elegance much more than cheerleading :-) * Thanks Neil * Of course, all the cheerleading leads to more libraries which makes me more productive and happier, so its all a win in the end. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: file name letters
Hi Ouch, the well-known case-preserving but case-ignoring default behaviour of MacOS X's file system bites someone again. In short: Mac OS X's filesystem ignores case when opening a file, but preserves case when creating. FWIW, Windows has exactly the same behaviour. Technically you can change to case sensitive at format time, but I've never actually seen anyone running in this mode. Thanks Neil ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: [Haskell-cafe] Hawiki articles
Hi Bring back HaWiki! I couldn't agree more! We built up an incredible array of articles, by fantastic authors with stunning content - which we then deleted... I learnt much from the old wiki, and it would be a shame if others didn't get that opportunity. Of course it should be static only, big warnings of out of date content etc. - but it should be available for years to come. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Hawiki articles
Hi There are two entirely separate issues in this thread - let's not confuse them. 1) The old HaWiki content is good and unavailable. I want it made available, in whatever form is appropriate. Please :-) 2) Licensing - the old content cannot be dumped onto the new wiki. My personal view is who cares. There are numerous license violations within the Haskell community (I can think of 4 off the top of my head), but in general everyone is working for the same purpose, and its just pesky laws getting in the way - not violating peoples intent. I realise that this will be a minority opinion, and that its probably a bad idea to follow my opinion on this. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Derived instances
Hi I have just discovered Generics in Haskell and, writing some code with GHC, I wondered about the portability of my code. If you use Data.Generics, your code is not portable - no other Haskell implementation supports the necessary bits. I can use the -fgenerics to automatically derive instances for Something. But I could wish to have the generated source code for those instances (like, I suppose, the DrIft tool does), for instance, to compile my source with another compiler, which does not support Generics. Use Derive, which does support Generics: http://www-users.cs.york.ac.uk/~ndm/derive/ If you care about portability, and are not using some of the more advanced features of Generics, then you might want to take a look at Uniplate: http://www-users.cs.york.ac.uk/~ndm/uniplate/ Uniplate does generic traversals, but will work on all Haskell 98 compilers, to some degree - read the paper for exact details. Thanks Neil ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: hello
Hi hello thanks for my new subscription, do I need to pay a fee or is it free to join ? This is a mailing list. Its totally free, and for the purpose of discussing future improvements to the Haskell standard. If you just wait, you'll find interesting discussions flowing into your inbox. If you would rather just have general Haskell discussions, then the haskell-cafe mailing list might be a more appropriate venue. Thanks Neil ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: [Haskell-cafe] defining mapPairs function
Hi Alexteslin, I just came across with this question on the exam and can not think of implementing it. mapPair :: (a - a - a) - [a] - [a] such that mapPairs f [x1, x2, x3, x4...] = [f x1 x2, f x3 x4,...] I would implement this using direct recursion. As a starting point, the standard definition of map is: map f [] = [] map f (x:xs) = f x : map f xs You should be able to start at this and modify it to give the behaviour you require. If you want further hints, you might want to try the Haskell IRC Channel, which is good for these kind of questions. http://www.haskell.org/haskellwiki/IRC_channel Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] defining mapPairs function
Hi mapPairs :: (a - a - a) - [a] - [a] mapPairs f [x] = [x] mapPairs f [] = [] mapPairs f (x:xs) = f x (head xs) : mapPairs f (tail xs) It looks like it works, but you can get a better version by changing the last line: mapPairs f (x:y:zs) = ... - left as an exercise, but no need for head or tail. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Ideas
Hi - Blogging software. (Because there isn't enough of it in the world yet.) Hope (google: Haskell Hope) - A wiki program. (Ditto.) Flippi (google: Haskell Flippi) - A general CMS. Hope - An interactive function plotter. (GNUplot is nice, but it can't plot recursive functions...) None that I know of. - A graphical programming tool. (You add boxes and put in lines, it constructs a program that you can run.) You mean a programming tool with a horrible syntax and user interface? If you want to remove the joy from programming, just use Ada. Alternatively, use PureData, which can be extended with Haskell, and gives boxes and lines. http://haskell.org/haskellwiki/AngloHaskell/2007#Abstracts Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Ideas
Hi Flippi (google: Haskell Flippi) ...and yet haskell.org uses WikiMedia? (Which is written in something bizzare like Perl...) Yes, but WikiMedia is a result of years of work, Flippi is a lot less. Wikipedia uses WikiMedia - its a tried and proven solution. - A graphical programming tool. (You add boxes and put in lines, it constructs a program that you can run.) Have you ever played with KLogic? You draw boxes and lines, and it makes some logic. (As in the digital electronics sense of logic.) I have some (very expensive) software called Reaktor. You draw boxes and lines, it does DSP algorithms. You build synthesizers and effects boxes with it. That sounds exactly like PureData - you can also do graphics as well with PureData, the demo I saw was very cool. Of course, PureData is written in C with Haskell as an extension language. The last two ideas you mentioned require a graphical user interface, which is an area of Haskell which is comparatively weak, compared to the rest of Haskell. Thanks Neil PS: Apologies to Andrew for 2 copies of this message. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell on the Playstation 3? :-)
Hi Another option would be to port the yhi bytecode interpreter to run on PalmOS. I tried this, but I ran into three problems: 1. libgmp dependency This is no longer an issue, we now have a flag to not require libgmp, which makes type Integer = Int 2. build system requires Python (scons). Still alas, but we'd like to fix it. If PalmOS is really un-POSIX compatible, it may be easier to write a custom compiler that compiles YHC or GHC Core to PalmOS. Well, the first time you try to write a compiler from Core - ??? is difficult, but the second time around is a lot easier ;) We have compilers from Yhc Core to everything nowadays. One to Lisp shouldn't be too tricky, if someone wanted to take that direction. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: [Haskell] Issue about use of WinHugs
Hi I'm new to WinHugs, what's wrong with isUpper of my WinHugs? Nothing. The book/tutorial you are going from is out of date. Before using the isUpper/isLower functions you first have to type :load Char: Hugs :load Char Hugs filter isUpper ABCDEfgh ABCDE The :load Char loads the Char module into scope, which provides the isUpper function. The haskell@ mailing list is mainly for announcements, so I've sent this email to haskell-cafe@ - which is the right place for questions like this. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC optimisations
Hi Its (==) isn't reflexive (is it transitive? probably, at least if there aren't too many optimizations, but floating-point transitive equality isn't very useful). It's not even referentially transparent in all cases. a == b may fail while the double's are in the high precision registers, and then succeed later on in the program once they are truncated. I think you have to specify -fexcess-precision with GHC to get this behaviour. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Graph reduction [Was: Where is StackOverflow on the Wiki?]
Hi Yeah, the precise details may vary, even :) But for teaching, an automatic tool that does graph reduction would be great. I don't mind if it's sloppy (directly apply definitions pattern matching VS everything is a lambda abstraction) and only does simply typed lambda calculus (no type applications, no type classes). Well come ON people, there's *got* to be enough big-wigs on this list to put *something* together! ;-) It's been done, but never got to a released state: http://haskell.org/haskellwiki/Haskell_Equational_Reasoning_Assistant http://www.cs.york.ac.uk/fp/darcs/proof/ (screenshot: http://www-users.cs.york.ac.uk/~ndm/temp/proof.png) And neither of them will be of much use to a beginner. Haskell is too complex to reason about formally for a beginner, and reasoning informally isn't very easy to do - because its not a precise description of what to do. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC optimisations
Hi Other rules that could be interesting are: forall a b. fromInteger a + fromInteger b = fromInteger (a + b) forall a b. fromInteger a * fromInteger b = fromInteger (a * b) This is wrong, since the class function can do what it wants. Imagine: instance Num String where (+) = (++) fromInteger x = show x 1 + 2 :: String this expression now goes from 12 to 3 by applying this rule. You need to be incredibly careful if there are any classes floating around. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC optimisations
Hi If Num obeys ring axioms, fromInteger is a perfectly fine ring-homomorphism. (It's also the first or second homomorphism taught.) Does Int obey these axioms? I'm thinking that assuming properties about things such as numbers is very likely to go wrong very quickly. Monads you might be able to get away with, Numbers you probably can't. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] help understanding lazy evaluation
Hi factors :: Int - [Int] factors n = [x | x - [1..n], n `mod` x == 0] prime :: Int - Bool prime n = factors n == [1, n] My vague intuition said we either need factors or we don't, we do because we need to perform the test, so we compute it. That's wrong, so a posteriori the explanation must be something like this: The key point is that factors doesn't either compute all or none, it may compute part of the value. It does this by computing something like: _:_ 1:_ 1:_:_ where _ is the unevaluated bit, i.e. it computes one bit of the result at a time. Equals also has this property, it can be defined as: a:as == b:bs = a == b as == bs [] == [] = True _ == _ = False If you have (1:_) == (2:_) then the match will fail instantly. That's a lot of *context* about that particular evaluation of factors, in particular step puzzles me. Can anyone explain how lazy evaluation fits there? I suspect the key is the implementation of == together with the fact that list comprehensions are lazy themselves, is that right? Everything is lazy, to all subparts. You might get along better with a reasoning more of the form to compute anything, this expression will demand this expression - rather than your someone knows we'll need. If you follow example derivations you'll see that there is always a very clear idea of what needs to happen next for the computation to proceed, which explains the laziness quite naturally. [*] Which notation do you use for functions in text? is f() ok? Sure, although a little unusual for Haskell where f() means f applied to the empty tuple. Some people use |f| (generally those who use latex), but generally it can be inferred from the context what is a function Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC optimisations
Hi Wait, you're saying that ghc can produce pure c-code, that doesnt contain any assembly code, and that runs as fast as ghc code that does contain assembly? No. It can produce pure C code (unregistered), but to get high performance it processes the output assembly afterwards (registered). Sooo if I was feeling evil, could I take this c-code and pipe it into something that turns it into C#??? You might be able to. Much easier would be to use Yhc and pass the --dotnet flag which generates .NET binaries natively. macros (or any macros at all perhaps...), this becomes non-trivial, but otherwise I think most things in C can be mapped fairly trivially to C#? (It's a one-way mapping of course, eg delete in C is simply dropped when mapped to c#). There isn't going to be much free/delete, its all a garbage collected heap. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generic data constructor in pattern?
Hi Peter, liftV1 f (V x y) = V (f x) (f y) liftV2 f (V x1 y1) (V x2 y2) = V (f x1 x2) (f y1 y2) liftM1 f (M x y) = M (f x) (f y) liftM2 f (M x1 y1) (M x2 y2) = M (f x1 x2) (f y1 y2) Both pairs of lift functions have almost identical implementations. Can I merge these somehow? Using the Uniplate library the first already has a name, its called descend. The second does not, but could be implemented in Uniplate if you wanted. descend2 :: Biplate a b = (b - b - b) - a - a - a descend2 f a b = a2 (zipWith f as bs) where (as, a2) = uniplate a (bs, b2) = uniplate b For full details see the website: http://www-users.cs.york.ac.uk/~ndm/uniplate/ Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Newbie question: Where is StackOverflow on the Wiki?
Hi sum (enum 1 10) = sum' 0 (enum 1 10) = ... sum' 36 (9 : enum (9+1) 10) = (sum' $! (36+9)) (enum (9+1) 10) = sum' 45 (enum (9+1) 10) = sum' 45 [] = 45 (I need to find some way to automate making these trails :) ) Yes! We'd need such an automatic tool for the wikibook, too. The problem is that Haskell is ridiculously complex, and the small step interpretation is much harder than you'd think. For example, sum may well be defined as foldl' (+) 0, which is a CAF, so gets reduced once. The 0 won't actually be a 0, but will be fromInteger 0, which will correspond to looking up an item in the dictionary and applying it. Dictionaries especially make the simple interpretation completely wrong. It's easy to do informally, but once you start being more precise, its very complex. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] is there a way to patch the build-depends line of a cabal file without breaking backwards compatibility?
Hi Distribution/Simple/InstallDirs.hs:267:36: Not in scope: `dropDrive' [EMAIL PROTECTED]:~/installs/cabal-head/caballs -l `which ghc` lrwxrwxrwx 1 root root 31 2007-08-20 11:08 /usr/local/bin/ghc - /usr/local/bin/ghc-6.7.20070816 You'll need to upgrade the filepath library as well, since cabal depends on it, and a very recent version at that. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] List comprehension desugaring
Hi, The Haskell desugaring for list comprehensions is given in: http://haskell.org/onlinereport/exps.html#list-comprehensions All the rules seem to be left to right rewrites, apart from the second one, which seems to be right to left. Is there some deep reason for this, or is this accidental. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: List comprehension desugaring
Hi Sorry for the noise, I've now realised they are a left to right rewrite system, the second rule is required to set up the base case. Thanks Neil On 8/19/07, Neil Mitchell [EMAIL PROTECTED] wrote: Hi, The Haskell desugaring for list comprehensions is given in: http://haskell.org/onlinereport/exps.html#list-comprehensions All the rules seem to be left to right rewrites, apart from the second one, which seems to be right to left. Is there some deep reason for this, or is this accidental. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ghc 6.7 /6.8
Hi will 6.7 be released, or will only 6.8 be ? 6.7 is the HEAD branch, 6.8 will be released. -in that case, does the 6.8 branch means the freeze is on, and what would be the target for 6.8 ? Q3/2007 ? Q4 ? Soon, in the next month - according to latest targets/estimates. ( context: I am impatient to test the new debbuger, but too newby to go and fix all the lib..hoping it may help me undertand haskell execution.. ) The new debugger looks dead cool, I can't wait for it either! Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: hyperlinked haskell 98 grammar
Hi Peter, A nice application. One suggestion: I would have implemented it so that a maximum on one backward hyperlink menu could be visible at once - try clicking on two RHS's and you'll get too menus visible at the same time. You also might want to mail this out on haskell-cafe - which is more appropriate for general haskell things, ghc-users is more for direct issues with GHC (although haskell-cafe can also be used for those) Thanks Neil On 8/16/07, Peter Hercek [EMAIL PROTECTED] wrote: Hi, I was improving my Haskell knowledge lately and I created a small dhtml application which allows browsing of Haskell 98 grammar. I contains both forward and backward hyperlinks. By backward hyperlink I mean that you can click on an a production head and you get a popup list box where you can navigate to any production using the nonterminal. If you like it please save and use your local copy since the server cannot handle much load. http://www.hck.sk/users/peter/ Peter. ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: hyperlinked haskell 98 grammar
Hi In addition, perhaps this should be relocated to haskell.org, if your server is not suitable for a large volume of users. I think it should also be integrated somewhere (perhaps a link from the HTML report, and certainly on the wiki) Thanks Neil On 8/16/07, Neil Mitchell [EMAIL PROTECTED] wrote: Hi Peter, A nice application. One suggestion: I would have implemented it so that a maximum on one backward hyperlink menu could be visible at once - try clicking on two RHS's and you'll get too menus visible at the same time. You also might want to mail this out on haskell-cafe - which is more appropriate for general haskell things, ghc-users is more for direct issues with GHC (although haskell-cafe can also be used for those) Thanks Neil On 8/16/07, Peter Hercek [EMAIL PROTECTED] wrote: Hi, I was improving my Haskell knowledge lately and I created a small dhtml application which allows browsing of Haskell 98 grammar. I contains both forward and backward hyperlinks. By backward hyperlink I mean that you can click on an a production head and you get a popup list box where you can navigate to any production using the nonterminal. If you like it please save and use your local copy since the server cannot handle much load. http://www.hck.sk/users/peter/ Peter. ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: [Haskell-cafe] Pattern matching articles/explanations
Hi So what I noticed that A Gentle Introduction to Haskell mentioned that wild-cards are useful in constructors. For example: head (x:_) = x So, does that offer any performance benefits over: head (x:xs) = x No. They are exactly the same. _ simply means a new unique name. Or is it primarily to indicate to the coder that xs is useless? Yes I get the impression it has a very similar meaning to the irrefutable pattern in regards to not evaluating it when the function is called. Or am I way off? Way off :-) Nothing to do with irrefutable patterns, or demand of evaluation, its got two uses: 1) Indicate to the reader that this argument is never used 2) Save you coming up with a name for the argument Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: Guihaskell and PropLang 0.1
Hi Dougal, It's cross platform, and will use GHC and Hugs if they are already installed. Thanks Neil On 8/15/07, Dougal Stanton [EMAIL PROTECTED] wrote: On 15/08/07, Asumu Takikawa [EMAIL PROTECTED] wrote: == GuiHaskell Guihaskell is a graphical REPL using PropLang, a GUI combinator library built on top of Gtk2hs, which aims to be an IDE for Haskell written in Haskell. It's still rough around the edges, so think of this as an alpha release. As such, I'd appreciate any feedback very much! Looks interesting. I couldn't see anywhere whether it has any OS dependencies. Is it cross platform? Cheers, D. ___ 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: Unexpected boxing in generated code
Hi Simon, OK this is an interesting one. Here's the smallest program that demonstrates the problem. foreign import ccall unsafe stdio.h getchar getchar :: IO CInt f56 :: State# RealWorld - Int - Int f56 s v2 = case (unIO getchar s) of (# s' , v6 #) - case v2 of I# _ - f56 s' v2 GHC says this is lazy in v2, which it obviously isn't. Why? Because there's a special hack (introduced after an earlier bug report) in the strictness analyser to account for the fact that a ccall might exit the program. Suppose instead of calling 'getchar' we called 'exit'! Then f56 is not strict in v2 any more. One could imagine an analysis for definitely does not exit. But it only really makes sense for IO-ish things. Why not demand that all unsafe foreign imports do not exit the program? If your foreign call does exit the program, then its unlikely to be performance critical. All unsafe FFI functions can then have their strictness analysed as before. Thanks Neil ___ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
Re: [Haskell-cafe] Re: Navigating Haddock
Hi a) use hoogle (haskell.org/hoogle). You can use hoogle to find functions by types. But I don't know haw to create a query such as ... - Document - ... Hoogle unfortunately doesn't do that very well, although that would be a great feature. Wait for version 4 :-) - I've added _ 's for wildcard types, _ - Document - _ would give you what you want. I've also made it so a search for Document can give you all the types which involve document in any way. But I think that Text.XML.HaXml isn't indexed by Hoogle anyway? Version 4 will be capable of indexing all of hackage, so hopefully that can be done. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Type without a data constructor?
Hi I by mistake defined a type which did not specify a data constructor So the question is what are types with no constructors good for? A simple example would be appreciated. They are called phantom types, and can be used for ensuring properties at the type level. I wrote about them in a blog post: http://neilmitchell.blogspot.com/2007/04/phantom-types-for-real-problems.html There are probably better references, but that's the easiest for me to find ;-) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How odd...
Hi If you just use Catch (http://www-users.cs.york.ac.uk/~ndm/catch/): foo x | x 0 = ... | x == 0 = ... | x 0 = ... This gives an error. Something identical to this code is in Data.FiniteMap, and indeed, when using floats and NaN's (or just silly Ord classes) you can cause Data.FiniteMap to pattern match error. See section 6.3 of the draft paper on the Catch website for details. myNot x | x == True = False | x == False = True This is determined to be exhaustive. as it is (in general) undecidable whether all patterns are covered. In general, yes. But its possible to do a lot better than GHC's current warnings (I'm not saying its worth changing, though) Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: monad subexpressions
Hi Perhaps we need to cool this thread down a little bit, and refocus. I personally choose never to use ++ as anything but a statement, since my brain works that way. Other people find different things natural, so can pick what they choose. The one thing you can guarantee is that discussing it isn't going to result in anyone changing their opinion! The thread started out on monad subexpressions, with request for helpful thoughts as to what could be done with them, and how we can treat them syntactically. Does anyone have any further thoughts on the syntax? We started with 4 suggestions, and as far as I can tell, are left with only one (- ...). This is the time for people to have new and clever thoughts, and possibly shape the future of (what I think) will be a very commonly used Haskell syntax. For the record, my comments on (- ...) where not objections, but merely thoughts out loud, and I could certainly see myself using that syntax in a day to day basis. Thanks Neil On 8/3/07, Mirko Rahn [EMAIL PROTECTED] wrote: rewrite *p++=*q++ in haskell? it's one of C idioms. probably, you don't have enough C experience to understand it :) Maybe, but how can *you* understand it, when the standard is vague about it? It could be A: *p=*q; p+=1; q+=1; B: *p=*q; q+=1; p+=1; C: tp=p; tq=q; p+=1; q+=1; *tp=*tq; ...and so on. Which is the right version? result is that currently C code rewritten in Haskell becomes much larger and less readable. Larger should not be that issue and readability depends on the reader as your C example shows. Some Haskellers would very quickly recognize some common idioms, where others need some help... /BR -- -- Mirko Rahn -- Tel +49-721 608 7504 -- --- http://liinwww.ira.uka.de/~rahn/ --- ___ 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] Re: monad subexpressions
Hi Thinking on the semantic issue for the moment: Can you use (-) outside of a do block? b f (- a) What are the semantics of do b f (- a) where does the evaluation of a get lifted to? Given: if (- a) then f (- b) else g (- c) Do b and c both get monadic bindings regardless of a? if (- a) then do f (- b) else g (- c) Does this change to make b bound inside the then, but c bound outside? Does this then violate the rule that do x == x Can you combine let and do? do let x = (- a) f x Our best guess is that all monadic bindings get floated to the previous line of the innermost do block, in left-to-right order. Monadic expressions in let statements are allowed. Outside a do block, monadic subexpressions are banned. Despite all these complications, it's still a great idea, and would be lovely to have! Thanks Neil and Tom On 8/3/07, Neil Mitchell [EMAIL PROTECTED] wrote: Hi Perhaps we need to cool this thread down a little bit, and refocus. I personally choose never to use ++ as anything but a statement, since my brain works that way. Other people find different things natural, so can pick what they choose. The one thing you can guarantee is that discussing it isn't going to result in anyone changing their opinion! The thread started out on monad subexpressions, with request for helpful thoughts as to what could be done with them, and how we can treat them syntactically. Does anyone have any further thoughts on the syntax? We started with 4 suggestions, and as far as I can tell, are left with only one (- ...). This is the time for people to have new and clever thoughts, and possibly shape the future of (what I think) will be a very commonly used Haskell syntax. For the record, my comments on (- ...) where not objections, but merely thoughts out loud, and I could certainly see myself using that syntax in a day to day basis. Thanks Neil On 8/3/07, Mirko Rahn [EMAIL PROTECTED] wrote: rewrite *p++=*q++ in haskell? it's one of C idioms. probably, you don't have enough C experience to understand it :) Maybe, but how can *you* understand it, when the standard is vague about it? It could be A: *p=*q; p+=1; q+=1; B: *p=*q; q+=1; p+=1; C: tp=p; tq=q; p+=1; q+=1; *tp=*tq; ...and so on. Which is the right version? result is that currently C code rewritten in Haskell becomes much larger and less readable. Larger should not be that issue and readability depends on the reader as your C example shows. Some Haskellers would very quickly recognize some common idioms, where others need some help... /BR -- -- Mirko Rahn -- Tel +49-721 608 7504 -- --- http://liinwww.ira.uka.de/~rahn/ --- ___ 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] Re: Re: monad subexpressions
Hi Can you combine let and do? do let x = (- a) f x Right. In effect, as a matter of fact, the notation x - a would become equivalent to let x = (- a) Hmm, interesting. Consider: let x = 12 let x = (- x) Currently, in let x = ... the x is in scope on the right hand side. Now it isn't. Changing the order of evaluation with syntactic sugar seems fine, changing the lexical scoping seems nasty. Perhaps this is a reason to disallow monadic expressions in a let. Our best guess is that all monadic bindings get floated to the previous line of the innermost do block, in left-to-right order. Monadic expressions in let statements are allowed. Outside a do block, monadic subexpressions are banned. Sure. SPJ mentioned that you wouldn't promote (- x) past a lambda. I'm not convinced (it seems to fall into the same category as the if statement), but it's worth considering. I'm not convinced either, a nice concrete example would let people ponder this a bit more. What is nice to note is that all your answers to my questions matched perfectly with what I thought should happen. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: monad subexpressions
Hi
do { do { a; b}; c }
is still the same as
do { a; do { b; c } }
yes? no? perhaps? sometimes? how long did it take you?
I'm not entirely sure I understand the point here. The monad laws are
defined in terms of = and return. They have never had anything to do
with do, let, or -. All of the monad laws still hold.
The Monad laws have never been defined in terms of do notation, but
they have always held with do notation since it was simply basic sugar
for and =. Now do notation is no longer as simple, and the laws
do not hold on do, only on the desugared version. We have lost the
ability to manipulate do quite as easily, and gained a more compact
expression of monadic actions.
I think the trade off is worth it, but others may not.
Thanks
Neil
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Re: [Haskell-cafe] Re: Re: Re: monad subexpressions
Hi let x = 12 let x = (- x) Okay, so the desugaring process wouldn't terminate in that case! One could either: (a) try to retain the equivalence in theory, but make it illegal to use x in a monadic subexpression when defining x; (b) we could abandon my claim that they are equivalent. This example isn't intended to be about termination of the desugaring, or about types etc - the only point is to note the change in the lexical scoping rules that (-) gives. I'll try and state my concern more clearly: let x = a In this expression, x is available for use within a, since let is recursive. This allows us to write: let xs = paws : xs With the end result that xs is bound to [paws,paws,paws,paws... Now consider: let x = (- a) With the proposed desugaring we obtain: temp - a let x = temp Now x is NOT in scope within the expression a! We have changed the static lexical scoping, and only within the brackets. This behaviour is (in my opinion) horrid. A quick poll of people in my office lead us all to believe that this issue means you should not be allowed (-) within a do's let statement. This leads us to a second problem, floating these monadic expressions outside any binding: do case x of [] - return 1 (y:ys) - f (- g y) Here, the proposed desugaring does not work, since y is not in scope where we move the element to. Perhaps this leads to the conclusion that monadic subexpressions should not be allowed inside any binding group, including let, case or lambda. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Re: Re: monad subexpressions
Hi if you write : let x = (-a):x is it possible that is desugars into : temp -a let x = temp:x that would'nt work ? That would work, since 'a' doesn't refer to 'x'. I can't think of a real example where it becomes an issue, but the scope within 'a' has changed. Also : do case x of [] - return 1 (y:ys) - f (- g y) Is it not possible that is desugars to do case x of [] - return 1 (y:ys) - g y = \temp - f temp See the rule about always binding to the previous line of a do block. This case then violates that. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Re: Re: monad subexpressions
Hi do case x of [] - return 1 (y:ys) - g y = \temp - f temp See the rule about always binding to the previous line of a do block. This case then violates that. I assumed that the example was equivalent to : do case x of [] - return 1 (y:ys) - do f (- g y) Shouldn't the rule work then ? If the do was inserted, then yes, this would work. Without it, it doesn't. Perhaps this makes a restriction to not inside case/let/lambda not that severe, since usually an additional do could be inserted. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] positive Int
Hi
Catch (www.cs.york.ac.uk/~ndm/catch) can infer that certain uses of
numbers fit into the {Neg, Zero, One, Pos} abstraction - so for
example it can infer that length returns {Zero, One, Pos}, but not
Neg. If you then do:
xs !! length ys
It will detect that length ys is natural, and will be safe. However,
if you pass any arbitrary value as the index to !! it will warn of a
possible pattern match error.
You can of course use type Nat = Int, and write additional
documentation, even if this documentation isn't a static guarantee.
Thanks
Neil
On 8/2/07, brad clawsie [EMAIL PROTECTED] wrote:
as far as i know, the haskell standard does not define a basic Int
type that is limited to positive numbers.
would a type of this kind not potentially allow us to make stronger
verification statements about certain functions?
for example, 'length' returns an Int, but in reality it must always
return a value 0 or greater. a potential counter-argument would be the
need to possibly redefine Ord etc for this more narrow type...
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Re: [Haskell-cafe] Perfect example
Hi I know that Audrey Tang (the Pugs project) has used hamming numbers for this, see http://www.perl.com/lpt/a/959 Thanks Neil On 8/2/07, Jon Harrop [EMAIL PROTECTED] wrote: Any suggestions for a perfect example that uniquely demonstrates the benefits of the Haskell language compared to other languages? -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. OCaml for Scientists http://www.ffconsultancy.com/products/ocaml_for_scientists/?e ___ 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] monad subexpressions
Hi Chris, I've heard Simon (Peyton-Jones) twice now mention the desire to be able to embed a monadic subexpression into a monad. I think this is a fantastic idea, please do so! $( expr ) -- conflicts with template haskell ( - expr ) -- makes sense, and I think it's unambiguous Other ideas: ``expr`` -- back-ticks make sense for UNIX shell scripters (| expr |)-- I don't think anything uses this yet This final (| one |) looks way too much like template haskell, it has the feel of template haskell, even if it isn't yet in the syntax. Your (- proposal) feels a bit like an operator section - I'm not sure if that is a good thing or a bad thing, but for some reason feels slightly clunky and high-syntax overhead, perhaps because of the inevitable space between the - and expr, and that ()- are all fairly high semantic value currently in Haskell, while this extension should blend in, rather than stand out. The `` syntax is clever, and I like it, but I worry that its quite a long way from the current use of ` as infix, although I'm not sure if that is a particular issue given - (negation/subtraction) and -- (comment) couldn't be more different. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] positive Int
Hi It will detect that length ys is natural, and will be safe. However, if you pass any arbitrary value as the index to !! it will warn of a possible pattern match error. I hope catch doesn't actually think that's safe, because it's not - set ys = xs = [1,2,3,4,5], you'll get an index out of range error. (Yes, I deliberately chose a list longer than 4 elements). Of course, it doesn't, I abstracted for the sake of simplicity :-) The !! operator has two entirely separate pattern match errors, one is negative index, one is ran off the end of the list. The length xs will satisfy the first one by proving that length is always a natural, but the only proof of the other condition is that either the list is infinite, or that the index is Zero/One, and the list is bigger than this. Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
