Re: [Haskell-cafe] [Haskell] ANNOUNCE: tardis
On Tue, Aug 7, 2012 at 7:04 AM, Dan Burton danburton.em...@gmail.com wrote: As a side note, since the code base is relatively small, it can also serve as a simple demonstration of how to use a cabal flag in conjunction with CPP to selectively include swaths of code (see Control/Monad/Tardis.hs and tardis.cabal). Eep, your API changes based on compile-time settings. I think this is a bad idea, because other packages cannot depend on a flag, so realistically other packages cannot depend on the instances existing, so they're nearly useless. UndecidableInstances is excessively maligned and usually fine anyway. If it compiles, it won't go wrong. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANNOUNCE: notcpp-0.0.1
On Sun, Apr 15, 2012 at 7:14 PM, Steffen Schuldenzucker sschuldenzuc...@uni-bonn.de wrote: On 04/13/2012 10:49 PM, Ben Millwood wrote: I'm pleased to announce my first genuinely original Hackage package: notcpp-0.0.1! http://hackage.haskell.org/package/notcpp [...] Why is it scopeLookup :: String - Q Exp with n bound to x :: T = @scopeLookup n@ evaluates to an Exp containing @Just x@ , not scopeLookup :: String - Q (Maybe Exp) with n bound to x :: T = @scopeLookup n@ evaluates to Just (an Exp containing @x@) ? Shouldn't n's being in scope be a compile time decision? That would also make the openState: runtime name resolution has its drawbacks :/[1] a compile time error. -- Steffen [1] http://hackage.haskell.org/packages/archive/notcpp/0.0.1/doc/html/NotCPP-ScopeLookup.html This way minimises the amount the user has to know about Template Haskell, because the user can just splice the expression immediately and then operate on it as an ordinary value. The design you suggest would require messing about in the Q monad to construct the expression you wanted based on whether you got a Nothing or a Just, which in my view is more awkward. I can see how your version would be useful too, though – in particular I can move the error call to a report call, which throws a compile-time error as you say. I'd be happy to expose both next version ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANNOUNCE: haskell-src-meta 0.3
It seems I wasn't subscribed to haskell-cafe so this reply didn't get through (sorry for sending this twice, Felipe). I've now subscribed with delivery turned off for -cafe so if you want me to read a response be sure to address me directly :) On Tue, Jan 4, 2011 at 5:08 AM, Ben Millwood hask...@benmachine.co.uk wrote: On Tue, Jan 4, 2011 at 4:02 AM, Felipe Almeida Lessa felipe.le...@gmail.com wrote: On Tue, Jan 4, 2011 at 1:49 AM, Ben Millwood hask...@benmachine.co.uk wrote: haskell-src-meta is a package originally written by Matt Morrow to provide a translation from the syntax tree provided by haskell-src-exts to template haskell syntax. Essentially this allows TH code to be parsed from a string, and is used in several quasiquoter libraries. Are the purposes of haskell-src-meta and haskell-src-exts-qq the same? What are the main differences? Cheers! =) -- Felipe. If you look at the source for Language.Haskell.Exts.Translate from haskell-src-exts-qq [1], you can see that it was actually taken from an earlier version of haskell-src-meta, presumably during the long period for which HSM was not maintained. I guess you could say that it inlined the key parts of haskell-src-meta into its codebase, but I hope that the maintainer can feel more confident about using the haskell-src-meta package as a dependency now. My understanding of the *purpose* of haskell-src-exts-qq is that it is for easily generating ASTs for use with haskell-src-exts, whereas haskell-src-meta is for generating ASTs for use with template haskell. [1] http://hackage.haskell.org/packages/archive/haskell-src-exts-qq/0.3.0/doc/html/src/Language-Haskell-Exts-Translate.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Monads and Functions sequence and sequence_
The actual, entire, complete definitions of sequence and sequence_ are (or at least, could be): sequence [] = return [] sequence (m:ms) = do x - m xs - sequence ms return (x:xs) -- or, equivalently: sequence' = foldr (liftM2 (:)) (return []) sequence_ [] = return () sequence_ (x:xs) = do x sequence_ xs -- or: sequence'_ = foldr () (return ()) They're defined once for all monads, not once for each monad, so in some sense they behave the 'same' in that they use the Monad instance in the same way. It's just like, say, sort :: Ord a = [a] - [a] might do different computations to compare elements depending on what 'a' is, but always produces a sorted list regardless of that detail. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Package regex-posix-0.94.2 suddenly failed to link
On Sat, Oct 30, 2010 at 11:56 AM, Arnaud Bailly arnaud.oq...@gmail.com wrote: Hello, All of a sudden, the package regex-posix-0.94.2 failed to link after i installed a couple of other packages (http, json). When I try to reinstall it, I got the folowing errors: D:\projets\crete1941cabal install --global --reinstall --enable-documentation regex-posix-0.94.2 Resolving dependencies... Configuring regex-posix-0.94.2... Preprocessing library regex-posix-0.94.2... Text\Regex\Posix\Wrap.hsc:107:19: regex.h: No such file or directory This is the crucial line in this particular build failure, everything else is fluff. I really, really wish gcc would learn to stop compiling after a header file was missing, since it almost always results in large amounts of nonsense errors. It looks like cabal-install or possibly Cabal can't find your headers. I notice you're compiling with an explicit --global option - why is that? Is it currently installed locally or globally? You can use 'ghc-pkg list regex-posix' to check. You can also enable global installs and documentation by default by editing your config file, whose location is given in the output of cabal --help. You can also specify extra-include-dirs there if you need to do that. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] who's in charge?
On Fri, Oct 29, 2010 at 5:06 AM, Steve Severance st...@medwizard.net wrote: whenever I here any open source community (yeah...everyone not just haskell) tell beginners to contribute a package I always scratch my head with a little bit of wonder. Would you really want a package that someone like me who is still trying to figure out how to utilize haskell's features would build? Do you want my outrageous non-use of the Monads that haskell offers? Can I just take up this point and say, yes I do. It's much easier to fix a bad library than write a good one :P Besides, I'd think that often what Haskell developers lack is time more than skill - there are plenty of tasks that could be done without advanced knowledge of deep abstractions, if only someone could put aside a few weekends for them. For example, writing low-level FFI bindings is almost mechanical (i.e. requires basically no actual ingenuity) with the right tools, but it takes time and effort, so libraries go unbound. In short, you do not need a PhD to write a decent and useful library! Just open a github and give out commit access like confetti and everything will be fine :) I also think that it's a good idea to review Haskell packages more thoroughly, but I think shiny-new-hackage is going to help a little in that regard with reverse dependencies prominently visible on package pages. I also think that it should be convention to link every hackage package with a page on the wiki for discussion (perhaps creating a new namespace in mediawiki for this purpose). This would be as simple as adding an autogenerated link to hackage's template. This is not a new idea, but it's yet to be popularised, and I think it needs backing from Hackage itself to do that. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] type class design
On Fri, Oct 29, 2010 at 1:33 PM, Tillmann Rendel ren...@informatik.uni-marburg.de wrote: Note that the case of (==) and (/=) is slightly different, because not only can (/=) be defined in terms (==), but also the other way around. The default definitions of (==) and (/=) are mutually recursive, and trivially nonterminating. This leaves the choice to the instance writer to either implement (==) or (/=). Or, for performance reasons, both. I find these sorts of defaults deeply unsatisfying: particularly, suppose I do newtype Foo = Foo Integer deriving (Eq, Show) instance Num Foo where Foo a + Foo b = Foo (a + b) fromInteger = Foo expr = Foo 3 - Foo 2 That I haven't defined implementations for (-) or negate will not even get me a compiler warning, let alone a static error: it will just stack overflow or spin endlessly on expr. This kind of bug is notoriously difficult to track down. I'm not sure how to handle this better, though. A compiler that automatically calculated minimal complete definitions would be nice, but relatively complicated. It might be more sensible to just take all efficiency methods out of classes, and use a mechanism like rewrite rules to give an efficient implementation where possible. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Current thinking on CompositionAsDot issue in haskell prime?
On Thu, Oct 28, 2010 at 11:12 AM, John Smith volderm...@hotmail.com wrote: On 28/10/2010 10:15, Alexander Kjeldaas wrote: Hi haskellers. Reading through the Haskell Prime suggestions, one that caught my eye is the CompositionAsDot issue. I'm especially thinking of the Pro issue: * Paves the way for using . as the selection operator in improved record or module systems Here's the wiki page: http://hackage.haskell.org/trac/haskell-prime/wiki/CompositionAsDot Personally I think function composition is what Haskell is all about and it is absolutely essential that the syntax for it be lightweight. If we think using . as qualification as well as composition is confusing, I'm much more inclined to say using it as qualification was a mistake. The comment on the wiki page about $ being more common in reality is not even close to true for my own code, and I don't think I'm unusual in that regard. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Test command line programs
On Tue, Oct 26, 2010 at 5:11 PM, Dupont Corentin corentin.dup...@gmail.com wrote: Hello again café, I have a command line program that takes input from various handles (actually network sockets) like this: s - hGetLine h etc. I'd like to unit test this. How can I do? If all you ever do in some part of the code is read from the socket, consider passing an IO action to do that into your function, instead of the handle itself. Then: a. you can easily replace the IO String with (return testdata), or a read from an MVar you feed data into, or whatever else you like. b. you can statically guarantee the function doesn't do anything unexpected to the handle, like closing it or seeking or setting a buffering option. c. you will probably not have to write as much, saving on keyboard wear. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskellers.com skills list moderation?
On Tue, Oct 19, 2010 at 2:32 PM, Michael Snoyman mich...@snoyman.com wrote: Algorithmic Problem Solving I think this needs to go, because I'm really having a hard time imagining any programmer who doesn't do this. High Assurance Software Development This sounds vague to me and/or the same as other skills (cf. Formal Verification). Again, I'm not sure how many people would describe their software as low assurance. Robotics and Automation Would be tempted to drop Automation from here. Web development (HTML, CSS and Javascript) I wonder if these parentheses are necessary, or if they hint at the fact that this isn't really one skill. I have a suspicion that being competent at website and stylesheet *design* (i.e. knowledge of good design principles and application to HTML/CSS) is an entirely different sort of thing from *implementation* in terms of JavaScript technologies like AJAX and JSON and who-knows-what. Overall, I think it would be nice to have a consistent idea about how concrete or abstract we allow skills to be, and as someone else mentioned what the target audience is for them. We have skills that relate to specific libraries and then skills that are nebulous and abstract. Maybe we could ask a narrower question, or have two fields: what can you use, and what interests you. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Proving stuff about IORefs
On Sun, Oct 17, 2010 at 11:15 AM, Malcolm Wallace malcolm.wall...@me.com wrote: The problem with the code you originally posted was that it looked like this: f r = do r' - something f r' something else -- this is dead code That is, the computation is non-terminating, because f simply calls itself recursively, with no base case. Regards, Malcolm He was using ==, not =, it was a statement of equality not a definition :) Much like one might say that sort xs == sort (reverse xs). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] An interesting paper from Google
On Fri, Oct 15, 2010 at 9:28 PM, Andrew Coppin andrewcop...@btinternet.com wrote: I'm still quite surprised that there's no tool anywhere which will trivially print out the reduction sequence for executing an expression. You'd think this would be laughably easy, and yet nobody has done it yet. I tried to do something a bit like this: http://github.com/benmachine/stepeval but it could be charitably described as crude: has three failing testcases and a bagful of unimplemented functionality. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Pronouncing Curry and currying
On Fri, Oct 8, 2010 at 8:56 AM, Petr Pudlak d...@pudlak.name wrote: thanks for both the explanation (Donn) and the sound sample (Luke). Unfortunately, hurry is pronounced differently in British and US English [1], so again I was a little bit confused :-). But Luke's sound sample made it clear for me. I think curry is too - I would still pronounce it to rhyme with hurry even though I have a British accent. I think in the presence of accents it's not always sensible to look for a single correct pronunciation - clarity of communication is the only really important thing, and that will probably be maintained whatever you do with that vowel. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Eta-expansion destroys memoization?
On Thu, Oct 7, 2010 at 1:44 PM, Luke Palmer lrpal...@gmail.com wrote: The section works because (a %^) (for some operator %^) is short for (%^) a and (%^ a) is short for flip (%^) a. Sections don't expand into lambdas. According to the report they do: http://haskell.org/onlinereport/exps.html#sections http://haskell.org/onlinereport/haskell2010/haskellch3.html#x8-33.5 but GHC is different, I think: http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/syntax-extns.html#postfix-operators I'm not sure if the significance of this difference is explored anywhere, but notice that: ghci (() `undefined`) `seq` () *** Exception: Prelude.undefined ghci (`undefined` ()) `seq` () () ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Coding conventions for Haskell?
On Tue, Sep 28, 2010 at 5:55 AM, Evan Laforge qdun...@gmail.com wrote: I write haskell and python in a proportional font and it hasn't yet let to tabs, so no pain so far :) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe I like writing proportional haskell, but I haven't worked out how to do let-blocks in ways that look nice - the tactic I use for do and where of breaking the line immediately tends to look a little odd, especially if you're only making about one binding. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Applicative instances for Monads
On Sat, Sep 25, 2010 at 3:01 AM, Gregory Crosswhite
gcr...@phys.washington.edu wrote:
==
import Control.Applicative
import Control.Concurrent
import Control.Concurrent.MVar
newtype AIO a = AIO {unAIO :: IO a}
instance Monad AIO where
return = AIO . return
(AIO x) = f = AIO (x = unAIO . f)
instance Functor AIO where
fmap f (AIO x) = AIO (fmap f x)
instance Applicative AIO where
pure = return
(AIO mf) * (AIO ma) = AIO $ do
f_box - newEmptyMVar
forkIO (mf = putMVar f_box)
a_box - newEmptyMVar
forkIO (ma = putMVar a_box)
f - takeMVar f_box
a - takeMVar a_box
return (f a)
==
This idea is pretty neat :) I think it should be found a place on the
wiki, or maybe even Hackage. The way in which it interacts with
exceptions, especially async exceptions, could be odd though, so it'd
be worth checking it pedantically adheres to the rules.
To summarize: on the one hand every Monad has a generic instance for
Applicative, and yet on the other hand this instance is often arguably not
the correct one because it ignores the fact that the second computation is
independent of the first, which is a fact that can be exploited given
additional knowledge about the structure of the Monad.
I bring this up because there has been talk here of automatically having
instances of Monad also be instances of Applicative, and what bugs me is
that on the one hand this makes perfect since as every Monad can also be
viewed as an Applicative, and yet on the other hand not only is there often
more than one natural way to define an Applicative instance for selected
Monads but furthermore the generic instance is often an inferior
definition because it ignores the structure of the Monad.
I think what we learn from this is not that the Monad-based instance
of Applicative is necessarily the wrong one, but rather that there
is often more than one reasonable instance for a type, each suitable
for different uses. There are times when parallelisation is not a
priority, but determinism is, in which case we'd *want* the sequencing
of Monad even in the Applicative instance.
Often we use newtypes to distinguish between them (see: ZipList), and
if we accept that the Monad-based instance is always a useful one (and
if the Monad instance itself is useful I think it is) it makes sense
for it also to be the default one, so that we can have ap and *
always mean the same thing in the same context.
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Re: [Haskell-cafe] Problems with installing type-level package
2010/9/19 Николай Кудасов crazy.fiz...@gmail.com: Hi, cafe, I have a stange thing when trying to install type-level package: $ sudo ./Setup.hs configure Configuring type-level-0.2.4... $ sudo ./Setup.hs build Preprocessing library type-level-0.2.4... Building type-level-0.2.4... [1 of 8] Compiling Data.TypeLevel.Num.Reps ( src/Data/TypeLevel/Num/Reps.hs, dist/build/Data/TypeLevel/Num/Reps.o ) [2 of 8] Compiling Data.TypeLevel.Num.Sets ( src/Data/TypeLevel/Num/Sets.hs, dist/build/Data/TypeLevel/Num/Sets.o ) [3 of 8] Compiling Data.TypeLevel.Num.Aliases.TH ( src/Data/TypeLevel/Num/Aliases/TH.hs, dist/build/Data/TypeLevel/Num/Aliases/TH.o ) [4 of 8] Compiling Data.TypeLevel.Num.Aliases ( src/Data/TypeLevel/Num/Aliases.hs, dist/build/Data/TypeLevel/Num/Aliases.o ) Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Loading package syb ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package containers-0.2.0.1 ... linking ... done. Loading package pretty-1.0.1.0 ... linking ... done. Loading package template-haskell ... linking ... done. Generating and compiling a zillion numerical type aliases, this might take a while $ [...] It looks like the Setup.hs build step is only compiling 4 of 8 modules? Does this stage appear to exit successfully? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO Put confusion
On Wed, Sep 15, 2010 at 12:45 AM, Chad Scherrer chad.scher...@gmail.com wrote: Hello, I need to be able to use strict bytestrings to efficiently build a lazy bytestring, so I'm using putByteString in Data.Binary. But I also need random numbers, so I'm using mwc-random. I end up in the IO Put monad, and it's giving me some issues. To build a random document, I need a random length, and a collection of random words. So I have docLength :: IO Int word :: IO Put Oh, also putSpace :: Put My first attempt: doc :: IO Put doc = docLength = go where go 1 = word go n = word return putSpace go (n-1) I think you misunderstand, here, what return does, or possibly . This function generates docLength random words, but discards all of them except for the last one. That's what the operator does: run the IO involved in the left action, but discard the result before running the right action. The IO action 'return x' doesn't do any IO, so 'return x a' does nothing, discards x, and then does a, i.e. return x a = a Unfortunately, with this approach, you end up with a one-word document. I think this makes sense because of the monad laws, but I haven't checked it. Yes, the above equation is required to hold for any monad (it is a consequence of the law that 'return x = f = f x') Second attempt: doc :: IO Put doc = docLength = go where go 1 = word go n = do w - word ws - go (n-1) return (w putSpace ws) This one actually works, but it holds onto everything in memory instead of outputting as it goes. If docLength tends to be large, this leads to big problems. Here you're using the from the Put monad, which appends lazy ByteStrings rather than sequencing IO actions. The problem is that the ordering of IO is strict, which means that 'doc' must generate all the random words before it returns, i.e. it must be completely done before L.writeFile gets a look-in. It turns out the problem you're trying to solve isn't actually simple at all. Some of the best approaches to efficient incremental IO are quite involved - e.g. Iteratees. But your case could be made a great deal easier if you used a pure PRNG instead of one requiring IO. If you could make word a pure function, something like word :: StdGen - (StdGen, Put) (which is more or less the same as word :: State StdGen Put), then you'd be able to use it lazily and safely. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Curious why cabal upgrade parsec not installing latest version
On Thu, Sep 16, 2010 at 4:00 AM, Ivan Lazar Miljenovic ivan.miljeno...@gmail.com wrote: Because Parsec-3 apparently still has some speed regressions compared to Parsec-2 (I'm not qualified to note whether its design is slow or if you have to use it differently to get good performance out of it), so many developers prefer to stick to Parsec-2 for this reason. I thought Parsec 3.1 had pretty much caught up with the performance of parsec 2? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Idea for hackage feature
On Fri, Sep 17, 2010 at 1:44 AM, Ivan Lazar Miljenovic ivan.miljeno...@gmail.com wrote: On 17 September 2010 03:18, Henning Thielemann My suggestion is to move the Unsafe modules to a new package 'unsafe'. Then you can easily spot all dirty packages by looking at reverse dependencies of 'unsafe'. Hooray, yet another supposedly stand-alone library that GHC will depend on and thus can't be upgraded anyway, so there's no real advantage of making it stand-alone (after all, doesn't base use unsafeInterleaveIO or something for lazy IO?). Well, it's not like we plan on regularly fiddling that API :) The clever thing about this suggestion is that most packages don't *export* equivalent power to unsafePerformIO even if they import it (inlinePerformIO from bytestring is a notable exception) so you can easily see from a library's *immediate* dependencies whether it could potentially do anything naughty or not. Also, it's implementable entirely with existing technology, although we'll probably want a major base version bump to remove the modules. When discussing this sort of taint I think it's important not to forget that the FFI can be just as bad. For a start, one common use of unsafe functions is to provide a pure API to a foreign library (as is done in RWH), and clearly in such cases the proof of correctness cannot exist in the language because it depends on properties of libraries, which may not even be linked until runtime. Secondly, FFI imports are almost as bad safetywise as System.IO.Unsafe, and twice as impossible to prove correct. So your taint measure should take into account use of that extension, too. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Scraping boilerplate deriving?
On Wed, Sep 15, 2010 at 2:11 PM, Kevin Jardine kevinjard...@gmail.com wrote: I do think that defObj(MyType) looks a bit cleaner than $(defObj MyType) I believe as of GHC 6.12 you no longer need the $() around top-level splices. So that would just be: defObj MyType ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Cleaning up threads
On Tue, Sep 14, 2010 at 9:44 PM, Mitar mmi...@gmail.com wrote: Hi! On Tue, Sep 14, 2010 at 9:04 PM, Gregory Collins g...@gregorycollins.net wrote: That's surprising to me -- this is how we kill the Snap webserver (killThread the controlling thread...). Yes. This does work. The only problem is that my main thread then kills child threads, which then start killing main thread again, which then again kills child threads and interrupt cleanup. This sounds wrong. Why is the main thread sending more than one kill? Handlers for some exception shouldn't run more than once unless you set them up that way. Are you perhaps being tripped up by the issue whereby when the main thread dies, the RTS just shuts down even if other threads are running? You might find you need some kind of maybe MVar-driven mechanism to keep the main thread alive until all else is definitely dead. Maybe this behaviour should be considered a bug, I don't know. It would be nice if after a forkIO threads were effectively equal. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Do expression definition
On Mon, Sep 13, 2010 at 8:21 AM, Alexander Kotelnikov sa...@myxomop.com wrote:
And, also, would it make any difference if
do {p - e; stmts} = let ok p = do {stmts}
ok _ = fail ...
in e = ok
is redefined as e = (\p - do {stmts})?
This is the magic that allows pattern-match failure in a do expression
to return a normal result. Notice that fail and not error is
called - each Monad has its own fail method, so that for example:
uncons :: [a] - Maybe (a, [a])
uncons xs = do { (x:xs) - return xs; return (x, xs) }
evaluates to Nothing rather than causing an exception when xs is empty.
That this implementation detail ends up in the Monad class is regarded
by many as untidy, though.
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Re: [Haskell-cafe] Disable LINE Pragma handling in GHC
On Fri, Sep 10, 2010 at 7:40 AM, JP Moresmau jpmores...@gmail.com wrote: I suppose even using GHC for building and something else (haskell-src-exts?) for code handling would leave us with compilation messages at the wrong place. I don't quite understand your use case so I'm not sure it helps, but for what it's worth haskell-src-exts allows you to ignore line pragmas while parsing: http://hackage.haskell.org/packages/archive/haskell-src-exts/1.9.0/doc/html/Language-Haskell-Exts-Parser.html#t:ParseMode ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Handling platform- or configuration-specific code (or, my CPP complaints)
Good evening, cafe, Having recently taken on maintenance of a package that depends on template-haskell, I've been in some discussion with users and dependencies of my package about how best to write a library that works with multiple incompatible versions of a dependency. The two main approaches that I'm aware of are: 1. Use CPP and the macros defined by Cabal to conditionally include or exclude code for each version. 2. Use Cabal file conditionals to select hs-source-dirs containing those parts of the code (or even TH to help generate those parts of the code) that are specific to each configuration. In my discussion with others and examination of existing libraries, it seems to me that 1. is the preferred option, but I personally can think of a number of reasons to prefer the second option: * CPP was not designed for Haskell, and even cpphs still shows symptoms of it, so we have to worry about things like its handling of string gaps or single quotes. * CPP is primarily a textual manipulation tool, rather than a symbolic one, which makes it even more easy to produce malformed code with it than with TH or similar. * On that note, it's difficult to statically analyse a file using CPP: parser tools like haskell-src-exts don't support it and it's not at all obvious how or if they ever could. With Cabal choosing source files based on configuration, all the source is valid, normal Haskell and is easy for computers and humans to understand. * It may require some significant digging into each source file to establish what configurations must be tested, or to add a new supported configuration or remove an old one, if they are chosen based on CPP conditional compilation. When Cabal chooses what is compiled it is fairly explicit what is chosen and what could be, and adding a new configuration is - at least in theory - as simple as adding a new file and conditional. * It's just not very pretty! Haskell code has a sort of aesthetic that I don't think CPP macros share - different function application syntax, for example. Of course the trouble is that when your conditional compilation is on the module level there are some things which just can't be done without code duplication, and there's a tendency for small and often quite incidental bits of a function definition to suddenly be in another module instead of where they're used. So I wonder what people think of the use of CPP in Haskell code, what alternatives people can propose, or what people hope to see in future to make conditional compilation of Haskell code more elegant and simple? I once pondered whether it would be a good idea to somehow make available to TH splices information from Cabal or GHC's configuration, so that one could do something like this: myFunctionDecl = if packageVersion template-haskell = Version [2, 4] [] then [d| unTyVarBndr (PlainTV n) = n; unTyVarBndr (KindedTV n _) = n |] else [d| unTyVarBndr = id |] This exactly wouldn't make sense because KindedTV wouldn't exist in the earlier version so the quotation would object, but one could imagine something similar being useful. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to catch exception within the Get monad (the Binary package)
On Tue, Sep 7, 2010 at 2:45 PM, Dimitry Golubovsky golubov...@gmail.com wrote: unThrow a = unsafePerformIO $ (E.evaluate a = return . Right) `E.catch` (\e - return $ Left e) -- or perhaps the right argument of catch could be just (return . Left)? bm2mb :: a - Maybe a bm2mb a = case unThrow a of Left (e::SomeException) - Nothing Right a - Just a Philosophically these functions are Nasty because they violate referential transparency. In particular it's possible for the same expression to throw different exceptions each time it's run depending on how it's optimised, what other threads are doing, if the user presses ctrl-C, etc. etc. See the spoon package: http://hackage.haskell.org/package/spoon which alleviates this a little by only catching some kinds of exception, and not telling you which it caught. It still violates monotonicity (I believe), so purists will be upset, but practically it can be useful for when editing the source code to provide explicit exceptions (which is ideally what you'd do) is not an option. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Restricted type classes
I have only one thing to add to this discussion: On Fri, Sep 3, 2010 at 5:16 AM, Ivan Lazar Miljenovic ivan.miljeno...@gmail.com wrote: 2b) Is it OK to promote functions that use a class to being class methods? When I was discussing this in #haskell several people mentioned that defining something like liftA2 for the Set instance of (restricted) Applicative would make more sense than the default * (since (a - b) isnt' an instance of Ord). One problem with defining both * and liftA2 in the class in terms of each other is that omitting them from instances is no longer a compile-time error, nor is it even an obvious runtime error - it's frequently an infinite loop, which is unpleasant. Though I understand that it's nice to be able to choose the methods you want to define, I think static error detection is nice too, so I tend to avoid providing extra class methods with defaults unless the alternative is much worse. On that note, why *do* we have missing instance methods filled in with bottoms, so that even without defaults, it's a warning rather than an error? It seems like quite an un-Haskelly thing to do. I know there may be some cases where you do not need to define a particular method, but imo you should be required to explicitly opt out of it if that's the case. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] creating a type based on a string
On Thu, Sep 2, 2010 at 9:31 PM, Andrew U. Frank fran...@geoinfo.tuwien.ac.at wrote: I have a user input (string) and need to select one of two types. depending what the input is. is this possible? data A data B data X n = X String op :: String - X n op a = X a :: X A op b = X b :: X B this does obviously not compile. is there a way to achieve that the type X A is produced when the input is a and X B when the input is b? thank you for help! andrew ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Here's another way of not quite doing what you want: op :: String - Either (X A) (X B) op a = Left (X a) op b = Right (X b) which is roughly how I translate the recent discussion about type-level validity certification: http://www.haskell.org/pipermail/haskell-cafe/2010-August/082899.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Handling absent maintainers
2010/8/17 Geoffrey Mainland mainl...@eecs.harvard.edu: On 08/17/2010 12:28, Ben Millwood wrote: 2010/8/17 Jonas Almström Duregård jonas.dureg...@gmail.com: Hi, Has there been any progress with this package? Like you I have also tried to contact Matt and like you I have ended up making my own version of src-meta :). When someone assumes maintainership of this package I would like to discuss integrating some additions I made to the Translation module. /Jonas Hi Jonas, Sorry about the delay. In my version of haskell-src-meta I basically replaced the entire Language.Haskell.TH.Instances.Lift module with derivations from the th-lift package whose latest version currently doesn't build [1]. I had been in contact with the maintainer of the package who has prepared a version 0.5 and was waiting for them to upload it so that I could use it as a dependency, but I haven't heard from them in a week or so. I'll try emailing them again, and as soon as 0.5 is on hackage I will go ahead with my upload. I suppose since there's no reason why I deserve maintainership more than you you could just go ahead and upload your version instead. You can see exactly what I've done at my github repository [2]. Yours, Ben Millwood [1] http://hackage.haskell.org/package/th-lift-0.4 [2] http://github.com/benmachine/haskell-src-meta I'm also waiting on a 6.12-compatible version of haskell-src-meta so I can release a version of my package for quasiquoting C/CUDA. I pulled Mark's changes to create a version that maintains compatibility with version 2.3 of the template-haskell library (and therefore with GHC 6.10). Is there any chance we can keep backwards-compatibility? ;) I'm happy to help with patches and/or maintainership, of course. Geoff I imagine maintaining compatibility across template-haskell versions might be tricky. Prior to template-haskell-2.4.0.0, I believe there was no TH support for kinds, and class contexts were simply Types rebranded (not supporting equality constraints). So some not-inconsiderable sections of code would need duplication, or we'd need to deal with CPP which always struck me as a fragile and inelegant solution. Of course, I welcome the patches to prove me wrong :) or well, if you just want to take maintainership yourself, the package is really as much yours as it is mine at this stage. Ben ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A cabal odyssey
On Wed, Aug 18, 2010 at 8:31 AM, Evan Laforge qdun...@gmail.com wrote: I was trying to turn on --global by default user-install: False I think it's not completely a stupid idea to have profiling default off. I personally do not really enjoy the fact that I compile everything three times nowadays :) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANNOUNCE: haskell-src-meta 0.1.0
Hello cafe, The haskell-src-meta package was originally written by Matt Morrow, to provide a translation from the syntax tree parsed from String by the haskell-src-exts package to Template Haskell's representation of Haskell source code, making possible a variety of interesting quasiquoters and metaprogramming. However, while the package was still fairly embryonic and incomplete, Matt disappeared from the haskell community, and his work no longer compiles with the latest versions of template haskell and GHC. Many people still had use for it, so there was some discussion about its future in a haskell-cafe thread [1] resulting in a fork [2]. After I and at least one other person duplicated the work of updating haskell-src-meta, ignorant of the fork, it was decided that maintaining forks was a less-than-ideal solution, so I volunteered to take up maintainership of a package that had by now been untouched for some ten months. Hence, haskell-src-meta-0.1.0, from a new maintainer (hello!). The functionality changes in this release are minimal, but nonzero: its primary purpose is to let the original work be used with updated TH. However, there have been some changes and additions since 0.0.6: * The previously somewhat lengthy Language.Haskell.TH.Instances.Lift is now mostly generated with template haskell using the th-lift package. * Contexts, kinds, bang patterns, unboxed word literals, newly supported in template-haskell-2.4.0.0, are used where necessary. * Use of haskell-src-exts is brought in line with newer versions. * Thanks to a patch by Jonas Duregard, a new ToDecs class has been added to handle cases where more than one Dec must be returned. I don't have any big plans for future releases: there is a lot that could do with tidying up, and some small aspects of the syntax conversion that aren't covered, but I mostly just intend to ensure it keeps building and that anyone with patches to contribute has a prompt response. How You Can Help: the easiest way is to tell me that I've put the version constraints too tightly on some dependency or other -- I have erred on the conservative side. Any other comments on the package in general are appreciated too, since this is the first time I've ever uploaded anything to Hackage. There is a github repository at http://github.com/benmachine/haskell-src-meta if you want to submit patches. Yours sincerely, Ben Millwood [1] http://www.haskell.org/pipermail/haskell-cafe/2010-July/080390.html [2] http://hackage.haskell.org/package/haskell-src-meta-mwotton ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Handling absent maintainers
2010/8/17 Jonas Almström Duregård jonas.dureg...@gmail.com: Hi, Has there been any progress with this package? Like you I have also tried to contact Matt and like you I have ended up making my own version of src-meta :). When someone assumes maintainership of this package I would like to discuss integrating some additions I made to the Translation module. /Jonas Hi Jonas, Sorry about the delay. In my version of haskell-src-meta I basically replaced the entire Language.Haskell.TH.Instances.Lift module with derivations from the th-lift package whose latest version currently doesn't build [1]. I had been in contact with the maintainer of the package who has prepared a version 0.5 and was waiting for them to upload it so that I could use it as a dependency, but I haven't heard from them in a week or so. I'll try emailing them again, and as soon as 0.5 is on hackage I will go ahead with my upload. I suppose since there's no reason why I deserve maintainership more than you you could just go ahead and upload your version instead. You can see exactly what I've done at my github repository [2]. Yours, Ben Millwood [1] http://hackage.haskell.org/package/th-lift-0.4 [2] http://github.com/benmachine/haskell-src-meta ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A cabal odyssey
On Mon, Aug 16, 2010 at 8:27 PM, Andrew Coppin andrewcop...@btinternet.com wrote: The amusing part is, if you sudo cabal install so it has permission to put the installed files into place, it then uses root's configuration file instead. *sigh* Well anyway, I managed to work around that. But... Cabal *still* fails to find Alex or Happy, even though they're now in the search path. Oh, wait. They're in *my* search path. They're not in root's search path. [...] It sounds to me like your life would be a lot easier if you knew about cabal-install's root-cmd configuration parameter! Open your .cabal/config file and uncomment and set: root-cmd: sudo Now cabal-install will take up root permissions when and only when necessary :) I only found this out after asking #haskell about similar problems. Perhaps it should be more prominent somehow. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Unwrapping long lines in text files
On Sat, Aug 14, 2010 at 2:38 AM, michael rice nowg...@yahoo.com wrote: The program below takes a text file and unwraps all lines to 72 columns, but I'm getting an end of file message at the top of my output. How do I lose the EOF? Michael While many other people have shown you why you need not necessarily answer this question, I think it'd be helpful for you to hear the answer anyway. Your message is being produced because you are trying to getLine when there is no input left. This raises an exception, which, because it is not handled by your program, prints a diagnostic message and exits. Strangely, it prints this before the output of your program - this isn't terribly important, but for the sake of completeness, it's because of the different buffering characteristics of stdout and stderr, which confusingly mean that even though your program produces output and then produces an error, the error is printed immediately while the output waits until the program is terminated to be produced. I think. Something like that, anyway. So, how do you avoid the exception? You can use System.IO.isEOF [1] to check if there is input available on the standard input handle: main = do eof - isEOF when (not eof) realMain -- when from Control.Monad, see also: unless where realMain = do realStuff Or you can let getLine throw the exception, but catch it and deal with it yourself, rather than letting it kill your program. Catching it is a little less simple, but is considerably more flexible and powerful. The exceptions situation in Haskell is somewhat complicated by the fact that the mechanism used by haskell98 has been improved upon, but the new extensible mechanism is incompatible with the old so both have to hang around. Have a look at Control.Exception [2] and System.IO.Error [3]. In either case you have 'catch' as a sort of basic operation, and more convenient things like 'try' which sort of turn an exception into a pure Either result. I'd do something like this: main = do result - try getLine case result of Left err - return () -- or do something diagnostic Right - putStrLn main Right line - doStuffWith line On a more general note, your main function looks a little suspect because it *always* recurses into itself - there's no termination condition. A question nearly as important as how does my program know what to do is how does it know when to stop :) [1] http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/System-IO.html#v%3AisEOF [2] http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/Control-Exception.html [3] http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/System-IO-Error.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Preview the new haddock look and take a short survey
On Thu, Aug 5, 2010 at 2:35 PM, Dino Morelli d...@ui3.info wrote: One thing I haven't seen anyone else comment on is the width of the new docs. I have a large (26) monitor and use the browser full-screen (with xmonad, so even more screen space). When I load these pages, particularly the non-frame one, something like 50% of my screen real-estate is empty whitespace on either side of the doc content. There is also wasted space in the frames version, just a little less of it. I wish the docs were using that space like the current Haddock does. Is the plan to use a fixed width like this? Please say no, it's a disappointing trend that you see everywhere. Like Twitter's web interface, for instance, very narrow. Yeah, I wrote about this in my survey response. It seems to me that if I find text in a narrow page more readable, I can easily just resize my browser window, this doesn't need to be enforced by the webpage itself. I'm also not so enthusiastic about tabbed synposis. I'm not convinced you can easily use the synopsis and doc text simultaneously, so I don't see any reason for it to be apart from the text body. Simplicity is a virtue :) I do think that in terms of colours, fonts etc. it's prettier, but the fixed max width and kind of gimmicky synopsis tab are steps backward in my opinion. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Handling absent maintainers
On Thu, Aug 5, 2010 at 5:07 AM, Erik de Castro Lopo mle...@mega-nerd.com wrote: The permissiveness of hackage uploads suggests that Hackage needs to start using something like GPG signing and GPG webs of trust. The Debian project has stuff like this in place and I'm sure this community could learn a lot from what Debian is currently using. Erik It's worth mentioning that Hackage accounts aren't just given out for free - one has to specifically request them, and they can presumably just as easily be revoked. Thanks Mark, I'll request a Hackage account within the next couple of days and then make the upload. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Handling absent maintainers
On Thu, Aug 5, 2010 at 6:38 PM, Ben Millwood hask...@benmachine.co.uk wrote: It's worth mentioning that Hackage accounts aren't just given out for free - one has to specifically request them Er, to clarify, I mean signup can't be automated, because account creation is done by a human. Not that hackage charges for accounts or something. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Handling absent maintainers
On Sun, Jul 18, 2010 at 3:02 AM, Mark Wotton mwot...@gmail.com wrote: I've uploaded haskell-src-meta-mwotton, using the development version. It seems to work fine for my applications. It's a bit of a hack, but I can't think of a better way to do it for now. mark -- 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 I've just come up against one of the drawbacks of this approach - having needed haskell-src-meta for a personal project, I downloaded the source and updated it to work with GHC 6.12, fixed various bits and bobs, and only now found out that much of that work had already been done elsewhere :) Matt Morrow has been missing for a long time and I think it's reasonable to suppose he won't suddenly spring out of the darkness to fix things for us. I propose that someone just take up maintainership of the package. I am quite willing to do this with my version, or Mark if you think you'd like to keep a closer eye on your dependencies you could do it instead. I further propose that we should write up a haskellwiki page about absent maintainers and what the community thinks is reasonable in terms of attempting contact before assuming them missing, presumed gone. This kind of depends also on how big an indignity we consider it to be if someone updates a package while the maintainer is just on holiday or something. So we need to decide on: first, who will take haskell-src-meta, and second, what we think is good as a more general policy. I would think the process would go something like: 1. email maintainer, wait 2 weeks for reply 2. email cafe and maintainers of reverse dependencies with proposed changes, wait a week or so for people who know the maintainer to show up or other people to object to your changes 3. chomp package ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Constructor question
On Sat, Jul 31, 2010 at 2:32 PM, Ivan Lazar Miljenovic ivan.miljeno...@gmail.com wrote: Forcing; it means that the values are evaluated (up to WHNF) before the Complex value is constructed: http://www.haskell.org/ghc/docs/6.12.1/html/users_guide/bang-patterns.html Actually, this isn't a bang pattern: the 'a' here is not a pattern, it's a type variable. Strictness flags in data declarations are a haskell98 feature. See: http://www.haskell.org/onlinereport/haskell2010/haskellch4.html#x10-680004.2 the paragraph on Strictness Flags a little way down that page. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Laziness question
On Sat, Jul 31, 2010 at 4:56 PM, michael rice nowg...@yahoo.com wrote: From: http://en.wikibooks.org/wiki/Haskell/Laziness Given two functions of one parameter, f and g, we say f is stricter than g if f x evaluates x to a deeper level than g x Exercises 1. Which is the stricter function? f x = length [head x] g x = length (tail x) Prelude let f x = length [head x] Prelude let g x = length (tail x) Prelude f undefined 1 Prelude g undefined *** Exception: Prelude.undefined Prelude So, g is stricter than f? Wouldn't both functions need to evaluate x to the same level, *thunk* : *thunk* to insure listhood? f x = length [head *thunk* : *thunk*] g x = length (tail *thunk* : *thunk*) Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Notice the two different kinds of brackets being used in f versus g :) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Laziness question
On Sat, Jul 31, 2010 at 5:59 PM, michael rice nowg...@yahoo.com wrote: OK, in f, *length* already knows it's argument is a list. In g, *length* doesn't know what's inside the parens, extra evaluation there. So g is already ahead before we get to what's inside the [] and (). According to the types, we already know both are lists. The question is, of course, what kind of list. But since both still have eval x to *thunk* : *thunk*, g evaluates to a deeper level? Michael I think this question is being quite sneaky. The use of head and tail is pretty much irrelevant. Try the pointfree versions: f = length . (:[]) . head g = length . tail and see if that helps you see why f is lazier than g. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Definition of List type?
On Fri, Jul 30, 2010 at 7:50 PM, Gregory Crosswhite gcr...@phys.washington.edu wrote: The reason why this definition never actually appears is because it defines the constructors using operators rather than names, which is not allowed in vanilla Haskell. (There is an extension, TypeOperators, however, that does allow this.) Nope: see Data.Complex.Complex; only infix *type* constructors are nonstandard. The thing about lists that makes them impossible to define in normal Haskell is the [a] syntax, which is some kind of outfix type constructor, which no amount of currently available extensions will allow. In addition, the constructor [] for the empty list isn't a normal constructor, syntactically, because it doesn't start with an uppercase character or a colon. Basically, lists are so ubiquitous in Haskell that they have their own special syntax, which cannot be defined like any other data type. It is simple, as others have said, to define a new data type that works identically to lists. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Definition of List type?
On Fri, Jul 30, 2010 at 8:54 PM, michael rice nowg...@yahoo.com wrote: Thanks all, Now that I have a (very) rudimentary understanding of Haskell, I figured I'd back up and have a closer (conceptual) look at type definitions to see what they have in common, and just happen to pick Maybe and List. I also noticed Maybe has a list of Instances Monad Maybe Functor Maybe Typeable1 Maybe MonadFix Maybe MonadPlus Maybe etc. while List has none, at least I don't see any in Data.List. Same reason? From Learn You A Haskell: If a type is a part of a typeclass, that means it supports and implements the behavior the typeclass describes. I'm way out on a limb here, but isn't Monad a typeclass? and if, as we say above, that Maybe is an instance of Monad, wouldn't there have to be instance Monad Maybe where return = ... -- return for Maybe = = ... -- bind for Maybe etc. somewhere? Where? It's not in Data.Maybe. Is there some kind of scheme for defining this stuff, i.e., this goes here, that goes there? It *is* in Data.Maybe: http://hackage.haskell.org/packages/archive/base/4.2.0.1/doc/html/src/Data-Maybe.html Generally speaking a type class instance should go either where the type is defined, or where the class is defined, although there are exceptions. In any case, if you can get the instance loaded in ghci, you can find out where its defined: ghci :i Maybe data Maybe a = Nothing | Just a -- Defined in Data.Maybe instance (Eq a) = Eq (Maybe a) -- Defined in Data.Maybe instance Monad Maybe -- Defined in Data.Maybe instance Functor Maybe -- Defined in Data.Maybe instance (Ord a) = Ord (Maybe a) -- Defined in Data.Maybe instance (Read a) = Read (Maybe a) -- Defined in GHC.Read instance (Show a) = Show (Maybe a) -- Defined in GHC.Show ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Memory and Threads - MVars or TVars
On Thu, Jul 29, 2010 at 3:49 AM, Eitan Goldshtrom thesource...@gmail.com wrote: Perhaps you guys could help me with Cabal now though? I'm trying to install Orc but it wants base=4.2 and =4.3 and I have 4.1 after installing the latest release of GHC. Cabal won't upgrade the base. It complains about a dependency to integer-simple. Anyone know what that's about? The latest version of GHC (6.12) comes with base 4.2. In general, base versions are tied to compiler versions, and you can't upgrade base on its own. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] MonadLib usage
On Sun, Jul 18, 2010 at 5:59 PM, Emil Melnikov emilm...@gmail.com wrote: On 2010, July 18, 23:27 Ivan Lazar Miljenovic ivan.miljeno...@gmail.com wrote: When discussing a similar issue with Manuel Chakravarty, he convinced me that cunning newtype deriving is actually rather bad in practice and shouldn't be used as there's a lack of proofs or some such (I can't remember the arguments, but I remember being convinced by them :p). Hmm... I can't imagine how it is possible, since new and original types are isomorphic. Can you give me some pointers to this discussion (links or keywords)? They are isomorphic, but distinct. This trac ticket is relevant: http://hackage.haskell.org/trac/ghc/ticket/1496 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] bug in ghci ?
On Thu, Jul 8, 2010 at 3:45 PM, Daniel Fischer daniel.is.fisc...@web.de wrote: Well, I made the suggestion of emitting a warning on instance declarations without method definitions. That would be comparatively easy to implement (even with an additional check to only emit the warning if the class defines any methods) and catch many (if not most) cases. Unfortunately, it would catch some perfectly valid cases, see the list of instances for flat datatypes here: http://hackage.haskell.org/packages/archive/deepseq/1.1.0.0/doc/html/src/Control-DeepSeq.html This demonstrates that there is at least one (admittedly probably not much more than one) case where a class with methods would have a default implementation that was total and valid in some cases. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Is my code too complicated?
On Mon, Jul 5, 2010 at 2:41 PM, Ertugrul Soeylemez e...@ertes.de wrote: Yes, there is some performance loss because of wrapping/unwrapping, but I think this loss is neglible for most applications. And I'd ask anyway. This is a discussion thread after all. =) Pretty much all monad transformers are implemented as newtypes, so the wrapping and unwrapping operations themselves should get compiled into nothing, I think. It may be that the extra type faff makes inlining or other arcane optimisations less straightforward, but I see no reason to assume that monad transformers are necessarily even slightly slower than explicitly-constructed amalgamations. In my experience, something like ReaderT Params (StateT SessionData IO) a may *look* scary, but in all your code you just use ask and put and get anyway and they all work like magic - the difficult bits are generally speaking hidden in your type synonyms and run function. But then, my largest haskell projects have never been more than a thousand or so lines, so perhaps it's just an issue of scale. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: [Haskell] ANNOUNCE: Sifflet visual programming language, release 0.1.7
On Sat, Jun 12, 2010 at 9:05 PM, Daniel Fischer daniel.is.fisc...@web.de wrote: On Saturday 12 June 2010 21:43:25, gdwe...@iue.edu wrote: so I go on declaring in my cabal file that my package depends on P = A.B.C, even though I *might* be using new features of P that wouldn't work with P == A.B.C. If you're changing the code to use new features of P, you have to raise the lower bound of the dependency. If you don't use any new features, it would continue working with P-A.B.C, so you can leave the lower bound. It's not always easy to remember or tell when you break compatibility with old versions. I came up with this idea for cabal-install to aid in development: http://hackage.haskell.org/trac/hackage/ticket/661 which I think would help, but it's not been written yet. Maybe add yourself to the CC list if you're eager to see it happen. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: GATD and pattern matching
On Fri, Jun 11, 2010 at 12:46 AM, Felipe Lessa felipe.le...@gmail.com wrote: eqTypeable :: (Typeable a, Eq a, Typeable b, Eq b) = a - b - Bool eqTypeable x y = case cast y of Just y' - x == y' Nothing - False ...or indeed: eqTypeable x y = cast x == Just y ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Using the ContT monads for early exits of IO ?
On Thu, Jun 10, 2010 at 8:57 PM, Maciej Piechotka uzytkown...@gmail.com wrote: Error monad seems not to be a semantic solution as we exit on success not failure. Which is really why the Either monad should not necessarily have Error associations :) If you forget about the fail method, the Monad (Either e) instance doesn't need the e to be an error type. Alternatively, if even Error is more information than you need, you could use MaybeT: http://hackage.haskell.org/package/MaybeT which allows you to just stop. Given you're using it with IO it'd be easy to write a result to an IORef before terminating the computation, so it's of equivalent power, if slightly less convenient. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Problems with Haskell Platform
On Wed, Jun 2, 2010 at 12:37 PM, Ivan Lazar Miljenovic ivan.miljeno...@gmail.com wrote: Is there a way of making Cabal install dependencies using the system package manager, then? For example, I might ask Cabal to install package A. Package A depends on B and C. A package for B can be downloaded through APT, but there are no APT-installable candidates for A and C. Are you saying that Cabal can download and install B using APT, then download A and C from Hackage? If you mean cabal-install, then no, there's no integration on either side. It's worth noting, though, that cabal-install doesn't track installed packages itself, GHC does. So if you install a library from APT and it writes to the package.conf correctly, cabal-install will be able to find it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] TDD in Haskell
On Tue, May 25, 2010 at 12:36 PM, Ionut G. Stan ionut.g.s...@gmail.com wrote: Oh, and a small off-topic question? Is it considered a good practice to use implicit imports in Haskell? I'm trying to learn from existing packages, but all those import all statements drive me crazy. It's pretty common but I don't like it. Whenever I have to debug someone else's code usually the first thing I'll do is make all the import lists explicit so I can work out where a troublesome function is coming from, and that's a bit of a waste of time. In some cases though it can get a little silly. Here is an import statement from one of my projects: import Language.Haskell.Exts ( Alt (Alt), Binds (BDecls), Decl (PatBind, FunBind, InfixDecl), Exp (App, Case, Con, Do, If, InfixApp, Lambda, LeftSection, Let, List, Lit, Paren, RightSection, Tuple, Var, XPcdata), GuardedAlt (GuardedAlt), GuardedAlts (UnGuardedAlt, GuardedAlts), Literal (Char, Frac, Int, String), Match (Match), Op (ConOp, VarOp), Pat (PApp, PInfixApp, PList, PLit, PParen, PTuple, PVar, PWildCard), Name (Ident, Symbol), QName (Special, UnQual), QOp (QConOp, QVarOp), Rhs (UnGuardedRhs), SpecialCon (Cons), SrcLoc (), -- (SrcLoc), Stmt (Generator, LetStmt, Qualifier), preludeFixities, prettyPrint ) ...there comes a certain point where one can probably leave the biggest import implicit, on the basis that if it's from nowhere else, it's probably there. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bug with [Double]
On Wed, May 19, 2010 at 10:57 AM, Serguey Zefirov sergu...@gmail.com wrote: PS Rationals: Prelude [1,1+2/3..10] :: [Rational] [1 % 1,5 % 3,7 % 3,3 % 1,11 % 3,13 % 3,5 % 1,17 % 3,19 % 3,7 % 1,23 % 3,25 % 3,9 % 1,29 % 3,31 % 3] Same result. This sounds like a bug to me. The section of the Haskell Report that deals with the Enum class mentions Float and Double, not Rational, and there's really no sensible reason why Rationals would exhibit this behaviour given that they don't have rounding error. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GADTs and Scrap your Boilerplate
On Sun, May 16, 2010 at 2:34 AM, Tom Hawkins tomahawk...@gmail.com wrote: I got the GADT data DataBox where DataBox :: (Show d, Eq d, Data d) = d - DataBox [snip] but I can't figure out how to implement gunfold for DataBox. The error message is Text/XML/Generic.hs:274:23: Ambiguous type variable `b' in the constraints: I had a similar difficultly in Atom making a GADT a member of Eq. At one point I had my head wrapped around the reason for the problem, but now it escapes me. However, I remember the solution: I created a function to convert the GADT into another, unGADT type, which was then used to compute (==). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Have you tried using StandaloneDeriving (and DeriveDataTypeable)? According to http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/deriving.html#stand-alone-deriving you can't derive instances for GADTs normally but a standalone derivation will at least attempt to make an instance as if it was an ordinary data type. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO (Either a Error) question
On Sun, May 9, 2010 at 7:27 AM, wren ng thornton w...@freegeek.org wrote: The only examples I can think of where we'd want 'fail'-able patterns are entirely pedagogical (and are insignificantly altered by not using 'fail'-able patterns). I can't think of any real code where it would actually help with clarity. You're not a fan of e.g. catMaybes xs = [x | Just x - xs] or the do-notation form: catMaybes xs = do Just x - xs return x then? (I actually prefer foldr (maybe id (:)) [] but that's probably just me :) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] IO (Either a Error) question
On Sat, May 8, 2010 at 3:26 AM, John Meacham j...@repetae.net wrote: What counts as unfailable? (x,y) probably, but what about data Foo = Foo x y If we don't allow it, we add 'magic' to tuples, which is a bad thing, if we do allow it, there are some odd consequences. adding another constructor to Foo will suddenly change the type of do notations involving it non locally. said constructor may not even be exported from the module defining Foo, its existence being an implementation detail. All in all, it is very hacky one way or another. Much more so than having 'fail' in Monad. This is an interesting point, but I still disagree. A data type having constructors added or changed *is* going to break code in clients using it, or at least make GHC spit out a bunch of non-exhaustive warnings. It's then a good idea, I think, that people are forced to re-examine their use sites which don't obviously handle the new failing case. Presumably if they were really really sure then just a few well-placed ~s would make the problem go away. (i.e. to answer your question, pattern matching against any single-constructor data type should be unfailable in my opinion). On Sat, May 8, 2010 at 7:16 AM, Ivan Lazar Miljenovic ivan.miljeno...@gmail.com wrote: As I said in another email: does not the x - Nothing itself call fail as it expects x to be an actual value wrapped in Just? No, the propagation of Nothings is done solely by the definition of = for Monad, and doesn't need fail at all. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Would it be evil to add deriving Typeable to newtype Q?
On Thu, May 6, 2010 at 4:05 AM, Ivan Miljenovic
ivan.miljeno...@gmail.com wrote:
Re-CC'ing -cafe:
On 6 May 2010 12:54, Leonel Fonseca leone...@gmail.com wrote:
I wasn't aware of GeneralizedNewtypeDeriving.
I just edited the source file Language.Haskell.TH.Syntax
and left:
newtype Q a = Q { unQ :: forall m. Quasi m = m a }
deriving Typeable
Hang on, is Q something actually in the template-haskell library? In
that case, you can't just do deriving (Typeable) .
{-# LANGUAGE DeriveDataTypeable, StandaloneDeriving #-}
import Data.Typeable
import Language.Haskell.TH
deriving instance Typeable1 Q
-- Sorted :)
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Re: [Haskell-cafe] {-# LANGUAGE DeriveApplicative #-} ?
On Thu, May 6, 2010 at 8:55 AM, Pavel Perikov peri...@gmail.com wrote: Hi, list!. Now in 6.12.1 we have DeriveFunctor, DeriveFoldable and DeriveTraversable. This greatly simplifies the reuse structure style of programming. Some structure (not just _data_ structure) got captured in ADT and can be reused for various purposes. Wouldn't it be nice to have the ability to derive Applicative as well? It shouldn't be more difficult than deriving Functor but will provide exciting possibilities. Just think about liftA2. The difference is that there is at most one law-abiding instance of Functor for each type, whereas there are in principle multiple possible instances for Applicative for a type. E.g. the following: instance Applicative [] where pure x = [x] fs * xs = concatMap (\f - map f xs) fs instance Applicative [] where pure = repeat (f:fs) * (x:xs) = f x : fs * xs _ * _ = [] are both law-abiding instances (although only one has a corresponding law-abiding Monad, I believe). Which should GHC choose? It's worth noting, though, that there are other derivable classes that don't have a single implementation. It's a question of trading off complexity of the compiler versus saved effort in code versus additional clarity in code, I think. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mixing map and mapM ?
On Thu, May 6, 2010 at 11:51 AM, Bill Atkins watk...@alum.rpi.edu wrote: Almost - liftM modificationTime has type Status - IO EpochTime. Like other IO functions (getLine, putStrLn), it returns an IO action but accepts a pure value (the modification time) ghci :m +Control.Monad System.Posix.Files ghci :t liftM modificationTime liftM modificationTime :: (Monad m) = m FileStatus - m System.Posix.Types.EpochTime where m = IO in this case. Also, I like this style: import Control.Applicative (($)) blah = do times - mapM (PF.modificationTime $ PF.getFileStatus) filenames ... The $ operator evaluates to fmap so it's a cleaner way to apply a pure function to an IO value. Usually I'd agree but in fact PF.getFileStatus is not an IO value, but an IO function, so you need to map over its result: mapM ((PF.modificationTime $) . PF.getFileStatus) filenames but then you lose the convenience of the $ as an infix operator, so mapM (liftM PF.modificationTime . PF.getFileStatus) filenames is probably clearer in this case. Or, if you're feeling particularly silly: mapM (fmap fmap fmap modificationTime getFileStatus) filenames ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mixing map and mapM ?
On Thu, May 6, 2010 at 12:37 PM, Bill Atkins watk...@alum.rpi.edu wrote: Just curious: why does getModificationTime take an IO FileStatus rather than a FileStatus? It doesn't. getModificationTime is a pure function (think of it like a record accessor). liftM makes it take IO FileStatus because that is what liftM is for :) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Location of library documentation with 6.12.1?
On Sat, May 1, 2010 at 1:41 PM, Roly Perera roly.per...@dynamicaspects.org wrote: I can't for example find Control.Monad.State. I guess I'm missing something obvious about how things are organised? The following places might therefore be of interest: http://hackage.haskell.org/package/mtl http://hackage.haskell.org/package/monads-tf http://hackage.haskell.org/package/monads-fd ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Location of library documentation with 6.12.1?
On Sat, May 1, 2010 at 3:33 PM, Ivan Lazar Miljenovic ivan.miljeno...@gmail.com wrote: You need to know which library they're in. You can use the ghc-pkg tool to do this: $ ghc-pkg find-module Control.Monad.State /usr/lib/ghc-6.12.1/package.conf.d monads-fd-0.0.0.1 mtl-1.1.0.2 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Functional Dependencies conflicts
On Sat, Apr 17, 2010 at 9:50 PM, Daniel Fischer
daniel.is.fisc...@web.de wrote:
{-# LANGUAGE OverlappingInstances, [...]
but with caution:
quicksilver using OverlappingInstances is the haskell equivalent of
buying a new car with high safety rating and replacing the air bags
with poison gas, pouring lubricating oil all over the brake pads,
cutting the cable to the parking brake, and gluing broken glass shards
all over the steering wheel.
:)
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Re: [Haskell-cafe] Functional Dependencies conflicts
On Sun, Apr 18, 2010 at 12:45 AM, Daniel Fischer
daniel.is.fisc...@web.de wrote:
Am Sonntag 18 April 2010 01:23:07 schrieb Ben Millwood:
On Sat, Apr 17, 2010 at 9:50 PM, Daniel Fischer
daniel.is.fisc...@web.de wrote:
{-# LANGUAGE OverlappingInstances, [...]
but with caution:
quicksilver using OverlappingInstances is the haskell equivalent of
buying a new car with high safety rating and replacing the air bags
with poison gas, pouring lubricating oil all over the brake pads,
cutting the cable to the parking brake, and gluing broken glass shards
all over the steering wheel.
:)
Wow. Makes me wonder what quicksilver says about IncoherentInstances.
Actually, I found that quote while grepping my logs for this one:
@quicksilver undecidable just turns of the termination checker,
which can be find if you're doing something clever outside of the
heuristics of the GHC termination checker.
@quicksilver overlapping actually shatters the language into tiny
inconsistent pieces
@quicksilver and incoherent files off the edges of the pieces so
they don't even fit together any more.
and now I should probably stop using the words of other people
possibly without their knowledge :P
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Re: [Haskell-cafe] ghc package problem
On Fri, Apr 16, 2010 at 2:40 PM, Phyx loneti...@gmail.com wrote: So same error. This isn't just limited to HSE though, it can't find packages like random,time etc either. Keeps reinstalling them on every cabal install. C:\Users\Phyx\AppData\Roaming\ghc\i386-mingw32-6.13.20100320\package.conf.d: QuickCheck-2.1.0.3 WinDll-0.1.9 cpphs-1.11 ghc-paths-0.1.0.6 haskell-src-exts-1.9.0 haskell98-1.0.1.1 mtl-1.1.0.2 random-1.0.0.2 syb-0.1.0.3 tar-0.3.1.0 time-1.2.0.1 So every package you are having problems with is in your user package list? Perhaps there is some disagreement in either Cabal or GHC or both as to where those packages/that package file should be? Perhaps try a cabal install --global haskell-src-exts and see if that makes a difference, or attach --user to a configure command. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] dependent types
On Sun, Apr 11, 2010 at 10:54 PM, Jason Dagit da...@codersbase.com wrote:
Or, you could use witness types:
data Vehicle classification = Vehicle { ... }
mkCar :: Vehicle Car
mkTruck :: Vehicle Truck
Then you would export the smart constructors, (mkCar/mkTruck) without
exporting the Vehicle constructor.
moveVehicle :: Vehicle c - Simulation ()
carsOnly :: Vehicle Car - ...
In the witness type version you may find that defining Vehicle as a GADT is
even better:
data Vehicle classification where
mkCar :: ... - Vehicle Car
mkTruck :: ... - Vehicle Truck
A minor point of syntax: GADT constructors should begin with a capital letter.
A more important point: GADT constructors are useful for some things
but aren't appropriate for this case because they aren't smart -
they can't take an Int parameter and then make either a Car or a Truck
depending on its value. A smart constructor will need to account for
the possibility that the parameter isn't valid:
mkCar :: Integer - Maybe Vehicle
mkCar weight
| weight carWeight = Nothing
| otherwise = Car { weight = weight }
-- or
mkVehicle :: Integer - Vehicle
mkVehicle weight
| weight carWeight = Truck weight
| otherwise = Car weight
Even with GADTs, you can't pattern match on the constructors unless
you export them, in which case you have to allow for the possibility
that someone might construct an invalid value of the type, so you
might then also need smart field labels that retrieve values from
the Vehicle without allowing you to set them as well (as you would be
able to if you exported the field label itself).
Personally I think this approach is all rather OO. The way that seems
most natural to me is:
moveVehicleAcrossBridge :: Bridge - Vehicle - Maybe Move
moveVehicleAcrossBridge bridge { maxWeight = max } vehicle { weight = w }
| w max = Nothing
| otherwise = {- ... moving stuff ... -}
so you just test the properties directly as and when they are
interesting to you.
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Re: [Haskell-cafe] Re: Haskell.org re-design
On Wed, Apr 7, 2010 at 2:22 AM, Thomas Schilling nomin...@googlemail.com wrote: I have set a maximum width on purpose so that it doesn't degrade too badly on big screens. I've never really trusted this argument - it's not required that the browser window occupy the entire screen, so why not let the user choose how wide they want their text? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Where are the haskell elders?
On Mon, Apr 5, 2010 at 3:35 PM, Kim-Ee Yeoh a.biurvo...@asuhan.com wrote: Something I've noticed is the phenomenon of Help Vampires [1] on this list. Amy Hoy: As soon as an open source project, language, or what- have-you achieves a certain notoriety—its half-life, if you will— they swarm in, seemingly draining the very life out of the community itself. Hmm. I admire her sentiments in a way - especially not blaming people for asking bad questions - but I don't like labels or stereotypes and I don't like condescension, so I would not be happy with this method being adopted in general. Especially You're a help vampire which I read as I'm going to bundle you with a bunch of other people who annoy me and make you read a patronising blog post to understand why you've just been compared to a villainous mythological creature. But I guess it's preferable to people losing patience entirely, so do whatever you have to, I suppose. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell.org re-design
On Fri, Apr 2, 2010 at 12:40 AM, Christopher Done chrisd...@googlemail.com wrote: That's true, it's a nice idea but in practice it's hard to know where to focus. I've gone with a left nav. I've built up the HTML which is cross-browser (ie6/7/8/opera/firefox/safari/chrome compat), still need to add some bits but I can tomorrow import it into a wikimedia skin. It's kind of easy to re-shuffle now that I've built it. http://82.33.137.16/haskell-website/ Feedback would be appreciated. There isn't a lot of visual separation between the nav bar and the main content. I think a border or background colour change might be nice. Also, when I let my firefox window fill the screen there's whitespace on the left and right, when I share my screen with another window the site doesn't fit horizontally - it doesn't adjust well to changing window widths. Also, in the nav bar it should be clearer when an item is linewrapping and when it is next in the list - on the left it looks like we have * The Haskell Platform * Glasgow Haskell * Compiler * ... so, bullet points or adjusted vertical spacing might help there. Also still quite grey. But I do like the focus on current events - the first impression you get visiting that page is that Haskell is alive and well, and people are using and developing it right now. The pictures of Real People smiling and huddling together really do help the friendly image we've managed to acquire (and should guard with utmost vigil, in my opinion). I think that the About and Learning sections of the original website are good section titles, and would work well on the navbar. The easiest thing to do on visiting the website is read about why Haskell is so great, and where to find out how to use it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Haskell.org re-design
On Fri, Apr 2, 2010 at 4:36 AM, Ben Millwood hask...@benmachine.co.uk wrote: The easiest thing to do on visiting the website is read about why Haskell is so great, and where to find out how to use it. Uhm, I meant the easiest thing *should be* reading about... Sorry about that. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: If wishes were horses...
On Sat, Mar 13, 2010 at 3:19 AM, wren ng thornton w...@freegeek.org wrote: The usual approach I've seen is not to distinguish strict and lazy datatypes, but rather to distinguish strict and lazy functions, e.g. by having two different arrows: (-) for lazy functions and (!-) for strict ones.[1] But what about the laziness properties of e.g. the maybe function? ghci maybe undefined id (Just ()) () ghci maybe () undefined Nothing () ghci maybe undefined id Nothing *** Exception: Prelude.undefined ghci maybe () undefined (Just ()) *** Exception: Prelude.undefined It's clear that no type signature for maybe is going to tell you about all these cases. It's similarly impossible to imagine a type signature that will tell you that take 3 is strict up to the third nested cons of the input list, and no further. In general, laziness behaviour can get complicated quickly and so I'm not convinced that the type signature is a good home for that information. I suppose a function arrow that had the same effect as putting a ! pattern on the parameter to its left might not be a bad thing (although we could argue about the exact syntax and representation, as imo !- is neither intuitively obvious nor aesthetically pleasing), but it's never going to make seq and ! patterns (which can be applied on a single equation rather than the whole function, and in nested/lambda bindings) and so forth redundant. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] cabal install darcs fails on Mac OS X Snow Leopard
On Sun, Mar 7, 2010 at 3:44 PM, George Colpitts george.colpi...@gmail.com wrote: /usr/bin/hsc2hs: line 16: unexpected EOF while looking for matching `' /usr/bin/hsc2hs: line 17: syntax error: unexpected end of file Sounds like a problem with hsc2hs itself. When I installed ghc on Snow Leopard I followed the instructions at http://www.haskell.org/haskellwiki/Mac_OS_X It says: Many packages need hsc2hs. To make it work correctly you need similar hackery: * Open /usr/bin/hsc2hs * Insert --cflag=-m32 --lflag=-m32 before $tflag My best guess is that you mistyped this step and dropped a double quote, or something. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Prelude.undefined
On Wed, Mar 3, 2010 at 6:15 AM, Ivan Miljenovic ivan.miljeno...@gmail.com wrote: On 3 March 2010 16:11, Tom Hawkins tomahawk...@gmail.com wrote: -Wall only complains about shadow bindings, defined but not used, and no type signature. But no unmatched patterns. Yes it does: one of the options it brings in is -fwarn-incomplete-patterns which tells you if you've missed a pattern match. http://www.haskell.org/ghc/docs/latest/html/users_guide/options-sanity.html I think he meant that it doesn't because he doesn't have any :) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] how to get a string from file
On Wed, Mar 3, 2010 at 6:30 PM, Thomas DuBuisson thomas.dubuis...@gmail.com wrote: On Wed, Mar 3, 2010 at 10:26 AM, Pradeep Wickramanayake prad...@talk.lk wrote: getItemFile :: IO String This says getItemFile is an action that returns a string. No arguments. getItemFile test = ... And your implementation obviously requires a file path as an argument. You wanted a type signature of: getItemFile :: FilePath - IO String or perhaps more simply (FilePath is just an alias for String): getItemFile :: String - IO String Cheers, Thomas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Or maybe you just want: getItemFile = do test - readFile input.txt return test i.e. exactly what you have but without the argument. This is actually exactly the same as getItemFile = readFile input.txt ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to understand `|` in this code snippet ?
On Sat, Feb 27, 2010 at 9:29 AM, Brandon S. Allbery KF8NH allb...@ece.cmu.edu wrote: On Feb 27, 2010, at 04:07 , zaxis wrote: xxxMain = do timeout - getEnv xxx_TIMEOUT case timeout of Just str | [(t, _)] - reads str - do addTimeout t (hPutStrLn stderr *** TIMEOUT _exit 1) return () _ - return () ... What does the `|` mean in Just str | [(t, _)] - reads str ? Is it a logical `or` ? It's a guard. Same as with function definitions (in fact, function definitions of that form are converted to case expressions). In fact it seems to be a pattern guard, which (until recently) are (were) a non-standard extension: http://www.haskell.org/ghc/docs/latest/html/users_guide/syntax-extns.html#pattern-guards ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] datakind declaration
On Tue, Feb 23, 2010 at 1:08 AM, Paul Brauner paul.brau...@loria.fr wrote: Hello, I remember seeing something like typedata T = A | B somewhere, where A and B are type constructors, but I can't find it in the ghc doc. Have I been dreaming or is it some hidden feature ? Paul ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe This is similar to what you are thinking of: http://hackage.haskell.org/trac/ghc/wiki/KindSystem ...but it's not implemented (yet). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Proper round-trip HughesPJ/Parsec for Doubles?
On Wed, Feb 24, 2010 at 1:24 PM, Christian Maeder christian.mae...@dfki.de wrote: 1. break the line after do (to avoid a layout change when change name or arguments of float' or rename the variable e) You can also break it immediately before do, which I think is sometimes more clear. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] STArray newListArray
2010/2/21 Vojtěch Knyttl kny...@gmail.com: Hello, I am trying to create STArray with newListArray like this: la x y = newListArray ((1,1),(x,y)) [(x'+y') | x' - [1..x], y' - [1..y]] – but it does not work: No instance for (MArray a Field m) Notice that newListArray has a monadic return type: newListArray :: (MArray a e m, Ix i) = (i, i) - [e] - m (a i e) with MArray a e m requiring Monad m. So newListArray returns your STArray in a monad. That pretty much has to be the ST monad, but that's not in scope, so you need to import Control.Monad.ST before the MArray (STArray s) e (ST s) instance is usable. I tried to define the type like this, but it would not work either: la :: Int - Int - STArray (Int,Int) Field The kind for STArray here is slightly off. STArray needs three type parameters, not two. This is linked to the fact that ST is not a monad, but (ST s) is - the extra parameter is essential to ST, to ensure the mutability doesn't leak out. You probably want: la :: Integer - Integer - ST s (STArray s (Integer, Integer) Field) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC RTS question
On Sun, Feb 21, 2010 at 7:10 PM, Max Bolingbroke batterseapo...@hotmail.com wrote: You might be able to get somewhere by writing a custom main function in C and linking it in. According to http://haskell.org/ghc/docs/latest/html/users_guide/options-phases.html if a lib specified with the -l option during compilation contains a main, that will be used in preference to the one from HSrts. I think the neater way of doing this would be to use the FFI, with a foreign export declaration making your haskell main available to a wrapper C file, which would then initialise the RTS with a slightly-modified argc and argv. See http://www.haskell.org/ghc/docs/latest/html/users_guide/ffi-ghc.html for details on how to do that. I also think it's strange, though, that adding RTS hooks is not optional. GHC should support some method of disabling them, in my opinion. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A small oversight
I can't answer your question (about getting minBy into the libraries) but I thought I'd point out some tricks: On Sat, Feb 20, 2010 at 10:47 AM, Andrew Coppin andrewcop...@btinternet.com wrote: Also, constructions like sortBy (compare `on` foo) must surely be very common. Common enough that Data.Ord introduces comparing: comparing :: (Ord a) = (b - a) - b - b - Ordering comparing = (compare `on`) see http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Ord.html#v%3Acomparing But it would still be useful to have sortOn et al to capture the common technique when your sorting property is potentially expensive (sortOn length, for example): sortOn f = map fst . sortBy (comparing snd) . map (\x - (x, f x)) a technique which I believe is called a Schwarzian transform. Finally, take a look at this: newtype SwapOrd x = SwapOrd (unswap_ord :: x) deriving Eq instance Ord x = Ord (SwapOrd x) where compare x y = swap_ord $ compare x y swap_ord :: Ordering - Ordering swap_ord o = case o of EQ - EQ GT - LT LT - GT Just in case you wanted to sort things in reverse order. I think having swap_ord would be nice if nothing else... swap_ord (compare x y) = compare y x, so usually flip compare fills this requirement :) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Implementing unionAll
On Thu, Feb 18, 2010 at 8:07 AM, Evan Laforge qdun...@gmail.com wrote: And BTW again, here's something I've occasionally found useful: -- | Handy to merge or sort a descending list. reverse_compare :: (Ord a) = a - a - Ordering reverse_compare a b = case compare a b of LT - GT EQ - EQ GT - LT I wondered why there wasn't one of these in the standard library until someone pointed out to me that reverse_compare = flip compare which actually takes fewer characters to type :P ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Two GET HTTP requests
On Fri, Feb 5, 2010 at 5:51 PM, Chris Eidhof ch...@eidhof.nl wrote: Approach 2: I installed the 'download-curl' package, and tried again. This seems to fail on the following example: import Network.Curl.Download main = do x - openURI http://haskell.org; y - openURI http://haskell.org/hoogle; return () If I put a print statement around the second line of the do-statement it looks like openURI never returns. I think you're supposed to use withCurlDo: http://hackage.haskell.org/packages/archive/curl/latest/doc/html/Network-Curl.html#v%3AwithCurlDo with the curl library. Could be wrong about that, haven't tried it myself. Approach 3: I used the simpleHTTP function from the HTTP package. This crashed, after I dug a little deeper into the code, it threw an error on calling the parseURI function (openFile: no such file exists). I installed the latest network package and upgraded my HTTP package, and the parseURI error went away. I felt like I was almost there, and tried the following: simpleHTTP (getRequest http://haskell.org;) This failed with just the text Bus error. I searched the HTTPBis git repository, but couldn't find the text Bus error. I don't have a clue of how to fix this. Bus error is a message generated by the operating system. On OS X, it can mean a null dereference, which is very unusual. I'm not sure how you'd debug it either - the most common cause when you're talking about C applications is programmer error, but Network.HTTP is specifically designed to be pure Haskell, and it's not easy to induce a null dereference from Haskell. I'm a bit stuck here, I would love to help fix the errors, but don't know what would be the best place to begin. If anyone can point me in the right direction, I will try to patch at least one of these packages. I don't think anyone would blame you if you didn't manage it, none of those are particularly friendly errors. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Multi-Class monadic type?
On Wed, Feb 3, 2010 at 1:10 PM, Alexander Treptow alexander.trep...@googlemail.com wrote: testFunc :: (forall a. Conf a, MonadIO m = m a) - TestType At a guess, this function takes a tuple containing a forall a. Conf a and a MonadIO m = m a, which is not what you meant. As Miguel says, more parentheses are the answer. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Allowing hyphens in identifiers
On Wed, Dec 16, 2009 at 2:55 PM, Daniel Fischer
daniel.is.fisc...@web.de wrote:
unCamel :: String - String
unCamel ('':cs) = '' : inTag cs
unCamel (a:b:c:cs)
| isLower a isUpper b isLower c = a : '_' : toLower b : c : unCamel
cs
unCamel (a:bs@(b:cs))
| isLower a isUpper b = a : '_' : b : unCamel cs
| otherwise = a : unCamel bs
unCamel cs = cs
Excuse my pedantry, but: writeToList - write_toList.
I think the third equation needs to be:
unCamel (a:b:cs@(c:_))
| isLower a isUpper b isLower c = a : '_' : toLower b : unCamel cs
so that the third character is not ignored in subsequent parses.
By the way, I like camelCase because I think that in most cases you
*don't* want to break identifiers up into their component words - you
read and understand what the function does once, and then you use it
as a word in its own right. Any resemblance to actual English is
really just a mnemonic of sorts.
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Re: [Haskell-cafe] Re: Parallel foldl doesn't work correctly
On Sat, Dec 12, 2009 at 10:08 AM, Maciej Piechotka uzytkown...@gmail.com wrote: If operation is associative it can be done using divide et impera spliting list in half and operating on it pararerlly then split in half etc. I implemented something like this as an exercise: http://benmachine.co.uk/parconcat.hs It took me a little while to get everything to par as it should and I'm still not sure I'm doing it in the most efficient way, but there it is. (If the output is nonsense, you might try changing hPutStrLn stderr into putStrLn so that it's buffered and arrives in blocks). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Status of TypeDirectedNameResolution proposal?
On Sun, Nov 22, 2009 at 7:13 PM, Ketil Malde ke...@malde.org wrote: E.g. if module Foo.Bar isn't found in Foo/Bar.hs GHC could look in Foo.hs (which would just contain a concatenation of what would currently reside in Foo.hs and Foo/Bar.hs). The obvious question arising here is what if module Foo.Bar *is* found in Foo/Bar.hs as well as in Foo.hs - is the latter ignored? It doesn't sound like an insurmountable problem but one of the nicest things about the current module system is its simplicity and predictability, both of which are somewhat attacked by this proposal. Also, it sounds like your proposal would disallow definition of multiple top-level modules in a file, because we wouldn't know where to look for them. This is not necessarily unreasonable, but it's an unexpected special case. Presumably having the modules together in a file would also mean that they could only be compiled together and would produce a single .o or .hi file. Then you might ask whether the ABI or whatever is necessarily broken by a change to *any* of the modules involved, in which case modularisation starts to become purely about name qualification. Thinking about that, it's worth noting that importing one module twice with two different names works fine. This is not to say I'm against the proposal, but it's probably not as clear-cut as it sounds. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Where is `newTVarIO` defined ?
On Tue, Nov 24, 2009 at 11:11 PM, zaxis z_a...@163.com wrote: I cannot hoogle it. It appears in Pugs: run' (-d:rest) = do info - fmap Just (io $ newTVarIO Map.empty) let ?debugInfo = info run' rest Sincerely! - fac n = foldr (*) 1 [1..n] -- View this message in context: http://old.nabble.com/Where-is-%60newTVarIO%60-defined---tp26504967p26504967.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 http://hackage.haskell.org/packages/archive/stm/latest/doc/html/Control-Concurrent-STM-TVar.html The docs for it are in GHC.Conc but you probably wouldn't import it from there. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[Haskell-cafe] cursive to foldr
It looks quite neat to use the Maybe monoid here: import Data.Monoid searchList p = foldr (\x - if p x then mappend (Just [x]) else id) Nothing but it seems that the Maybe Monoid instance keeps this strict. I fiddled with this a bit, and came up with the following: instance (Monoid m) = Monoid (Maybe m) where mempty = Nothing -- as usual mappend (Just x) y = Just $ mappend x (fromMaybe mempty y) mappend Nothing y = y which results in the expected behaviour (it's unsatisfyingly asymmetric, since it should (but can't) produce a Just if the second argument is Just without pattern-matching on the first, but there is only so much one can do without involving scary things like unamb). I'd be interested in what people thought of the relative merits of this alternative Monoid instance, but perhaps that would be a subject for a different thread. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Help Haskell driving Medical Instruments
On Tue, Nov 10, 2009 at 5:04 AM, Philippos Apolinarius phi50...@yahoo.ca wrote: foreign import ccall rs232.h opencport opencport :: CInt - IO () foreign import ccall rs232.h closecport closecport :: CInt - CInt [...] Originally, I had the following line (that did not work properly): foreign import ccall rs232.h closecport closecport :: IO () I don't know why the latter line didn't work properly, but I'm pretty sure it's closer to the right answer than the former. If you don't have an IO type for your function, then Haskell is allowed to assume it is pure (has no side effects) and can then call it only when the result is needed, or multiple times if it likes, without affecting the meaning of the program. For a function that closes a handle this is clearly not the case. So I'm pretty sure your type signature needs to be in IO if you want to guarantee it is called at the right time; it might be worth elaborating on how the IO () version did not work, and how you used it. The way you are using it now would appear to work most of the time because the print statement will force the result to be evaluated, forcing the function to be called - but having a handle closed based on when an apparently irrelevant print statement runs or doesn't is obviously not ideal. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Medical Instruments - Jason
On Wed, Nov 11, 2009 at 6:00 PM, Philippos Apolinarius phi50...@yahoo.ca wrote: closecport :: Int - IO Int closecport n= return (fromIntegral (c_closecport (fromIntegral n))) The return here doesn't do what you think it does - semantically, the value of c_closecport is still considered pure and assumed to be referentially transparent, so multiple calls to closecport are allowed to share the value returned, or delay the call until the value is unwrapped, call it multiple times for each use of the value, or anything else. You need to use IO *directly* in the foreign import declaration so that the compiler knows that the function calls can't be shared or inlined or generally messed about with: the IO tells it that order of execution with respect to your other IO actions is important. This one looks the most right: foreign import stdcall unsafe rs232.h closecport closecport :: IO () so I think you need to look closer about why it wasn't working for you, and where or how you were using it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Interactive chatbot
Oops, I clicked reply instead of reply to all. Duplicating the message below. I suppose this means someone is going to get two copies of this. Sorry someone! On Thu, Nov 5, 2009 at 12:56 AM, Ben Millwood hask...@benmachine.co.uk wrote: On Wed, Nov 4, 2009 at 10:21 PM, Torsten Otto t-otto-n...@gmx.de wrote: When we read the user's input through t - getLine it is not possible to delete typos before hitting enter and thereby sending the input off to the system (at least in OS X, bash). I didn't find that terribly problematic, but of course it is a bit of a show stopper from their point of view. As people have said it's worth checking what buffering settings you are using (especially note that ghci changes some interesting settings in relation to how input is handled, and compiled code may behave differently), but it might also be worth checking the terminal application's preferences to see if there are settings related to the interpretation of the backspace key that you need to twiddle one way or the other. In particular, if you are finding that pressing delete makes ^H appear on the input line instead of deleting things, or if pressing ctrl-H deletes stuff where the delete key fails to do so, it might be a problem with your terminal rather than with your program. This is only based on what I vaguely remember from faffing with the Mac Terminal application some time ago when it wouldn't co-operate with screen, but it may be worth a look. yours, Ben Millwood ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
