October 2008 - Haskell-Cafe

ANNOUNCE: Data.IVar 0.1
by Luke Palmer 08 Oct '08

08 Oct '08

Several very elegant FRP approaches are emerging, most visibly FRP.Reactive, which rely on blocking on multiple variables at once, continuing when the *first* of them is available. . . inside an unsafePerformIO, so the beautiful STM "orElse" solution is not available. The current solution is to race threads against each other, and have the one that finishes first kill the other one. This is implemented, for example, in Data.Unamb. However, our empirical tests have shown that the GHC scheduler is not *quite* good enough to handle this efficiently, and ends up introducing too much latency and nondeterminacy. The Data.IVar<http://hackage.haskell.org/cgi-bin/hackage-scripts/package/data-ivar-0.1>module, just uploaded to hackage, provides an alternative to thread racing as a solution to this problem. It provides *write-once* variables which can be blocked on in parallel, without forking any threads or using STM (so it is safe to use in unsafePerformIO). Example usage from the documentation: import qualified Data.IVar as IVar import Control.Concurrent main = do iv <- IVar.new iv' <- IVar.new forkIO $ threadDelay 10000000 >> writeIVar iv' "my spoon is too big" let merger = IVar.read iv `mplus` IVar.read iv' print =<< IVar.nonblocking merger -- most likely "Nothing" print =<< IVar.blocking merger -- waits a while, then prints writeIVar iv' "i am a banana" -- throws error "IVar written twice" Enjoy! Luke

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RE: I'll do USB in Haskell - tips?
by Galchin, Vasili 08 Oct '08

08 Oct '08

Hello, I am also interested in SCSI, PCI, etc. I think the main problem/issue vis-a-vis USB/SCSI/PCI et. al. for Haskell is does (or will there exist) a Posix API and a Windows API for these buses/protocols? I read one URL but it will only support Linux. Ok .. maybe there API will be implemented for other Posix OS's like solaris, AIX, HP-UX, etc. but I wouldn't "bet the farm on it". I am not trying to be pesimistic but just save Marco some time in his life. Maybe Marco should talk to the people doing the Linux implementation for the "Posix" API and see how viable that API is before spending a year for nothing. Or Marco maybe you can be a trailblazer and come up with a better "Posix/Win32 USB API" and implement in Haskell? ;^) Very kind regards, Vasili

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Call for Contributions - Haskell Communities and Activities Report, November 2008 edition
by Janis Voigtlaender 08 Oct '08

08 Oct '08

Dear Haskellers, so much has happened in the Haskell world in the past months. Therefore, I would very much like to collect contributions for the 15th edition of the ================================================================ Haskell Communities & Activities Report http://www.haskell.org/communities/ Submission deadline: 31 October 2008 (please send your contributions to hcar(a)haskell.org, in plain text or LaTeX format) ================================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough -- please reconsider and submit an entry anyway! * If you are interested in any project related to Haskell that has not previously been mentioned in the HC&A Report, please tell me, so that I can contact the project leaders and ask them to submit an entry. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the May 2008 edition -- you will find interesting topics described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Janis (current editor) FAQ: Q: What format should I write in? A: The best format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/11-2008/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2008/hcar.sty that you can use to preview your entry. If you do not know LaTeX, then use plain text. If you modify an old entry that you have written for an earlier edition of the report, you should have received your old entry as a template already (provided I have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Do not worry about writing correct LaTeX, I will be able to handle your file. Please do not use HTML or even DOC. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg format, then. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. This corresponds to approximately one page, or 40 lines of text, with the above style and template. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or ``historic'' overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep entries up. There is no minimum length of an entry! The report aims at being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports over distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also ask the editor. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use it to write ``wishlist'' items for libraries and language features you would like to see implemented. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell the editor that there are no changes. The old entry will be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. -- Dr. Janis Voigtlaender http://wwwtcs.inf.tu-dresden.de/~voigt/ mailto:voigt@tcs.inf.tu-dresden.de

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Type classes question
by Roly Perera 08 Oct '08

08 Oct '08

Hi, I'm reasonably well versed in Haskell but fairly new to defining type classes. In particular I don't really understand how to arrange for all instances of X to also be instances of Y. It's quite possibly that my question is ill-posed, so I'll make it as concrete as possible: in the following code, I define a Stream class, with two instances, Stream1 and Stream2. How do I arrange for there to be one implementation of Functor's fmap for all Stream instances? I currently rely on delegation, but in the general case this isn't nice. I guess I'm either misunderstanding what it is I'm trying to achieve, or how to do this kind of thing in Haskell. Any help would be greatly appreciated. many thanks, Roly Perera {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, ExistentialQuantification, FunctionalDependencies #-} module Test where ------------------------------------------------------------------------------- -- Just some helpers. ------------------------------------------------------------------------------- -- Product map. prod :: (a -> b) -> (c -> d) -> (a, c) -> (b, d) f `prod` g = \(a, c) -> (f a, g c) -- Diagonal. diag :: a -> (a, a) diag x = (x, x) -- Mediating morphism into the product. both :: (a -> b) -> (a -> c) -> a -> (b, c) both f g = prod f g . diag ------------------------------------------------------------------------------- -- "Abstract" stream. ------------------------------------------------------------------------------- class Stream s a | s -> a where first :: s -> a next :: s -> s fby :: a -> s -> s -- I want every Stream to be a Functor. fmap_ :: Stream s' b => (a -> b) -> s -> s' fmap_ f = uncurry fby . both (f . first) (fmap_ f . next) ------------------------------------------------------------------------------- -- Implementation 1. ------------------------------------------------------------------------------- data Stream1 a = a :< Stream1 a instance Functor Stream1 where fmap = fmap_ instance Stream (Stream1 a) a where first (x :< _) = x next (_ :< xs) = xs fby = (:<) ------------------------------------------------------------------------------- -- Implementation 2. ------------------------------------------------------------------------------- data Stream2 a = forall b . S b (b -> a) (b -> b) instance Functor Stream2 where fmap = fmap_ instance Stream (Stream2 a) a where first (S x c _) = c x next (S x c i) = S (i x) c i fby y s = S (y, s) fst (uncurry (,) . both first next . snd)

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Darcs / Git
by Dominic Steinitz 08 Oct '08

08 Oct '08

Not really a Haskell question but I'm not sure where else to go. What's the preferred method of converting a darcs repository to git? And is there a way of converting from git to darcs? The reason I ask is that my colleague cannot get darcs to work on his Windows box. Thanks, Dominic.

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Linux binary dist problems
by Colin Fleming 07 Oct '08

07 Oct '08

Hi all, I just joined this list, I'm interested in learning Haskell to complement my day job language (Java). I was interested in trying to make a simple website using Haskell in order to have something to practise on, however in my host's shared environment I can't use the Linux binary packages - I get the same errors that a few people were talking about in September - firstly the "cannot determine current directory", and then after hacking the configure script the "[install] Error 136". It seems that this is probably an old libc, but this is an environment that I have almost no control over so I can't fix that. Are there any building from source options if I don't already have GHC? It looks from the porting guide that I might be able to make 6.6.2 with just a C compiler, can I then use that to build 6.8.3? Thanks for any help. As a few people have commented, it would be great to make all this easier, as it's a pretty big stumbling block to starting with Haskell, and people are likely to get discouraged pretty quickly. Cheers, Colin

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newbie questions (read, etc., with Data.ByteString.Lazy.Char8)
by Mike Coleman 07 Oct '08

07 Oct '08

Hi, I could use a little help. I was looking through the Real World Haskell book and came across a trivial program for summing numbers in a file. They mentioned that that implementation was very slow, as it's based on String's, so I thought I'd try my hand at converting it to use lazy ByteString's. I've made some progress, but now I'm a little stuck because that module doesn't seem to have a 'read' method. There's a readInt method, which I guess I could use, but it returns a Maybe, and I don't see how I can easily strip that off. So: 1. Is there an easy way to strip off the Maybe that would allow an equivalently concise definition for sumFile? I can probably figure out how to do it with pattern matching and a separate function--I'm just wondering if there's a more concise way. 2. Why doesn't ByteString implement 'read'? Is it just that this function (like 'input' in Python) isn't really very useful for real programs? 3. Why doesn't ByteString implement 'readDouble', etc.? That is, why are Int and Integer treated specially? Do I not need readDouble? Thanks, Mike -- lazy version (doesn't compile) -- file: ch08/SumFile.hs import qualified Data.ByteString.Lazy.Char8 as L main = do contents <- L.getContents print (sumFile contents) where sumFile = sum . map read . L.words

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Network.URI bug? (2.2.0.0)
by Bárður Árantsson 07 Oct '08

07 Oct '08

Hi all, I'm having a bit of difficulty with Network.URI -- unless I misunderstand the documentation, the attached demo program is supposed to produce: http://localhost:9000 http://localhost:9000/d1 http://localhost:9000/d1/d2 but it produces http://localhost:9000 http://localhost:9000/d1 http://localhost:9000/d2 Is this some sort of bug or am I not understanding the documentation correctly? Cheers, -- Bardur Arantsson <bardurREMOVE(a)THISscientician.net> The thing that's depressing about tennis is that no matter how good I get, I'll never be as good as a wall. Mitch Hedberg

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Random question
by Iain Barnett 07 Oct '08

07 Oct '08

Hi, I have a function, that produces a random number between two given numbers rand :: Int -> Int -> IO Int rand low high = getStdRandom (randomR (low,high)) (Naively) I'd like to write something like take (rand 1 10 ) [1..10] and see [1,2,3,4] ... or anything but nasty type-error messages. I'm reading about 6 tutorials on monads simultaneously but still can't crack this simple task, and won't pain you with all the permutations of code I've already tried. It's a lot, and it ain't pretty. Would anyone be able to break away from C/C++ vs Haskell to help? Just a point in the right direction or a good doc to read, anything that helps will be much appreciated. Regards Iain

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Inferred type is less polymorphic than expected, depends on order
by Jason Dagit 07 Oct '08

07 Oct '08

Originally I sent this to glasgow-haskell where I was hoping someone by the name of Simon would comment on the error message. No one commented at all, so I'm resending to haskell-cafe. Thanks! I was wondering if someone could help me understand why reordering the case statements changes the type inference for this code. 1) I find the error message a bit confusing. 2) I don't understand why it's a problem in one order and not the other. I've tried to send this as literate haskell in hopes that you can just copy and paste to a file and run the example. This happens with or without GADTs, this version doesn't have them but they don't turn out to make any difference. \begin{code} {-# LANGUAGE ExistentialQuantification, RankNTypes #-} module Main where data Sealed a = forall x. Sealed (a x) -- Or equivalently: -- data Sealed a where -- Sealed :: a x -> Sealed a \end{code} Originally, I noticed this in a monad context...The original was much more complicated. But, we can simplify it even more, so keep reading. goodOrder :: Monad m => (forall y z. p x y -> q y z -> q x z) -> m (Sealed (p x)) -> (forall b. m (Sealed (q b))) -> m (Sealed (q x)) goodOrder f mx my = do Sealed x <- mx Sealed y <- my return (Sealed (f x y)) badOrder :: Monad m => (forall y z. p x y -> q y z -> q x z) -> m (Sealed (p x)) -> (forall b. m (Sealed (q b))) -> m (Sealed (q x)) badOrder f mx my = do Sealed y <- my Sealed x <- mx return (Sealed (f x y)) Several helpful people in #haskell helped me converge on this greatly simplified version below. \begin{code} f :: p x y -> q y z -> q x z f = undefined \end{code} \begin{code} badOrder :: (Sealed (p x)) -> (forall b. (Sealed (q b))) -> (Sealed (q x)) badOrder sx sy = case sy of Sealed y -> case sx of Sealed x -> Sealed (f x y) \end{code} \begin{code} goodOrder :: (Sealed (p x)) -> (forall b. (Sealed (q b))) -> (Sealed (q x)) goodOrder sx sy = case sx of Sealed x -> case sy of Sealed y -> Sealed (f x y) \end{code} \begin{code} main = return () \end{code} This gives the error: $ ghc --make Reorder.lhs [1 of 1] Compiling Main ( Reorder.lhs, Reorder.o ) Reorder.lhs:52:29: Inferred type is less polymorphic than expected Quantified type variable `x' is mentioned in the environment: y :: q x x1 (bound at Reorder.lhs:51:24) When checking an existential match that binds x :: p x2 x The pattern(s) have type(s): Sealed (p x2) The body has type: Sealed (q x2) In a case alternative: Sealed x -> Sealed (f x y) In the expression: case sx of Sealed x -> Sealed (f x y) After discussing this a bit, I think what may be happening in the badOrder case is that the existentially bound type of x is bound after the type `b' in the type of y, leading to the error message. I would appreciate help understanding this, even if the help is, "Go read paper X, Y, and Z." Thanks! Jason

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