- Haskell-Cafe - Haskell.org

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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New System.Process
by john lask 07 Oct '08

07 Oct '08

Something that has irked me in the past about System.Process is the inability to obtain an OS system handle from the haskell Process handle. Such a facility would greatly enhance the interoperabity of c and haskell libraries. Provision is made (although not standardised) to obtain OS specific handles from haskell File Handles which is very useful. The same should be provided for Haskell Process handles. The main barrier to such a facility is the variability between OS representations of such a type (aka windows Handles, unix PID). comments anyone. _________________________________________________________________

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announcing darcs 2.1.0pre3
by Eric Kow 07 Oct '08

07 Oct '08

Hi everybody, The third pre-release of darcs 2.1 is now available at http://darcs.net/darcs-2.1.0pre3.tar.gz Since the last pre-release, we have resolved two of the items on our work list and a recently discovered darcs 2 bug. On the other hand, we have discovered a new regression from darcs 2.0.2 and some performance issues in our recent work for issue971. We are confident that we can resolve these within the upcoming week, but since regressions are involved, I think it might be prudent to delay the release so that we at least have one week to test these changes. At the very latest, darcs 2.1 should be released on 17 October, but I hope we will be able to get it out soon after the tenth. We are now very close to the finish line. More news in a week! Upcoming release schedule ------------------------- 2008-10-10: darcs 2.1.0pre4 2008-10-17: darcs 2.1.0 Work remaining -------------- * Backing out http://bugs.darcs.net/issue971 (Petr Ročkai) We have observed a performance regression in the newer, more robust version of the darcs repair code and will be postponing this feature for next release. The work for this week consists of undoing the relevant changes and auditing the undo. * http://bugs.darcs.net/issue1095 Darcs sometimes crashes in darcs send. It is clear to us why the crash happens and how to fix it. The work for this week consists of creating a minimal test case for our regression testing suite and implementing the relevant changes. * http://bugs.darcs.net/issue1026 Improve the error messages for 'bug in get_extra' errors, which can happen if darcs is fooled into thinking that two patches are the same. Changes since the last pre-release ---------------------------------- * Bug Fix: Eliminate an error merging repeated conflicts in darcs-2 repositories (issue1043, David Roundy) * New Feature: Hide 'Ignore-this:' lines which will be generated by future versions of darcs to prevent patch-id collisions. (issue1102, Eric Kow, David Roundy) * Bug Fix: Support darcs repositories which have symbolic links in their paths (issue1078, Dmitry Kurochkin) * Bug Fix: Make ssh connection sharing (darcs transfer-mode) work with old-fashioned repositories (issue1003, David Roundy) -- Eric Kow <http://www.nltg.brighton.ac.uk/home/Eric.Kow> PGP Key ID: 08AC04F9

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I'll do USB in Haskell - tips?
by Mauricio 07 Oct '08

07 Oct '08

Hi, I'll need to use a lot of USB at work and, after looking around, it seems there's no general USB code done. 'libusb' and 'openusb' do not seem ready enough so that wrapping them would be easier than writing from scratch. (If you think I am wrong until here and have time to write me your opinion, I'll value it a lot.) So: I would like to (try to) write a good implementation of USB access in Haskell. I would like it to be really general, so that we could use all of USB without need to resort to something else; available to all possible environments and operating systems; allow easy testing of USB clients, maybe using "fake" USB devices that could simulate problems; do that using Haskell code to directly access operating system API. I've read the appropriate chapters of USB 2.0 standard reference. I'll greatly appreciate any information you can give me: sugestions on what would be "the Haskell way" of doing USB; good technical information on how USB is implemented in different OSes; warnings on problems I may have; wishes of good luck :) I hope in one year I'll be able to post a message here saying Haskell is the greatest language to interface with USB devices. Thanks, Maurício

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Haskell participating in big science like CERN Hadrian...
by Galchin, Vasili 07 Oct '08

07 Oct '08

Hello, One of my interests based on my education is "grand challenge science". Ok .. let's take the CERN Hadrian Accelerator. Where do you think Haskell can fit into the CERN Hadrian effort currently? Where do you think think Haskell currently is lacking and will have to be improved in order to participate in CERN Hadrian? Kind regards, Vasili

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Re: [Haskell-cafe] newbie questions (read, etc., with Data.ByteString.Lazy.Char8)
by wman 07 Oct '08

07 Oct '08

as usual, i forgot to use the magical "reply to all" and under the impression I'm still talking to the list had bothered dons personally (heresy/sacrilege/deathwish , i know) to rectify it a bit at least, i'm posting a summary, in hopes someone find it useful. ---------------------------- -- this doesn't work - space leak no matter which optimizations you use, with -O2 it crashes in 5 secs instead in 75 ;-) import qualified Data.ByteString.Lazy.Char8 as S main = print . go 0 =<< S.getContents where go n s = case S.readInt s of Nothing -> n Just (k,t) -> go (n+k) (S.tail t) -- slightly stricter version, notice those ! before parameters in go declaration and the fancy -XBangPatterns option to ghc which it requires ;-) -- let dons_bstr.hs = import qualified Data.ByteString.Lazy.Char8 as S main = print . go 0 =<< S.getContents where go !n !s = case S.readInt s of Nothing -> n Just (k,t) -> go (n+k) (S.tail t) ghc -XBangPatterns --make dons_bstr.hs time dons_bstr < nums real 1m8.686s user 0m0.015s sys 0m0.031s ghc -XBangPatterns -Onot -fstrictness --make dons_bstr.hs time dons_bstr.exe < nums real 1m10.607s user 0m0.031s sys 0m0.015s ghc -XBangPatterns -O2 --make dons_bstr.hs time dons_bstr.exe < nums real 0m1.500s user 0m0.015s sys 0m0.031s so althought the -fstrictness still seems to have no effect in this case, using -O2 the dons version is ~2 times faster then the bytestring/readInt version -O2

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