April 2009 - Haskell-Cafe

Data.ByteString woes
by David Carter 16 Apr '09

16 Apr '09

Hi, I am battling with Data.ByteString.Lazy.Char8, and currently losing. Any help gratefully received. Here's my code: import qualified Data.ByteString.Lazy.Char8 as B import Text.Regex.Posix ((=~)) test = B.pack "abc" =~ B.pack "b" :: Bool This works fine without the ".Lazy", and is also fine under GHC 6.8.1, but with GHC 6.10.1, it fails with: No instances for (Text.Regex.Base.RegexLike.RegexLike Text.Regex.Posix.Wrap.Regex B.ByteString, Text.Regex.Base.RegexLike.RegexMaker Text.Regex.Posix.Wrap.Regex Text.Regex.Posix.Wrap.CompOption Text.Regex.Posix.Wrap.ExecOption B.ByteString) I then thought I might work around the problem by converting lazy ByteStrings to strict ones in order to do the regex match. I discovered toChunks, which is advertised in the documentation as type ByteString -> [ByteString], but it actually appears to be of type toChunks :: ByteString -> [Data.ByteString.Internal.ByteString] and I can't see any way of converting a Data.ByteString.Internal.ByteString to a (strict) ByteString. Does anyone know whether this is (a) bug(s) in GHC, Data.ByteString, Text.Regex.Posix or (maybe most likely) my own limited understanding? And any ideas for fixes or workarounds (other than the obvious one of downgrading to GHC 6.8)? Thanks in advance David

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how to upgrade?
by Johannes Waldmann 16 Apr '09

16 Apr '09

Is there a nice way of upgrading ghc: I mean does cabal-upgrade know to install exactly the packages that I had with the previous ghc version? - J.W.

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Issues with running Ghci from emacs
by Daryoush Mehrtash 16 Apr '09

16 Apr '09

I am having problem running GHCI with Haskell files that include imports. I am running emacs22 on Ubuntu, with haskell-mode-2.4 extensions. I load my file (Fal.lhs in this case) in emacs. Then try to run the Ghci by doing C-c C-l. The result is shown below. Ghci fails to find the Draw.lhs which is also on the same directory as the Fal.lhs. Note: If I go to the directory where Fal.lhs is and run Ghci directly it all works fine. Any idea how I can get the interepreter to work form emacs? GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. *Prelude> :cd ~/.cabal/ Prelude> :load "../Documents/haskell/SOE/src/Fal.lhs" * ../Documents/haskell/SOE/src/Fal.lhs:76:9: Could not find module `Draw': Use -v to see a list of the files searched for. Failed, modules loaded: none. Prelude> Thanks, Daryoush

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Cabal and WinHugs
by Iavor Diatchki 16 Apr '09

16 Apr '09

Hello, What is the preferred way to install a cabal package so that it works with winhugs? When I tried "cabal install --user --hugs" I got an error that it could not find "hugsffi". I managed to get things working by manually downloading the package, and extracting the appropriate source directories to the "packages" folder of winhugs (i.e., basically skipping cabal) but this is kind of clunky. So to summarize, here are my two questions: 1. Am I doing something wrong, or does "cabal" not support WinHugs?, and 2. Does WinHugs have a user specific "packages" folder (so that each user can have their own set of packages)? Thanks for any info that anyone might have, Iavor

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Re: [Haskell-cafe] Enum to String, and back?
by michael rice 15 Apr '09

15 Apr '09

Thanks guys. It works like a charm. Michael --- On Wed, 4/15/09, Rahul Kapoor <rk(a)trie.org> wrote: From: Rahul Kapoor <rk(a)trie.org> Subject: Re: [Haskell-cafe] Enum to String, and back? To: Cc: haskell-cafe(a)haskell.org Date: Wednesday, April 15, 2009, 3:20 PM On Wed, Apr 15, 2009 at 3:13 PM, michael rice <nowgate(a)yahoo.com> wrote: > Using Show it is possible to establish a relationship between an enum type > and a String type to display it. > Can one as easily establish a reverse relationship, i.e., convert a String > type like "Red" back to its corresponding Color type? > Make it an instance of the Read type class. http://haskell.org/ghc/docs/latest/html/libraries/base/Prelude.html#t%3ARead That will allow you to write - read "Red" :: Color => Red HTH Rahul _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Brackets and Asynchronous Exceptions
by Andrew Gallagher 15 Apr '09

15 Apr '09

Hi, In a program I am writing, I have many locations where I acquire a resource, perform an operation with it, then release it. I have been using the 'bracket' function so that the "release" operation would be performed even if the operation threw an exception. This seems to work nicely. In the event of an asynchronous exception, however, is there a possible scenario where a release is not performed after an acquire? Looking at the example given in bracket documentation: bracket (openFile "filename" ReadMode) (hClose) (\fileHandle -> do { ... }) Is it possbile that an asynchronous exception could be raised in this thread after openFile executes but *before* the appropriate handlers are installed and the operation is run, preventing hClose from executing? If 'bracket' does not handle this case, should I be using the block/unblock functions to disable asynchronous exceptions: block (bracket (openFile "filename" ReadMode) (hClose) (\fileHandle -> do unblock ({ ... }))) Thanks, Andrew

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Mondrian and Haskell
by Vasili I. Galchin 15 Apr '09

15 Apr '09

Hello, Several days ago I posted a question about retargeting GHC to generate CIL so that Haskell could be a .NET language. One objection was Haskell "lazy" nature didn't fit well with .NET's CLR. I found a couple publications .... http://www.ddj.com/windows/184404967;jsessionid=0AVCEATVAEGKMQSNDLOSKH0CJUN… ... more popular presentation http://docs.msdnaa.net/ark_new/Webfiles/WhitePapers/ECOOP.pdf ... this paper seems to be saying that Mondrian's "lazy" nature fits in quite well Any opinions from Mondrian people "listening" in? What lessons were learned from the Mondrian experience that could speak to Haskell in particular GHC? Regards, Vasili

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Enum to String, and back?
by michael rice 15 Apr '09

15 Apr '09

Hi, Using Show it is possible to establish a relationship between an enum type data Color = Red | Blue | Green | Yellow | Orange | Brown | White | Black deriving (Show, Eq, Enum, Bounded) and a String type to display it. *Main> Red Red *Main> [Red,Green,Blue] [Red,Green,Blue] *Main> which is Red :: Color -> "Red" :: [Char] Can one as easily establish a reverse relationship, i.e., convert a String type like "Red" back to its corresponding Color type? So that "Red" :: [Char] -> Red :: Color Michael

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Maybe off-topic -- Writing contracts or software specifications
by Maurício 15 Apr '09

15 Apr '09

Hi, I'm an engineer, and as a programmer I'm just an amateur. This easied things to me, since I could take decisions about practices based on what made sense to me. But now I need to take responsability for some formal programming tasks, and I don't know which examples to follow. I need, for instance, to write a contract with a programmer we are hiring for a task. But the only example I have of such contracts seemed to me (as I said, an amateur. I may be completely wrong) impractical. It was a 150 pages document with every possible user action and every imaginable allowed consequences. But it would be easier to me write the software than such contract itself. Are there ways already accepted by practice on how to write software contracts? It's a small program to acquire and show data from a device, a one person, two months work. I like the guy we are going to hire, and much to my surprise he sugested using Haskell for the task. But he also do not have experience writing such documents. Thanks, Maurício

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Fast mutable arrays of ByteString?
by Maxime Henrion 15 Apr '09

15 Apr '09

Hello all, I have been rewriting a small utility program of mine from C to Haskell for fun. This tool reads lines from stdin or from files, shuffles them one or more times using the Fisher-Yates algorithm, and outputs the result to stdout. Since this algorithm is based on in-place updates, I've been storing my strings in a mutable array in the ST monad. Since it's holding strings I could not use an unboxed array. The resulting program works fine and seems to run at a decent speed, even though it is much slower than the original C version, slightly more so than I expected. While trying to optimize it using profiling, and playing with the number of shuffling passes, I noticed that this operation was responsible for a significant amount of the runtime, much more so than with the C version. I also noticed that the %GC time was around 56%. In order to do more tests, I wrote another version of this program which keeps the strings in a pure and immutable array, and stores the indices of this array in an unboxed mutable ST array. The shuffling is then done on this indices array instead of the strings array. This version runs much faster and only spends ~21% of its time in the garbage collector, at the cost of consuming more memory for the indices array. I'm attaching both versions of the code to this e-mail, and I'd be curious to hear about any possible improvements to it, and whether the performance of STArray of ByteString I'm observing corresponds to people's expectations. Thanks in advance, Maxime Henrion

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