March 2009 - Haskell-Cafe

Hidden module?
by Cristiano Paris 02 Mar '09

02 Mar '09

Cabalising hsc2hc I get (this is actually from manually building the package): --- [paris@bagend hsc2hs-0.67.20061107]$ runghc Setup.hs build Preprocessing executables for hsc2hs-0.67.20061107... Building hsc2hs-0.67.20061107... Main.hs:32:7: Could not find module `System.Process': it is a member of package process-1.0.1.0, which is hidden [paris@bagend hsc2hs-0.67.20061107]$ which I cannot understand. Please help! --- Cristiano

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Cabal and `main-is` field
by Manlio Perillo 02 Mar '09

02 Mar '09

Hi. I have started to use Cabal for a small project where I have to build several executables. The project layout is something like this: / bin/ a.hs b.hs src/ X.hs Y.hs The Cabal configuration file is something like: executable a build-depends: haskell98, base, ... main-is: a.hs hs-source-dirs: src bin other-modules: X This is perfectly reasonable, but it is odd, IMHO. I don't understand why `main-is` is relative to `hs-source-dirs`. What is the rationale? After all, there is only one executable that can be specified in the `executable` block. The configuration file should be, instead (IMHO, of course): executable a build-depends: haskell98, base, ... main-is: bin/a.hs hs-source-dirs: src other-modules: X Another "problem" I have noted, is that if I specify `main-is` field two times, no error is reported. Simply, Cabal uses the last directive. One last thing: is it possible to define "macros" in Cabal? In my package, I have n executables, that have many dependencies in common. I would like to avoid having to repeat the same dependencies for each executable block. Thanks Manlio Perillo

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ANNOUNCE: text and text-icu, fast and comprehensive Unicode support using stream fusion
by Bryan O'Sullivan 02 Mar '09

02 Mar '09

On behalf of the Data.Text team, I am delighted to announce the release of preview versions of two new packages: text 0.1 Fast, packed Unicode text support, using a modern stream fusion framework. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/text text-icu 0.1 Augments text with comprehensive character set conversion support and normalization (and soon more), via bindings to the ICU library. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/text-icu These packages fill out critical pieces of functionality for the Haskell platform, without compromising on either performance or safety. We are referring to these as *preview* releases because although the text package in particular has been quite heavily tested, it has not been thoroughly tuned, and we have not yet implemented a chunked lazy text representation suitable for streaming gigabytes of data. The APIs are pretty conventional, but are still subject to change. If you want to contribute, please get copies of the source trees from here: darcs get http://code.haskell.org/text darcs get http://darcs.serpentine.com/text-icu Please send bug reports and patches to your friendly Data.Text team: Tom Harper <rtharper(a)aftereternity.co.uk> Duncan Coutts <duncan.coutts(a)worc.ox.ac.uk> Bryan O'Sullivan <bos(a)serpentine.com>

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Left fold enumerator - a real pearl overlooked?
by Gü?nther Schmidt 02 Mar '09

02 Mar '09

Hi all, in the last few months I was looking for haskell database library, eventually settling for HDBC (thanks John btw). Takusen also caught my eye although I even failed installing it. Nevertheless a particular property of takusen, left-fold-enumerator, also sparked my interest and I tried to follow it up. I had the impression this is something of relevance but something that is not mentioned or made use of often. There are actually only a few references on the net and most of those by one person only, Oleg. There was one post from John Goerzen about Haskell HTTP libraries in which he hinted using left-fold enumerators. Anyway what I'm saying is that the whole topic is somewhat off the radar of the larger haskell community, how come, or am I merely overestimating its relevance and usefulness? Günther

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Basic problem in Haskell language design?
by Nicu Ionita 02 Mar '09

02 Mar '09

Hi, Today I found the following problem when writing a simple function: > -- Whole info from a word8 list to moves > movesFromWord8s :: [Word8] -> [Move] > movesFromWord8s (f:t:ws) = (f, t) : movesFromWord8s ws > moverFromWord8s _ = [] Here I made a small typo in the second equation, writing "mover..." instead of "moves..." for the name of the declared function. Of course this is an error, because I have non-exhaustive patterns in the function movesFromWord8s. But the compiler (here GHC 6.8.2 on WinXP) has in principle no chance to detect this mistake, I saw it only in QuickCheck (aka at run-time). I think this is a basic problem with the language design. In small programs it's not so bad, but in large ones this could be. What do you think about it? Are there possible solutions or workarounds? Nicu Ionita

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ANNOUNCE: tar 0.3.0.0
by Duncan Coutts 02 Mar '09

02 Mar '09

All, I'm pleased to announce a major new release of the tar package for handling ".tar" archive files. http://hackage.haskell.org/cgi-bin/hackage-scripts/package/tar This release has a completely new and much improved API. See the hackage page for the API documentation. There are high level "all in one" functions for extracting or creating .tar files. More interestingly, it is easy to make variants by composing a pipeline. For example extracting a ".tar.gz" file is just: > Tar.unpack dir . Tar.read . GZip.decompress =<< BS.readFile tar Or creating a ".tar.bz2" file: > BS.writeFile tar . BZip.compress . Tar.write =<< Tar.pack base dir In addition it provides a full api for inspecting and constructing tar files without having to pack or unpack to local files. The functions are lazy which allows large archives to be processed in constant space. It is based on the tar handling code that has been in use in the cabal-install program for the last year or so. It has been tested on a large number of real world .tar.gz files so compatibility should be pretty good. There is also an 'htar' tool which is essentially a demo program for the tar library. It implements the common subset of the command line interface of the standard tar program, including gzip and bzip compression. Thanks to Christian Maeder for feedback on pre-release versions. Duncan

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shuffle error
by friggin friggin 02 Mar '09

02 Mar '09

http://hpaste.org/fastcgi/hpaste.fcgi/view?id=1989#a1990 Is the algorithm wrong ar am I using it wrong? Also, is the second argument to shuffle supposed to eb random numbers, ie the function should be used in the IO Monad?

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ANNOUNCE: The Industrial Haskell Group
by Duncan Coutts 02 Mar '09

02 Mar '09

We are pleased to announce the creation of the Industrial Haskell Group (IHG). The IHG is an organisation to support the needs of commercial users of the Haskell programming language. For more information, please see http://industry.haskell.org/ Currently, the main activity of the IHG is a collaborative development scheme, in which multiple companies fund work on the Haskell development platform to their mutual benefit. The scheme has started with three partners of the IHG, including Galois and Amgen. More details are available at http://industry.haskell.org/collab If your company is interested in joining then please e-mail info(a)industry.haskell.org -- Duncan Coutts, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

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How to work with date?
by Magicloud Magiclouds 02 Mar '09

02 Mar '09

Hi, I am working on some calendar GUI thing. And I cannot find a library to operate the "date". Like, when `now <- getZonedTime`, how could I get all the dates of the week. Or when I got `today`, how to get the date of 10 days ago? Thanks. -- 竹密岂妨流水过山高哪阻野云飞

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Assignment of grayCode
by Anonymous Anonymous 02 Mar '09

02 Mar '09

Hello, I'm working on a project for my university. But I do not understand the assignment. It asks me to write a function that does the following: grayCode :: Int -> ([Char] -> Int, Int -> [Char]) well in my opinion this assignment tells me to write some function that gets one input (an integer) and result two functions in one tuple. One function goes from [Char] to Int and the other one goes from Int to [Char]. But in my opinion this function will stay the same forever... am I mistaken? The assignment should be: Write an efficient code grayCode :: Int -> ([Char] -> Int, Int -> [Char]) that, for values between 2 and 36 (included), creates a Gray Code. That is, a tuple of two functions. Thanks!

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