August 2009 - Haskell-Cafe

Typeclass for functions taking different kinds of strings
by Taru Karttunen 05 Aug '09

05 Aug '09

Hello It seems like a very common issue to have an API like: foo :: String -> Foo fooBS :: ByteString -> Foo fooLBS:: L.ByteString -> Foo is there currently a library that makes unifying them easy? Below is attached one try at this, does it make sense? I'm thinking of uploading it to Hackage but would like comments first. With the library the above code is transformed into: foo :: StringLike string => string -> Foo - Taru Karttunen

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Haskell interface files: Why used? What about same data in object files?
by Howard B. Golden 05 Aug '09

05 Aug '09

Hi, I am trying to understand the design of the Haskell interface files. Why are they a separate file rather than having the same data in the object file generated by the compiler? (Naively, it seems to me this would work also. Am I missing something?) Thanks. Howard B. Golden Northridge, California, USA

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powerSet = filterM (const [True, False]) ... is this obfuscated haskell?
by Thomas Hartman 05 Aug '09

05 Aug '09

on haskell reddit today powerSet = filterM (const [True, False]) is said to be beautiful / mind blowing. I just don't get it. I can play with transformations until I get powerSet [] = [[]] powerSet (x:xs) = let pxs = powerSet xs in map (x:) pxs ++ pxs which is understandable to me, but no matter how long I look at the original filterM definition it just doesn't click. Is this a uniquely haskell obfu, or is there a way of reading this definition that makes sense? If anybody agrees with me, care to throw out other examples of "obfuscated haskell considered harmful"?

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A problem with bytestring 0.9.1.4 "hGetBuf: invalid argument"
by kenny lu 05 Aug '09

05 Aug '09

Hi all, I've recently came across a problem when processing a large text file (around 2G in size). I wrote a Haskell program to count the number of lines in the file. module Main where import System import qualified Data.ByteString.Char8 as S -- import Prelude as S main :: IO () main = do { args <- getArgs ; case args of { [ filename ] -> do { content <- S.readFile filename ; let lns = S.lines content ; putStrLn (show $ length lns) } ; _ -> error "Usage : Wc <file>" } } I get this error, if I use the ByteString module, ./Wc a.out Wc: {handle: a.out}: hGetBuf: invalid argument (illegal buffer size (-1909953139)) Otherwise, it returns me the result. Another observation is that if I reduce the size of the file, the ByteString version works too. Is it a known limitation? Regards, Kenny A generator program that generate large file. (Warning, it is very slow, I don't know how to speed it up) -- generate a file module Main where import System import qualified Data.ByteString.Char8 as S l :: S.ByteString l = S.pack "All work, no fun, make Kenny a dull boy. " main :: IO () main = do { args <- getArgs ; case args of { [ n, fn ] -> do { let i = read n ; mapM_ (\s -> S.appendFile fn s) (take i $ repeat l) } ; _ -> return () } }

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Hackage Download Rankings August 2009
by Don Stewart 05 Aug '09

05 Aug '09

Total package downloads on Hackage, by August 1 2009, http://www.galois.com/~dons/hackage/august-2009/popularity-august-2009.html All packages by month: http://www.galois.com/~dons/hackage/august-2009/hackage-august-2009.html Raw data: http://www.galois.com/~dons/hackage/august-2009/hackage-downloads-august-20… Enjoy. -- Don P.S. For reference, here's the data from March, http://www.galois.com/~dons/hackage/popularity.html and analysis at the time http://www.galois.com/blog/2009/03/23/one-million-haskell-downloads/

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Cyclic data declarations
by Michal D. 04 Aug '09

04 Aug '09

I'm in the process of writing a toy compiler but I'm having some trouble trying to make my datatypes general. For example, using parsec I parse statements as: data Stmt = SIf Test [Stmt] [Stmt] | ... However, when it's time to create a control flow graph it would be nice to represent statements as (the Int's signify the node id's for either case of the if statement): data Stmt = SIf Test Int Int | ... So, in a eureka moment I decided that this should be allowable with the following declaration: data Stmt link = SIf Test link link | ... Ofcourse, the problem is trying to declare the resulting type for parsing: "parse -> Stmt [Stmt [Stmt ....]]". Any hints on whether there is a way to accomplish what I'm trying to do or do I have to bite the bullet and declare two seperate datatypes? I tried being clever and declaring a 'helper' type as "type StmtRec = Stmt [StmtRec]" but to no avail... GHC won't let it slide: "Cycle in type synonym declarations"! Cheers, Michal

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Split package organization (AntTweakBar, Cabal, darcs)
by Gregory D. Weber 04 Aug '09

04 Aug '09

I'm working on a Haskell binding for AntTweakBar, a light user interface for OpenGL applications (http://www.antisphere.com/Wiki/tools:anttweakbar) I have three questions about how to organize it as a Haskell package or packages, Cabal, and darcs. First, since AntTweakBar provides support for handling events from GLUT, GLFW, and SDL, as well as customizable event handling from other sources, would it be best to divide it into four packages -- AntTweakBar (core), AntTweakBar-GLUT, AntTweakBar-GLFW, and AntTweakBar-SDL? A few other "package groups" on Hackage have taken this approach of splitting according to the user interface: for example, grapefruit-ui-gtk, reactive-glut, though not on such a large scale as I am proposing. Advantages of four packages rather than one: -- Fewer build dependencies, for example, for users who want to use just SDL and not have to install GLUT or GLFW. -- If one of the build dependencies is broken (as for example GLFW is just at the moment with the latest OpenGL), users could still build the other AntTweakBar-* packages. Disadvantages: -- More packages to install, for those who want it all. -- I might seem to be grabbing an undue share of the package namespace. And it could get way out of hand if I further split the examples from the library, making AntTweakBar-(base|GLUT|GLFW|SDL)-(lib|examples) = 8 packages! Would that be a problem? -- Possible inconvenience for the developer (see second question). Second question, assuming the four-way split is the best way to package this: my impression is that a directory can have only one cabal package file and one Setup.hs file. (http://www.haskell.org/cabal/release/cabal-latest/doc/users-guide/authors.h…, "Creating a Package") So for example I could not have, in the same directory, AntTweakBar.cabal, AntTweakBar-GLUT.cabal, etc. That means the four packages would each have to have their own directories, and it is sometimes inconvenient to be jumping back and forth between them. Any good way around that? Third question, assuming four separate directories for the four Cabal package files. Putting all four into one darcs repo (i.e., the darcs repo would have one main directory and a subdirectory for each package) seems to make it easier to coordinate development of the related packages -- for example, a single 'darcs rec -a' takes care of recording the related changes in all four subdirectories. Any reason not to do it that way? -- ___ ___ __ _ / _ \ / _ \| | | | Gregory D. Weber, Associate Professor / /_\// / | | | /\ | | Indiana University East / /_\\/ /__| | |/ \| | http://mypage.iu.edu/~gdweber/ \____/\_____/\___/\__/ Tel. (765) 973-8420; FAX (765) 973-8550

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Re: ANN: yst 0.2.1
by spoon 04 Aug '09

04 Aug '09

This sounds great! I really like using yaml for web development. However, my last use case was for a non-techie, so I created a little desktop application for editing the "yamlbase", if you excuse the neologism. I think it worked quite well, because the user was then presented with a friendly looking system ( I even wrote a little calendar widget :) without having to spend the precious moneys on dynamic hosting. In the end, everyone was very happy. Actually, it was written in Ruby, and Shoes, and the data part was a lot less powerful than the yst query language there - but I think that an analogous tool would be useful; a GUI which creates yst scripts, would make yst a very powerful tool for non-techies. John

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funct.prog. vs logic prog., practical Haskell
by Petr Pudlak 04 Aug '09

04 Aug '09

Hi all, I'd like to convince people at our university to pay more attention to functional languages, especially Haskell. Their arguments were that (1) Functional programming is more academic than practical. (2) They are using logic programming already (Prolog); why is Haskell better than Prolog (or generally a functional language better than a logic programming language)? (1) is easier to answer, there are a lots of applications at HaskellWiki, or elsewhere around the Internet, written in Haskell, OCaml, etc. Still, I welcome comments on your experience, for example, if you have written some larger-scale application in Haskell (or another a functional language) that is not at HaskellWiki, and perhaps if/why you would recommend doing so to other people. (2) is harder for me, since I've never programmed in Prolog or another language for logic programming. I'd be happy if anyone who is experienced in both Prolog and Haskell could elaborate the differences, pros & cons etc. Thanks, Petr

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ANNOUNCE: The Haskell Platform 2009.2.0.2
by Don Stewart 04 Aug '09

04 Aug '09

We're pleased to announce the third release of the Haskell Platform: a single, standard Haskell distribution for everyone. The specification, along with installers (including Windows and Unix installers for a full Haskell environment) are available. Download the Haskell Platform 2009.2.0.2: http://hackage.haskell.org/platform/ The Haskell Platform is a single, standard Haskell distribution for every system, in the form of a blessed library and tool suite for Haskell distilled from the thousands of libraries on Hackage, along with installers for a wide variety of systems. It saves developers work picking and choosing the best Haskell libraries and tools to use for a task. When you install the Haskell Platform, you get the latest stable compiler, an expanded set of core libraries, additional development tools, and cabal-install – so you can download anything else you need from Hackage. What you get: http://hackage.haskell.org/platform/contents.html With regular time-based releases, we expect the platform will grow into a rich, indispensable development environment for all Haskell projects. Since the first release, there have been over 100,000 downloads of the installers. Thanks! -- The Platform Infrastructure Team

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