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About the import module
by xu zhang 06 Aug '09

06 Aug '09

Hi there, If I import a module and do not explicitly point out the entities I have imported. And I want the ghc to point out the entities automatically. Is there any method to do this? any methods to have the ghc point out the entities I import and export? Because there are so many files and I don't want to change them one by one, so I just want to find if there is a simple and automatic way to have the entities pointed out. Thank you in advance!

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Efficient functional idiom for histogram
by Paul Moore 05 Aug '09

05 Aug '09

How would I efficiently write a function in Haskell to count occurrences of unique elements in a (potentially very large) list? For example, given the list [1,2,3,4,5,3,4,2,4] I would like the output [[1,1], [2,2], [3,2], [4,3], [5,1]] (or some equivalent representation). Clearly, this won't be possible for infinite input, but I would like it to be as efficient as possible for (lazy) input lists containing many millions of elements. So an implementation based on group . sort is not going to work. In an imperative language like Python, I'd use a dictionary as an accumulator - something like for el in input: accums[i] = accums.get(i, 0) + 1 That's about as efficient as you can hope for (assuming an efficient dictionary implementation). How would I code something equivalent in Haskell? Thanks, Paul.

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Basic questions about concurrency in Haskell
by Thomas Witzel 05 Aug '09

05 Aug '09

Hello all, I'm new to Haskell, but have a good background in LISP/Scheme and do mostly C/C++ programming on a daily basis. I'm learning Haskell mainly because it provides facilities for concurrency on the language level, and I'm mainly interested in implementing parallel or massively parallel algorithms with Haskell. I have two questions that bother me. 1. Does the Haskell compiler make sure that there is no page sharing between threads, in order to avoid cache thrashing between cpus (a real killer on large SMP or ccNUMA systems) ? If so, are there functions/options to control this ? 2. I started with the very simple nfib example given in the manual for Control.Parallel (Section 7.18). On my systems using multiple cores makes the code actually slower than just using a single core. While the manual cautions that this could be the case for certain algorithms, I'm wondering whether this is the desired behaviour for this example. I'm using ghc 6.10.4 right now. Thank you for your help getting me started here. Thomas

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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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