January 2007 - Haskell-Cafe

Tracing a list comprehension
by Philippe de Rochambeau 18 Jan '07

18 Jan '07

Hello, is there a way to trace a list comprehension such as concat1 :: [[a]] -> [a] concat1 xss = [ x | xs <- xss, x <- xs ] I would like to see what gets stored in xs. Cheers, phiroc

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New (simple) Lazy Evaluation tutorial
by Andrew Wagner 18 Jan '07

18 Jan '07

Hi all, An interesting question came up in #haskell the other day, and I took the resulting discussion and wrapped it up into a simple tutorial for the wiki. Since I'm quite a newbie to haskell myself, I'd appreciate any double-checking of my logic and, of course, any other comments/suggestions. Thanks to Cale for his efforts on this problem, and to the original asker, whose name I can't recall. http://www.haskell.org/haskellwiki/Haskell/Lazy_Evaluation

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Re: [Haskell-cafe] Article review: Category Theory
by David House 18 Jan '07

18 Jan '07

On 18/01/07, Daniel Fischer <daniel.is.fischer(a)web.de> wrote: > Since I don't know whether I would have to register to modify the wiki or > something (and don't know what markup to use either), I just send you a few > corrections by e-mail. Thanks; I've corrected them all, with the exceptions of the ones shown below. http://en.wikibooks.org/w/index.php?title=Haskell/Category_theory&diff=7231… > perhaps index join/unit with the appropriate objects, e.g. > join_X . M(join_X) = join_X . join_M(X), to aid the newcomers? > the picture's left is the text's right. -- -David House, dmhouse(a)gmail.com

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re: Simple HTTP lib for Windows?
by brad clawsie 18 Jan '07

18 Jan '07

there is a Network.HTTP module, but it is not easy to use what you want is the equivalent of perl's LWP::Simple, which provides get() and head() functions i have heard that this is being worked on, in the meantime i personally use this wrapper: http://www.b7j0c.org/content/haskell-http.html that i wrote myself to provide simple get and head functionality with some trivial redirect support i use this module routinely so it should work, although i have only tested it on freebsd and linux no one else has ever looked at this so feel free to send critiques, it is mostly a hack of the get() example script distributed with the Network.HTTP module

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Announce: piggybackGHC 0.1
by Martin Grabmueller 18 Jan '07

18 Jan '07

Hello fellow Haskell Hackers, I announce release 0.1 of piggybackGHC, a small utility package for using GHC for lexing and parsing Haskell source code. Description: The modules in this package provide several functions for conveniently lexing and parsing Haskell modules. The library uses the GHC library for all the hard stuff, so all supported GHC extensions are available. Homepage: http://uebb.cs.tu-berlin.de/~magr/projects/piggybackGHC/doc/ Darcs: darcs get --partial http://uebb.cs.tu-berlin.de/~magr/projects/piggybackGHC/ Feedback appreciated! Have fun, Martin

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Currying
by phiroc＠free.fr 18 Jan '07

18 Jan '07

Hello, what is so great about "currying"? What are its uses, apart from letting one define functions with less parentheses? Many thanks. Phiroc

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mtl vs monads
by Conal Elliott 18 Jan '07

18 Jan '07

Are the mtl and monads (monadLib) packages both in active use? Is one being phased out? - Conal

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GHC performance of 64-bit
by Pedro Baltazar Vasconcelos 18 Jan '07

18 Jan '07

Hello all, I noticed that GHC generates slower code on an Linux amd64 bit platform than the 32-bit version on a cheaper 32-bit machine. CPUTime for running sieve of Erathostenes to generate 10,000 primes: Athlon XP 2800 (32-bit): 7.98 secs Athlon 64 3800 (64-bit): 10.29 secs This is using GHC 6.6 on the 64-bit machine and 6.4.1 on the 32-bit one. I googled around and could not find any information regarding degraded performance of ghc/haskell on 64-bit machines. Any ideas? Regards, Pedro

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What does the withProcessHandle_ function do?
by John Ky 18 Jan '07

18 Jan '07

I want to learn how to use FFI with Win32, so I'm looking through the GHC source code. I encountered the function terminateProcess :: ProcessHandle -> IO () terminateProcess ph = do withProcessHandle_ ph $ \p_ -> case p_ of ClosedHandle _ -> return p_ OpenHandle h -> do throwErrnoIfMinus1_ "terminateProcess" $ c_terminateProcess h return p_ which uses withProcessHandle_. What does it do? Thanks -John

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Partial application
by phiroc＠free.fr 18 Jan '07

18 Jan '07

Hello, could someone please give me an example of the "partial application" of the following curried function: add' :: Int -> Int -> Int add' a b = a + b Normally, add' 1 should work, but it doesn't. Many thanks. phiroc

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