March 2008 - Haskell-Cafe

Three Cabal/Hackage GSoC project proposals
by Duncan Coutts 20 Mar '08

20 Mar '08

All, I've proposed three GSoC proposals on different aspects of Cabal and Hackage: * Cabal 'make-like' dependency framework http://hackage.haskell.org/trac/summer-of-code/ticket/1552 * cabal-install package dependency resolution http://hackage.haskell.org/trac/summer-of-code/ticket/1553 * Hackage build reporting http://hackage.haskell.org/trac/summer-of-code/ticket/1554 This is in addition to the project that Neil proposed: * Hoogle 4 (using it as the main interface to the hackage website) http://hackage.haskell.org/trac/summer-of-code/ticket/1429 I'd like to invite interested people to help improve the proposals and to invite students to look at these and consider them. I'd also like other people interested in mentoring any of these projects to sign up. In particular, students who are interested should email the cabal-devel and/or libraries list (though they'd have to subscribe I think) to express their interest and work with us on improving the detail of their proposal. Duncan

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How to implement Read instance for user defined type
by Verma Anurag-VNF673 20 Mar '08

20 Mar '08

Hi, I am trying to understand how to define instances for Read class for a user defined type. This is the sample code I wrote, but I am not able to get it correctly. So, let me know what's wrong with this program: module Mark where data Mark = Mark Int deriving (Show) instance Read Mark where readsPrec _ str = [(Mark x, t') | ("mark",t) <- reads str, (x,t') <- reads t When I run it in ghci: *Mark> reads "mark 5" :: [ (Mark,String)] ( I am intentionally using mark in lowercase) [] Why is the output coming out as [] as against expected output [(Mark 5, "")] ? -Anurag

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Scoping in template haskell
by Lars Oppermann 20 Mar '08

20 Mar '08

Hi all, I am trying to get familiar with template haskell and I'm wondering whether the following is possible and if so, how it would be done properly... I have two templates, and I'd like to use one from the other. For instance the first one might be gen1 :: Int -> ExpQ gen1 i = [| "<<" ++ (show i) ++ ">>" |] Thus, I gould do > $(gen1 42) and I'd get "<<42>>" now I'd like to use gen2 in another generated expression so that I could so > $(gen2 [42, 66]) and get "<<42>><<66>>" My naive intention was to write gen2 :: [Int] -> ExpQ gen2 is = [| map (\x -> $(gen1 x)) is |] which gives me "Stage error: `x' is bound at stage 2 but used at stage 1 So I guess my actual question would be: how do I pass a variable bound outside a quotation into another splice used in that quotation? Bests, Lars

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Type parameters in type families
by Hugo Pacheco 19 Mar '08

19 Mar '08

Hi, I am trying to understand some differences of parameterizing or not some arguments of type families. I have some code such as *type family G a :: * -> * instance Functor (G Int) where fmap f (Left ()) = Left () fmap f (Right x) = Right (f x) ggg :: Functor (G a) => G a x -> G a x ggg = fmap id* and it works fine. However, I need to parameterize one extra argument (due to type equality): *type family F a x :: * class FunctorF d where fmapF :: (x -> y) -> F d x -> F d y fff :: (FunctorF a) => F a b -> F a b fff = fmapF id* This second scenario fails to compile because the compiler cannot unify types a and b with types d and x from the fmapF declaration. Is there any other option than passing dummy variables to fmapF? * class FunctorF d where fmapF :: d -> x -> (x -> y) -> F d x -> F d y fff :: (FunctorF a) => a -> b -> F a b -> F a b fff a b = fmapF a b id* Thanks, hugo

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Error handling in calculations
by hjgtuyl＠chello.nl 19 Mar '08

19 Mar '08

L.S., When playing with exceptions, I noticed the following strangeness: *Main> 1 / 0 Infinity *Main> 1 `div` 0 *** Exception: divide by zero This is in GHCi 6.8.2; WinHugs Sep 2006 gives: Main> 1 / 0 1.#INF Main> 1 `div` 0 Program error: divide by zero Is this difference between fractional and integral calculation intentional? It makes error handling more difficult. -- Met vriendelijke groet, Henk-Jan van Tuyl -- http://functor.bamikanarie.com http://Van.Tuyl.eu/ --

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libmad and os/x coreaudio wrappers
by Chris Waterson 19 Mar '08

19 Mar '08

Hi there! I've taken my first stab at writing some (admittedly minimal) libraries for Haskell, and would love to get feedback on them: * hmad: a wrapper for the libmad MP3 decoder. http://maubi.net/~waterson/REPO/hmad * CoreAudio: a wrapper for OS/X CoreAudio. http://maubi.net/~waterson/REPO/CoreAudio (You should be able to "darcs get" the above links, if you want.) I wrote the libmad wrapper to generate a "stream" (i.e., a lazy list) of audio samples. CoreAudio allows the input stream to be lazy, as well. So, here's a simple MP3 player: > module Main where > > import Sound.CoreAudio > import Codec.Audio.MP3.Mad > import qualified Data.ByteString.Lazy as B > import System > import System.IO > > main :: IO () > main = do files <- getArgs > mapM_ playFile files > > playFile :: String -> IO () > playFile file = > withBinaryFile file ReadMode $ \ inHandle -> > do xs <- B.hGetContents inHandle > samples <- decode xs > play samples I do have a couple questions... * The CoreAudio library requires its users to be compiled with "-threaded". Is there a way to specify that in the Cabal file? * I wanted to be able to generate a variety of streams from libmad. Besides stereo linear PCM data, it also seemed like it might be worth-while to produce a stream of MP3 frame headers, the unsynthesized frequency domain data, and so on. I tried to accomplish this with a the DecoderSink class, but I'm not sure I succeeded. Any thoughts here would be appreciated! I hope someone else finds these useful. The FFI was a joy to use once I figured it out... :) chris

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ghc 6.8.2 and timer_create error
by Anatoly Yakovenko 19 Mar '08

19 Mar '08

I have a gentoo box with ghc 6.8.2, and the binaries that ghc builds on that box do not work on redhat EL 4 or ubuntu 7.10. When I try to run the binary, i get an error: Main: timer_create: Invalid argument so is there any way to get ghc to build a binary that doesn't use timer_create? or is that something that is configured when ghc itself is built, if so, how do I turn that feature off? Thanks

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Satnam Singh of Microsoft Research
by Galchin Vasili 19 Mar '08

19 Mar '08

Hello, Recently Satnam Singh of Microsoft Research gave a talk at Google about concurrency, Haskell STM, etc. Was there a transcript of this talk? Thanks, Vasili

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Dynamic typing makes you more productive?
by Justin Bailey 19 Mar '08

19 Mar '08

>From a recent interview[1] with the guy leading Ruby development on .NET at Microsoft: "You spend less time writing software than you spend maintaining software. Optimizing for writing software versus maintaining software is probably the wrong thing to do. Static typing makes it harder to maintain software because it's harder to change it." Two years ago I would have agreed with that statement. Now - no way. Make the compiler work for you. I've done a lot of Ruby development and I would never use it for a project of more than 3 or 4 people. It's an awesome language but I don't think it would scale to programming "in the large." Any object can be modified at any time. Determining where a particular method comes from can be an exercise in Sherlockian deduction. Give an organization of 100 developers that much freedom and I can only imagine chaos would result. Justin [1] http://www.regdeveloper.co.uk/2008/03/17/ironruby_work_schedule/

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Re: [Haskell-cafe] build recent(ish) ghc on macos 10.3.9 powerpc?
by Uwe Hollerbach 18 Mar '08

18 Mar '08

Hello, haskellers, a few days ago I had asked about building recent ghc on macos 10.3.9. I have made a bit of progress along those lines, here's a small update on what worked and what didn't. Instead of trying to proceed with the porting procedure, I went back to an install: I ended up going to 6.2.2. After a bit of fiddling, I found that another bit of messing-about I had done to the machine was messing me up: I had upgraded gcc from 3.3.x to 3.4.6 by just downloading the sources and building. That produced a working gcc 3.4.6, but apparently the apple-aware ghc 6.2.2 and the apple-unaware gcc 3.4.6 didn't play nicely together. Once I disabled gcc 3.4.6, stuff worked again. Then I tried building ghc 6.4.2 using the installed 6.2.2. This took a lot of time, the compiler was able (as far as I could tell) to go through all of its own bootstrapping, but after I happily installed what I thought was a working ghc 6.4.2, the installed version was unable to do anything: it segfaulted while trying to do "ghc --version". Oops... Out of somewhat morbid curiosity, I tried running that inside the debugger, but gave up after several hours of elapsed time. Back to the drawing board. So I tried jumping directly from 6.2.2 to 6.6.1; that is, using 6.2.2 to build 6.6.1. That took another lot of time, but I'm happy to report that it has produced a working executable: it did proceed all the way through its bootstrapping, I verified that the executable in compiler/stage2 was able to run "ghc --version" in-place, and after I installed it, it was still able to do that; and (extra bonus! :-) ) it's also able to compile and link at least one small Haskell program written by me! Cool! I'm going to keep going forward and try to build 6.8.2, and I'll report back if that succeeds, but this is already pretty good progress... regards, Uwe

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