October 2010 - Haskell-Cafe

Systematic treatment of static arguments
by Stephen Tetley 17 Oct '10

17 Oct '10

Hello list The Monad and Applicative instances for functions are "equivalent" to the respective Reader vesions (I use "equivalent" along the lines of - operationally the same but without the type distinction / newtype). There is also the Monoid instance for functions which is pretty slick. Has anyone looked at variations with two or more static arguments though? For example mappend with two static arguments needs to be defined either as a new function: > mappendR2 :: Monoid a => (r1 -> r2 -> a) -> (r1 -> r2 -> a) -> r1 -> r2 -> a > mappendR2 f g = \x y -> f x y `mappend` g x y or an overlapping instance: > instance Monoid a => OPlus (r1 -> r2 -> a) where > f `mappend` g = \x y -> f x y `mappend` g x y Working in graphics I've found two static arguments comes up quite often - preliminarily most of my functions are functions from the DrawingContext to something (drawing context is an environment that tracks line width, stroke colour, fill colour, etc.): fn1 :: DrawingCtx -> a Many of my functions statically use a 'start' point as the only coordinate reference, so they are in a "coordinate free" style: fn2 :: Point -> DrawingCtx -> a Some functions even have have a third static argument, for example drawing arrowheads for lines seems nice if the angle of the line is parametric and not used explicitly: fn3 :: Radian -> Point -> DrawingCtx -> a The help from Applicative, Monad and Monoid is "used up" by the one static argument version, so I find that I have to introduce points (in the pointed / point-free sense not in the graphic sense) in the fn2 and fn3 versions where a point-free version might still be nice. Is there any prior work looking at sets of combinators for these higher arity functions - papers or code? I'd prefer not to introduce a whole new lexicon of combinator names if I can help it. Thanks Stephen

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Text.Regex Segfault
by Duane Johnson 17 Oct '10

17 Oct '10

I bumped into a segmentation fault in the regex library today and thought I'd warn others in case similar behavior is observed: Prelude> :m Text.Regex > Prelude Text.Regex> map read (splitRegex (mkRegex "\\|") "0|1|2|4") :: > [Int] > Loading package syb ... linking ... done. > Loading package array-0.2.0.0 ... linking ... done. > Loading package bytestring-0.9.1.4 ... linking ... done. > Loading package regex-base-0.72.0.2 ... linking ... done. > Loading package regex-posix-0.72.0.3 ... linking ... done. > Loading package regex-compat-0.71.0.1 ... linking ... done. > [0,1,2,4] > Prelude Text.Regex> Segmentation fault > Regards, Duane

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ghci - always show types
by Ozgur Akgun 17 Oct '10

17 Oct '10

Cafe, Just a quick question. Either I am hallucinating or there was a way of saying ghci to always show types. It was working as if you typed ":t it" after every line of input. Sorry, I searched but couldn't find the option via google. Hope someone here knows/remembers what I am talking about. Best, Ozgur

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UTF-8 problems when decoding JSON data coming from Network.HTTP
by Ionut G. Stan 16 Oct '10

16 Oct '10

Hi, I'm trying to decode this JSON response: http://github.com/api/v2/json/user/show/igstan As you can see, the "name" field contains a non-latin character: ț, and it appears that Text.JSON. can't decode this response when it comes from Network.HTTP. I've tried Network.HTTP.Enumerator too, but the problem persists. Here's a simple (hopefully) reproducible test case: http://gist.github.com/630319 If you load it in ghci and call "main", you'll see that it doesn't properly show the user name. Also, calling: request "http://github.com/api/v2/json/user/show/igstan" will display the respective character encoded in a way that I have no idea whether or not is correct (Unicode is not one of my strong points for the moment). Can anyone shed some light on this problem? Which package is the culprit? Thanks, -- Ionuț G. Stan | http://igstan.ro

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Golfing parsec code
by Jacek Generowicz 16 Oct '10

16 Oct '10

I'm trying to write a utility which is able to read Ratios from two distinct formats "2 / 3" -> 2 % 3 "4" -> 4 % 1 I'm sure that a skilled Haskell programmer could vastly improve on my code (do I even need Parsec?), so I've come to solicit advice. import Text.ParserCombinators.Parsec import qualified Text.ParserCombinators.Parsec.Token as P import Text.Parsec.Language (haskellDef) import Data.Ratio readFraction :: String -> Either ParseError (Ratio Integer) readFraction = parse ratio "-" lexer = P.makeTokenParser haskellDef integer = P.integer lexer natural = P.natural lexer whiteSpace = P.whiteSpace lexer ratio = try frac <|> integer' integer' = do i <- integer return $ fromInteger i frac = do whiteSpace num <- natural char '/' den <- integer return (num % den) f = readFraction " 3 / 2 " i = readFraction " 6 " main = print [f,i]

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downloading GHC
by Ketil Malde 16 Oct '10

16 Oct '10

I needed GHC on a new machine, and went to download a binary tarball. First, I go to http://haskell.org/ghc/download_ghc_6_12_3.html, which kindly suggests to get the Haskell Platform instead. Then, at http://hackage.haskell.org/platform/linux.html, I'm told that I first need GHC, and pointed back to the GHC download page. I'll manage, of course - but I think this is a bit more confusing than it need be. -k -- If I haven't seen further, it is by standing in the footprints of giants

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ANNOUNCE: DocTest-0.1.0
by Simon Hengel 16 Oct '10

16 Oct '10

I'm very excited to announce a new version of DocTest[1]. DocTest now uses Haddock for parsing of comments. Examples are now marked-up with Haddocks newly introduced support for "examples of interaction"[2][3]. A very basic example of usage is at [4]. DocTest is still experimental. Suggestions and patches are gladly welcome! Cheers, Simon [1] http://hackage.haskell.org/cgi-bin/hackage-scripts/package/DocTest [2] http://www.haskell.org/haddock/CHANGES.txt [3] http://www.haskell.org/haddock/doc/html/ch03s08.html#id566093 [4] http://haskell.org/haskellwiki/DocTest

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Ambiguous type variable
by Jacek Generowicz 15 Oct '10

15 Oct '10

-- Given a definition of view which is essentially a synonym for show: class View a where view :: a -> String instance View Int where view = show -- why does "show 2" compile, while "view 2" gives an -- 'Ambiguous type variable' error fine = view (2::Int) noProblem = show 2 ambiguousTypeVariable = view 2

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IORef memory leak
by Jim Snow 15 Oct '10

15 Oct '10

I'm having some trouble with excessive memory use in a program that uses a lot of IORefs. I was able to write a much simpler program which exhibits the same sort of behavior. It appears that "modifyIORef" and "writeIORef" leak memory; perhaps they keep a reference to the old value. I tried both ghc-6.8.3 and ghc-6.10.1. Is this a known limitation, or is this a ghc bug, or am I using IORefs in the wrong way? -jim module Main where import Data.IORef import Control.Monad -- Leaks memory leakcheck1 ior = do go 1000000000 where go 0 = return () go n = do modifyIORef ior (+1) go (n-1) -- Leaks memory leakcheck2 ior = do go 1000000000 where go 0 = return () go n = do x <- readIORef ior writeIORef ior (x+1) go (n-1) -- Runs in constant memory leakcheck3 ior = do go 1000000000 where go 0 = return () go n = do x <- readIORef ior go (n-1) main :: IO () main = do ior <- newIORef 0 leakcheck2 ior compiled with: ghc -O2 --make Leak.hs -o Leak

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Yi on Windows
by Peter Marks 15 Oct '10

15 Oct '10

Does anyone use Yi on Windows? I've managed to get it to build and run, but it doesn't seem to pick up a config file. This seems to be an issue identified over a year ago: http://code.google.com/p/yi-editor/issues/detail?id=269. This would seem to render the editor almost completely useless on Windows. The whole point is to have an editor you can customize in Haskell. Any suggestions appreciated. Thanks Peter

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