- Haskell-Cafe - Haskell.org

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 16 Oct '10

16 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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ANNOUNCE: darcs 2.5 release candidate 1
by Reinier Lamers 15 Oct '10

15 Oct '10

The darcs team would like to announce the immediate availability of darcs 2.5 release candidate 1 (also known as darcs 2.4.99.1 due to Cabal restrictions). If no blocking issues are found in the coming week, we will finally releease darcs 2.5. Important changes since darcs 2.4.4 are: * trackdown can now do binary search with the --bisect option * darcs always stores patch metadata encoded with UTF-8 * diff now supports the --index option * amend-record now supports the --ask-deps option * apply now supports the --match option * amend-record has a new --keep-date option * inventory-changing commands (like record and pull) now operate in constant time with respect to the number of patches in the repository * the push, pull, send and fetch commands no longer set the default repository by default * the --edit-description option is now on by default for the send command Changes since the last beta release are: * Fix a bug that let users add files outside the working copy directory * Fix excessive network usage when pulling from an old-fashioned repository * Make sure darcs builds on Windows with Haskell Platform 2010.2.0.0 If you have installed the Haskell Platform or cabal-install, you can install this beta release by doing: $ cabal update $ cabal install darcs-beta Alternatively, you can download the tarball from http://darcs.net/releases/darcs-2.4.99.1.tar.gz and build it by hand as explained in the README file. Kind Regards, the darcs release manager, Reinier Lamers

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Client-extensible heterogeneous types
by Jacek Generowicz 15 Oct '10

15 Oct '10

Greetings, I'm looking for dynamic dispatch on extensible sets of types. Here is a more detailed explanation of what I mean: ######################################################################## # Consider the following Python code as representative of something # you might see in Object-Orineted programs in all sorts of languages. # Create an abstract type from abc import ABCMeta, abstractmethod class Abstract: __metaclass__ = ABCMeta @abstractmethod def method(self): pass # Provide some reifications of the abstract type class Variant1(Abstract): def method(self): return "Variant 1 stuff" class Variant2(Abstract): def method(self): return "Variant 2 stuff" # Provide some utilities to process these data def heterogeneousProcessor(data): return [datum.method() for datum in data] # If you wrap all the above up in a library, clients can easily extend # it with their own new types which will still work within the # framework class ClientDefined(Abstract): def method(self): return "Client-defined stuff" heterogeneousContainer = [Variant1(), Variant2(), ClientDefined()] result = heterogeneousProcessor(heterogeneousContainer) ######################################################################## ------------------------------------------------------------------------ -- In Haskell, on the one hand, the heterogeneity is easily provided -- by algebraic data types data AbstractHeterogeneous = VariantHeterogeneous1 | VariantHeterogeneous2 methodHeterogeneous VariantHeterogeneous1 = "Variant 1 stuff" methodHeterogeneous VariantHeterogeneous2 = "Variant 2 stuff" heterogeneousProcessor dataa = [methodHeterogeneous datum | datum <- dataa] heterogeneousContainer = [VariantHeterogeneous1, VariantHeterogeneous2] resultHeterogeneous = heterogeneousProcessor heterogeneousContainer -- But in order to extend the set of variants, the client would have -- to modify the source code, changing the definiton of -- AbstractHeterogeneous and methodHeterogeneous, both of which belong -- to the library. ------------------------------------------------------------------------ -- On the other hand, the extensibility is easily provided by type -- classes -- Library code: class AbstractExtensible a where methodExtensible :: a -> String instance AbstractExtensible () where methodExtensible _ = "Variant 1 stuff" instance AbstractExtensible Char where methodExtensible _ = "Variant 2 stuff" -- Client extension: instance AbstractExtensible Int where methodExtensible _ = "Client-defined stuff" -- but in this case, there is no heterogeneity: you cannot create the -- equivalent of heterogeneousContainer above -- heterogeneousExtensibleContainer = ???? resultExtensible :: [String] resultExtensible = [methodExtensible (), methodExtensible 'a', methodExtensible (1::Int)] ------------------------------------------------------------------------ I can't see a Haskell solution which combines both of these orthogonal features without losing the benefits of the type system. (For example, I could create my own, weak, type system with tags to identify the type and maps to do the dispatch.) So my question is, what would be the Haskell approach to combining heterogeneity (as provided by variant (algebraic) types) with type-extensibility (as provided by type classes), without losing the benefits of Haskell's type system? I haven't looked into Template Haskell, but i guess that it is likely to provide a solution. But is there a solution in plain Haskell? Thanks.

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