September 2011 - Haskell-Cafe

Re: [Haskell-cafe] [Haskell] mapM with Traversables
by Marc Ziegert 30 Sep '11

30 Sep '11

Hi Thomas, this should be on the haskell-cafe or haskell-beginners mailing list. Haskell@... is mainly for announcements. You have: > f :: Monad m => > a -> m b > Data.Traversable.mapM :: (Monad m, Traversable t) => > (a -> m b) -> t a -> m (t b) So, if you define g with > g a = do Data.Traversable.mapM f a or in short > g = Data.Traversable.mapM f , then the type will be > g :: (Monad m, Traversable t) => > t a -> m (t b) instead of > g :: [a] -> m (Seq b) . Try using ghci to find these things out. It helps to get not confused with the types. Besides the missing "Monad" context, g misses a generic way to convert between different Traversables, which does not exist. You can only convert from any Traversable (imagine a Tree) "toList"; not all Traversables have a "fromList" function. For conversion, you might want to use Foldable and Monoid, fold to untangle and mappend to recombine; but any specific "fromList" function will surely be more efficient. Regards - Marc -------- Original-Nachricht -------- > Datum: Wed, 28 Sep 2011 17:27:58 -0600 > Von: thomas burt <thedwards(a)gmail.com> > An: haskell(a)haskell.org > Betreff: [Haskell] mapM with Traversables > Hi - > > I have a function, "f :: Monad m => a -> m b", as well as a list of a's. > I'd > like to produce a sequence (Data.Sequence) of b's, given the a's: > > g :: [a] -> m (Seq b) > g a = do Data.Traversable.mapM f a -- type error! > > I see that "Data.Traversable.mapM f a" doesn't work... is this like asking > the compiler to infer the cons/append operation from the type signature of > g? > > Do I need to write my own function that explicitly calls the "append" > functions from Data.Sequence or can I do something else that would work > for > any "g :: Traversable t, Traversable u => t a -> m (u b)" given "f :: a -> > m > b"? > > Thanks for any comments! > Thomas

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module -> package
by Roman Beslik 30 Sep '11

30 Sep '11

Hello. How can I find which installed package a specified module belongs to? ghci or cabal or ghc-pkg?

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Handy abbreviations for using SciTE with Haskell
by Amy de Buitléir 30 Sep '11

30 Sep '11

I often use SciTE for editing Haskell code. I wanted an easy way to type symbols such as "∀" (instead of "forall"), so I created a properties file that allows me to do that. I haven't seen anything similar online (I Googled first), so I made the code available online. You can find the file and instructions here: http://www.haskell.org/haskellwiki/Tips_for_using_SciTE_with_Haskell

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Re: [Haskell-cafe] Programming with arrows, exercises
by Sergey Mironov 30 Sep '11

30 Sep '11

2011/9/30 John Lask <jvlask(a)hotmail.com>: > On 30/09/2011 7:15 AM, Sergey Mironov wrote: >> >> Hello. I am reading "Programming with Arrows" by John Hughes (very >> helpful and interesting!), the book has an exercises requiring a >> module called Circuits for checking the answer. There should be things >> like class ArrowCircuit and various functions related to digital logic >> circuits simulation. Does anybody know where can I find one? >> >> Thanks, >> Sergey >> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> > > This was in the old arrows library I have attached a copy, it will probably > need modification to compile. > > You should be able to use the examples with the current arrows library with > a few mods. > Yes, looks quite old. Unfotinately, it doesn't match the book.. I found that module I was looking for is in Appendix! Should I be more attentive.. Anyway, thank you!

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Type family instances in Haddock?
by Sean Leather 30 Sep '11

30 Sep '11

Would it be useful/worthwhile to have type/data family instances show up in Haddock just as type class instances do? Regards, Sean

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Indentation problem using Happy
by Amy de Buitléir 30 Sep '11

30 Sep '11

I'm trying to use Happy for the first time, and I'm having trouble with this very simple example: ===== BEGIN: PolyParser.y ===== { module Main where import Char ( isAlpha, isDigit, isSpace ) } %name calc %tokentype { Token } %error { parseError } %token id { TokenId $$ } ',' { TokenCOMMA } wombat { TokenWombat $$ } %% Poly : id ',' wombat { parseError :: [Token] -> a parseError _ = error "Parse error" data Poly = Poly Int String deriving Show data Token = TokenId Int | TokenCOMMA | TokenWombat String deriving Show lexer :: String -> [Token] lexer [] = [] lexer (c:cs) | isSpace c = lexer cs | isAlpha c = lexVar (c:cs) | isDigit c = lexNum (c:cs) lexer (',':cs) = TokenCOMMA : lexer cs lexNum cs = TokenInt (read num) : lexer rest where (num,rest) = span isDigit cs lexVar cs = case span isAlpha cs of (wombat,rest) -> TokenWombat wombat : lexer rest (id,rest) -> TokenId id : lexer rest main = getContents >>= print . calc . lexer } ===== END: PolyParser.y ===== The resulting PolyParser.hs has an indentation problem (second-to-last line, below), so it won't compile. happyReduce_1 = happySpecReduce_3 4 happyReduction_1 happyReduction_1 _ _ _ = HappyAbsSyn4 (parseError :: [Token] -> a parseError _ = error "Parse error" Thank you in advance to anyone who can tell me where I've gone wrong.

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CFG specification and analysis directly in Haskell
by Anton Tayanovskyy 30 Sep '11

30 Sep '11

Hi, As a weekend hack, I just realized that Haskell has this wonderful DoRec syntax that among other things seems to be able to let the user express context-free grammars together with their processing rules in normal Haskell code, without template Haskell or anything like that, just like parser combinators. I am just wondering if this is this a known and useful result? Are there libraries doing similar stuff? I wrote up the Earley algorithm to demonstrate that one can in principle analyze the complete grammar (https://github.com/toyvo/haskell-earley) The result derivations `cheat` by using Data.Dynamic, but the result is quite pleasing, for example one can do: grammar :: G.Grammar (G.Rule Char E) grammar = do nat <- G.rule "NAT" [ fmap (\_ -> 0) (G.term '0') , fmap (\_ -> 1) (G.term '1') ] rec expr <- G.rule "EXPR" [ fmap Nat $ G.var nat , pure (\x _ y -> Add x y) <*> G.var expr <*> G.term '+' <*> G.var expr ] return expr Thanks, Anton

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Turn GC off
by Andreas Voellmy 30 Sep '11

30 Sep '11

Hi everyone, Is there a way to completely turn garbage collection off in the Haskell runtime system? I'm aware of the -A runtime option, but I'd like to completely turn it off, if possible. I'm OK with running the program until it runs out of memory, and I'm willing to recompile GHC if needed. Regards, Andreas

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Programming with arrows, exercises
by Sergey Mironov 29 Sep '11

29 Sep '11

Hello. I am reading "Programming with Arrows" by John Hughes (very helpful and interesting!), the book has an exercises requiring a module called Circuits for checking the answer. There should be things like class ArrowCircuit and various functions related to digital logic circuits simulation. Does anybody know where can I find one? Thanks, Sergey

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Cabal rebuilding of the C++ code for wxHaskell
by Dave Tapley 29 Sep '11

29 Sep '11

Hi all, I've been trying to resolve a compile time issue[1] with wxHaskell, and I thought I'd throw it open to see if anyone on cafe can help. Here's the crux of the issue: The Setup.hs for wxcore (the major component of wxHaskell) uses "simpleUserHooks", overriding only "confHook". However there is also "cleanHook", which is defined by simpleUserHooks to be: cleanHook = \p _ _ f -> clean p f, If you consult the source for clean[2] you'll see that it tries to "remove the whole dist/ directory rather than tracking exactly what files we created in there". I presume that's why we have to do a full re-build every time? To try and circumvent this I modified the definition of "main" in Setup.hs to this: main = defaultMainWithHooks simpleUserHooks { confHook = myConfHook, cleanHook = (\_ _ _ _ -> return ())} Unfortunately it still seems to re-build all the C++ on each 'install' from cabal. Not sure why? Dave, [1] http://sourceforge.net/mailarchive/message.php?msg_id=28099997 [2] Taken from http://www.haskell.org/ghc/docs/6.10.4/html/libraries/Cabal/src/Distributio… -- Cleaning clean :: PackageDescription -> CleanFlags -> IO () clean pkg_descr flags = do let distPref = fromFlag $ cleanDistPref flags notice verbosity "cleaning..." maybeConfig <- if fromFlag (cleanSaveConf flags) then maybeGetPersistBuildConfig distPref else return Nothing -- remove the whole dist/ directory rather than tracking exactly what files -- we created in there. chattyTry "removing dist/" $ do exists <- doesDirectoryExist distPref when exists (removeDirectoryRecursive distPref) -- these live in the top level dir so must be removed separately removeRegScripts -- Any extra files the user wants to remove mapM_ removeFileOrDirectory (extraTmpFiles pkg_descr) -- If the user wanted to save the config, write it back maybe (return ()) (writePersistBuildConfig distPref) maybeConfig where removeFileOrDirectory :: FilePath -> IO () removeFileOrDirectory fname = do isDir <- doesDirectoryExist fname isFile <- doesFileExist fname if isDir then removeDirectoryRecursive fname else if isFile then removeFile fname else return () verbosity = fromFlag (cleanVerbosity flags)

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