December 2008 - Haskell-Cafe

Dr Dobbs: Time to get good at functional programming
by Jeff Heard 08 Dec '08

08 Dec '08

Seen on Slashdot: http://www.ddj.com/development-tools/212201710;jsessionid=3MQLTTYJRPL3CQSND…

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Building "plugins" from Hackage
by Sol 08 Dec '08

08 Dec '08

Hello List, when I try to install the package "plugins" with cabal i get the following error. cabal: dependencies conflict: ghc-6.8.3 requires Cabal ==1.2.4.0 however Cabal-1.2.4.0 was excluded because plugins-1.3.1 requires Cabal ==1.4.* Is there a way to resolve this? Any ideas? Sol.

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Overlapping instances
by John Ky 08 Dec '08

08 Dec '08

Hi, I've got the following code which tries to implement a TShow class, which is equivalent to Show, except it is supposed to work on TVar types as well. import GHC.Conc createEngine :: String -> Int -> Int -> IO Engine createEngine name major minor = do tUsers <- newTVarIO [] return $ Engine { engineName = name , version = EngineVersion { major = major , minor = minor } , users = tUsers } class TShow a where tshow :: a -> IO String instance Show (TVar a) where show a = "%" instance (Show a) => TShow a where tshow a = return $ show a instance (Show a) => TShow (TVar a) where tshow ta = do a <- atomically (readTVar ta) return $ show a data User = User { userName :: String } deriving Show data EngineVersion = EngineVersion { major :: Int , minor :: Int } deriving Show data Engine = Engine { engineName :: String , version :: EngineVersion , users :: TVar [User] } instance TShow Engine where tshow a = do users <- atomically (readTVar (users a)) return $ "Engine { " ++ "engineName = " ++ show (engineName a) ++ ", " ++ "version = " ++ show (version a) ++ ", " ++ "users = %" ++ show users ++ " }" When I run it however, I get this: *Main> te <- createEngine "Hello" 1 2 *Main> s <- tshow te <interactive>:1:5: Overlapping instances for TShow Engine arising from a use of `tshow' at <interactive>:1:5-12 Matching instances: instance (Show a) => TShow a -- Defined at fxmain.hs:(26,0)-(27,27) instance TShow Engine -- Defined at fxmain.hs:(51,0)-(58,41) In a stmt of a 'do' expression: s <- tshow te I'm not seeing how instance (Show a) => TShow a in the above error message is applicable here since Engine is not an instance of Show. Why is it complaining? Thanks, -John

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Problem with System.Random.randoms
by S. Günther 08 Dec '08

08 Dec '08

Hi, I have a small problem with System.Random.randoms. I need a rather large number of random numbers but the following program consumes a huge amount of memory. I terminated it when it used up more than 2 Gb: module Main where import System.Random n :: Int n = maxBound main = do g <- getStdGen print $ length $ take n $ ((randoms g)::[Int]) On the other hand using take n $ [1..] it runs in constant space. Am I doing something wrong? Or should I just abandon randoms and use the more primitive functions in System.Random? Thanks in advance S. Günther

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Deriving something else?
by John Ky 08 Dec '08

08 Dec '08

Hi, I've defined a class and some instances, which I'm hoping would help me "show" values of types that may include transactional elements. class TShow a where tshow :: a -> IO String instance Show (TVar a) where show = "%" instance (Show a) => TShow a where tshow a = return $ show a instance (Show a) => TShow (TVar a) where tshow ta = do a <- readTVar ta return $ show a Having created a new class is it possible to do some magic so that it can be put it into a deriving clause? data Type = Type { field1 :: Int , field2 :: Int } deriving Show data AnotherType = AnotherType { field3 :: Int , field4 :: TVar Type } deriving *TShow* Thanks -John

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Animated line art
by Andrew Coppin 08 Dec '08

08 Dec '08

So, the muse has taken me. I'm going to attempt to produce some animated mathematical drawings involving lines and curves. Gtk2hs has a Cairo binding that should make rendering the stuff fairly straight-forward. So no problems there. Now, what I *could* do is write a new Haskell program for each drawing I want, hard-coding in the necessary imperative loops for updating coordinates, looping over multiple lines, etc. But that wasn't be very Haskell, would it? ;-) It seems that the "correct" course of action is to design a DSL for declaratively describing animated line art. Does anybody have ideas about what such a thing might look like?

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Is unsafePerformIO safe here?
by John Ky 07 Dec '08

07 Dec '08

Hi, Is the following safe? moo :: TVar Int moo = unsafePerformIO $ newTVarIO 1 I'm interested in writing a stateful application, and I wanted to start with writing some IO functions that did stuff on some state and then test them over long periods of time in GHCi. I was worried I might be depending on some guarantees that aren't actually there, like moo being discarded and recreated inbetween invocations of different functions. Thanks, -John

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Haskell haikus
by Gwern Branwen 07 Dec '08

07 Dec '08

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 Hi everyone. So today I finally got around to something long on my todo list - a compilation of all the Haskell haikus I've seen around! It is at http://haskell.org/haskellwiki/Haiku But I'm afraid I only have 5, and Google doesn't turn up any more. So: does anybody have a haiku I missed? Or even better, is anyone feeling poetically inspired tonight? :) - -- gwern -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (GNU/Linux) iEYEAREKAAYFAkk5ttoACgkQvpDo5Pfl1oJ+ygCfdSSBmbgyoOwkG53rKahF2Su1 84UAoIOrxGwe3u+WwnKxvKulq1AT4IJS =+2kH -----END PGP SIGNATURE-----

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Could FDs help usurp an ATs syntactic restriction?
by Nicolas Frisby 07 Dec '08

07 Dec '08

>From the error below, I'm inferring that the RHS of the associated type definition can only contain type variables from the instance head, not the instance context. I didn't explicitly see this restriction when reading the GHC/Type_families entry. Could perhaps the "a b -> bn" functional dependency of the TypeEq class lift this restriction for bn? This isn't my ball park, but that idea has my hopes up :). <haskell> {-# LANGUAGE TypeFamilies #-} import TypeEq -- Attempting to encapsulate TypeEq behind an associated type. class EQ a b where type BN a b instance TypeEq a b bn => EQ a b where type BN a b = bn </haskell> results in an error <ghci> /tmp/Test.hs:9:16: Not in scope: type variable `bn' Failed, modules loaded: none. </ghci>

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ANN: permutation-0.2
by Patrick Perry 07 Dec '08

07 Dec '08

Hi Everyone, I've uploaded a new version of the permutation library to hackage. Here is the package description: This library includes data types for storing permutations. It implements pure and impure types, the latter which can be modified in-place. The main utility of the library is converting between the linear representation of a permutation to a sequence of swaps. This allows, for instance, applying a permutation or its inverse to an array with O(1) memory use. . Much of the interface for the library is based on the permutation functions in the GNU Scientific Library (GSL). Here's the url: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/permutation Patrick

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