- Haskell-Cafe - Haskell.org

Cabal: defaultMainNoRead and source file location
by Mauricio 09 Dec '08

09 Dec '08

Hi, I've just seen this from Distribution.ModuleName (ghc 6.10): toFilePath $ ( simple "A.B.C" ) to which ghci answers: "A.B.C". Shouldn't it say "A/B/C"? The reason why I'm asking is that I've just created a Setup.hs with 'defaultMainNoRead', and 'Setup build' complains it can't find A.B.C. When I copy (move doesn't work) C.hs from <src_dir>/A/B to <src_dir>/A.B.C.hs, it doesn't complain, although it fails to generate my library saying: [1 of 1] Compiling A.B.C ( src/A/B/C.hs, dist/build/A/B/C.o) /usr/bin/ar: creating dist/build/libHSabc-0.a /usr/bin/ar: dist/build/A.B.C.o: No such file or directory Thanks for your tips, Maurício

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How to define Show [MyType] ?
by Dmitri O.Kondratiev 09 Dec '08

09 Dec '08

I am trying to define instance Show[MyType] so show (x:xs :: MyType) would return a single string where substrings corresponding to list elements will be separated by "\n". This would allow pretty printing of MyType list in several lines instead of one, as default Show does for lists. For example: data ShipInfo = Ship { name :: String, kind :: String, canons :: Int } deriving Show s1 = Ship {name ="HMS Fly", kind = "sloop", canons=16} s2 = Ship {name ="HMS Surprise", kind = "frigate", canons=42} -- Yet when I try to define: instance (Show ShipInfo) => Show [ShipInfo] where show (x:xs) = "<" ++ show x ++ ">" ++ show xs -- I get this error: Illegal instance declaration for `Show [ShipInfo]' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `Show [ShipInfo]' Failed, modules loaded: none. -- On the other hand this definition: instance (Show a) => Show [a] where show (x:xs) = "<" ++ show x ++ ">" ++ show xs -- Gives a different error: Duplicate instance declarations: instance (Show a) => Show [a] -- Defined at C:/wks/haskell-wks/ShowMatrix.hs:37:0 instance (Show a) => Show [a] -- Defined in GHC.Show Failed, modules loaded: none. -- Note the last error implicitly tells us that defining Show [a] is possible in principle! -- In fact it already defined in GHC.Show !!! -- How to define Show [MyType] ? Thanks! Dmitri

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Data.Map add only if not exist and return true if added
by John Ky 09 Dec '08

09 Dec '08

Hi, I'm looking for a function for Data.Map that will insert a new key and value into the map if the key doesn't already exist in the map. When the key already exists, I don't want the value updated in the map. Additionally, I want to know whether the key/value was inserted or not so that I can return that fact as a boolean from my function. I can't work out which function in Data.Map is suitable for this task and I wanted to avoid two lookups. Thanks, -John

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Why does "instance Ord Pat" causes <<loop>>
by Martin Hofmann 09 Dec '08

09 Dec '08

I am storing the TH data types 'Exp' and 'Pat' in Maps and Sets. As a first attempt to quickly get rid of typechecker's complaints I defined some naive instances of Ord for Exp and Pat. Now it took me about a week to realise, that 'instance Ord Pat' causes ghc to loop. Apparently, there is a reason, why Pat and Exp are not instances of Ord. What is so bad about it? If Pat and Exp should not be instances of Ord, maybe a note in the source code or haddock would be helpful. On the other hand, what would argue against a lexicographic ordering (apart from the inefficient implementation of the following one)? Following some literate code to reproduce the <<loop>> (or stack overflow in GHCi), by commenting and uncommenting the appropriate lines: > {-# OPTIONS_GHC -fglasgow-exts -fth #-} > module Test where > import Language.Haskell.TH > import Data.Set ------------------- naive Ord > instance Ord Exp > instance Ord Pat ------------------- lexicographic Ord instance Ord Exp where compare l r = compare (show l) (show r) instance Ord Pat where compare l r = compare (show l) (show r) ------------------- > mkVP s = VarP $ mkName s > mkVE s = VarE $ mkName s > rule1 = (,) [mkVP "x_14"] (mkVE "y_14") > rule2 = (,) [InfixP (mkVP "x1_15") '(:) (mkVP "x_16")] (InfixE (Just (mkVE "y1_15")) (ConE '(:)) (Just (mkVE "ys_16"))) > stack_overflow = fromList [rule1,rule2] Thanks, Martin

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Reading showables
by John Ky 09 Dec '08

09 Dec '08

Hi, Is there a way to read Showables? main = do putStrLn $ show $ read Thanks -John

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Dr Dobbs: Time to get good at functional programming
by Jeff Heard 09 Dec '08

09 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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