The SPECIALIZE pragma doesn't do what you think; those implementations are already as specialized as they get. You can enable OverlappingInstances, but the big problem is that it doesn't really work; consider this function:

foo :: Show a => a -> String foo x = out x

question = foo "hello"

What should "question" return? "hello", or "\"hello\""? With the "dictionary-passing-style" that GHC uses for typeclasses, "foo" will always call the Show a => Out a instance of out. But that's incorrect; question passes a String which has a specific implementation of "out", and that implementation doesn't get called. The right answer is to do this:

newtype UseShow a = UseShow a instance Show a => Out (UseShow a) where out (UseShow a) = show a

Now you can write

test1 = out (UseShow 1) -- "1" test2 = out (UseShow "hello") -- "\"hello\"" test3 = out "hello" -- "hello"

Feel free to use a shorter name for UseShow, of course :) I believe there is a proposal to allow you to declare ad-hoc superclasses of typeclasses; this would let you write -- notice reversed => to <= class Show a <= Out a where out :: a -> String out = show This means that every instance of Show is required to be an instance of Out; the default implementation of "out" for types that don't have an explicit instance is given. (In the dictionary-passing world, this means that every "Show" dictionary will contain an "Out" dictionary, instead of having to construct the "out" implementation as in your instance, it could contain a different "out" function that the default) But this isn't in the language currently. -- ryan 2009/6/5 Cetin Sert :

Now there's also a stackoverflow question for this: http://stackoverflow.com/questions/955711/specialization-in-type-classes-usi... Any help highly appreciated!

2009/6/5 Cetin Sert

...

module IOStream where

import System.IO import System.IO.Unsafe

class Out a where out :: a → String

instance Show a => Out a where out = show

instance Out String where {-# SPECIALISE out :: String → String #-} out = id

instance Out Char where {-# SPECIALISE out :: Char → String #-} out = \x → [x]

infixl 0 <<, ≪ (≪), (<<) :: Out a => IO Handle → a → IO Handle (<<)= (≪) h ≪ a = do s ← h hPutStr s $ out a return s

cout, cin, cerr :: IO Handle cout = return stdout cin = return stdin cerr = return stderr

endl :: String endl = "\n"

---

cetin@unique:~/lab/c/linking/demo$ ghci -fglasgow-exts iostream.hs GHCi, version 6.10.2: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Ok, modules loaded: IOStream. Prelude IOStream> cout << 22 << False 22False{handle: <stdout>} Prelude IOStream> cout << 22 << False << endl

<interactive>:1:0: Overlapping instances for Out String arising from a use of `<<' at <interactive>:1:0-26 Matching instances: instance (Show a) => Out a -- Defined in IOStream instance Out String -- Defined in IOStream In the expression: cout << 22 << False << endl In the definition of `it': it = cout << 22 << False << endl

o________________O

how can I specialise a type class function?

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