[Haskell-cafe] Re: specialization in type classes

[Cetin Sert]

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

2009/6/5 Cetin Sert <cetin.sert at gmail.com>

> 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 at 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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