2008/12/7 John Ky <[EMAIL PROTECTED]> > Thanks for the clarification. > > They're all the same, as you've explained: > > Prelude> putStrLn $ (show . read) "123" > *** Exception: Prelude.read: no parse
The "no parse" is an artifact of defaulting: Prelude> putstrLn $ (show . read) "()" () It's because ghci defaults the free type variables to (). The signatures for show and read are: show :: (Show a) => a -> String read :: (Read a) => String -> a So when you do show . read, you get String -> String, but where did the a's go? What type were they (what type is "read" trying to parse)? Ghci makes them (), because it's weird like that. A compiler will complain that there is an unconstrained type variable that it doesn't know what to do with. You can constrain it yourself with something like: showAs :: (Show a) => a -> a -> String showAs typ x = show (x `asTypeOf` typ) Then: Prelude> putStrLn $ (showAs (undefined::Int) . read) "123" 123 But this situation doesn't arise in practice, because usually you do something with a value you've read other than just printing it, and that something will determine the type to read. Luke
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