[Haskell-cafe] Reading showables
lrpalmer at gmail.com
Sun Dec 7 21:15:04 EST 2008
2008/12/7 John Ky <newhoggy at gmail.com>
> 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)
Prelude> putStrLn $ (showAs (undefined::Int) . read) "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.
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