[Haskell-cafe] Reading showables

Luke Palmer 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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