Thanks for the clarification.

They're all the same, as you've explained:

Prelude> putStrLn $ (show . read) "123"
*** Exception: Prelude.read: no parse

Prelude> putStrLn $ show $ read "123"
*** Exception: Prelude.read: no parse

Prelude> putStrLn $ (\x -> show (read x)) "123"
*** Exception: Prelude.read: no parse

-John

On Mon, Dec 8, 2008 at 12:08 PM, Jonathan Cast <jonathanccast@fastmail.fm> wrote:
On Mon, 2008-12-08 at 11:16 +1100, John Ky wrote:
> Hi Thomas,
>
> So "show . read" and "\x -> show (read x)" are actually mean different
> things?

No.  Of course not.  But there's no guarantee that

   show (read x) = x

either.

> Also, I never suspected that something like this should succeed:
>
> putStrLn $ (read . show) $ "!@#%$^DFD"

Of course it succeeds.  You put the `show' first; show always succeeds
and --- for the Show instances in the Prelude, plus some ---

  read (show x) = x

for finite, total x.

(Note that read . show /= show . read; they don't even have the same
type!

 show . read :: forall alpha. Show alpha => String -> String
 read . show :: forall alpha beta. (Read alpha, Show beta) => alpha ->
beta

NB: The reason why show . read is illegal should be screaming out at you
about now.  The caveat --- other than the one I mentioned above --- to
claims that read . show = id should also be screaming out.)

jcc