[Haskell-cafe] Reading showables

Jonathan Cast jonathanccast at fastmail.fm
Sun Dec 7 20:08:33 EST 2008

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


> 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

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

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.)


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