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