this is a bit ugly but would allow you to do if (x `isA` string) ... or (x `isA` int) ... int = int::Int string = string::String typeBind :: a -> ReadS a -> ReadS a typeBind _ x = x isA :: Read a => String -> a -> Bool isA s x = not (null (typeBind x (readsPrec 0) s)) -----Original Message----- From: Simon Marlow [mailto:simonmar@microsoft.com] Sent: 28 August 2001 15:53 To: Mark Carroll; haskell-cafe@haskell.org Subject: RE: Numeric literals
I also haven't yet worked out how to tell if a string is "read"able or not, yet - if (read "45")::Integer or whatever gives an error (e.g. if I'd put "af" instead of "45"), it seems to be pretty uncatchable, which is a pain. How do I tell if an instance of reading will work, or catch that it didn't?
In GHC, you can do this: import Exception do result <- catch (evaluate (read "foo" :: Int)) (\error -> ... ) but unfortunately read doesn't raise a useful exception (just error "Prelude.read: no parse"), so you can't filter it very easily. However I don't think that read is likely to raise any other exceptions. Cheers, Simon _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe