Dan Doel
There was talk of adding a readMaybe a while ago, but apparently it never happened.
As it is, you can use reads, "read s" becomes:
case reads s of [(a, rest)] | all isSpace rest -> <code using a> _ -> <error case>
which ensures that you have an unambiguous parse with only trailing whitespace. You can, of course, modify that if you don't care about ambiguity or trailing characters.
I was wondering about a more algebraic way of writing that; here's a version (that doesn't care about ambiguity) readMaybe :: Read a => String -> Maybe a readMaybe = join . fmap no_trailing_garbage . listToMaybe . reads where no_trailing_garbage = fmap fst . check (all isSpace . snd) check :: (MonadPlus m) => (a -> Bool) -> a -> m a check p a | p a = return a | otherwise = mzero I tried Hoogling for a function like check, but couldn't find it. Surely there's one in a library somewhere? It looks useful to me. (I'm rather taken by way the "check (all isSpace . snd)" part reads) Monad.guard comes close but fails to get the cigar; in fact guard b == check (const b) () So check is more general. Also, I don't see a singletonListToMaybe that one could use in place of listToMaybe to require unambiguity. Could do isSingleton [a] = True isSingleton _ = False and then use "listToMaybe . join . check isSingleton" -- aha! Another use for check! Jón [Footnote: I thought of writing "guard == flip (check . const) ()" but then realised it was pointless] -- Jón Fairbairn Jon.Fairbairn@cl.cam.ac.uk