Re: [Haskell-beginners] type ambiguity confusion

On Tuesday 15 June 2010 16:07:38, Stephen Tetley wrote:

On 15 June 2010 14:57, Thomas wrote:

...

However: Why does the ambiguity matter in the first place? The corresponding value is never used (\_ -> return Nothing).

Hi Thomas

GHC has to resolve all types, even if they are for values that are not used at runtime.

...

And there's another doubt: I got this example from a book (Real World Haskell, chapter 9) where actually the first version is used, without resolving the ambiguity in any way. And it does not show up in the errata list. So there's likely something I'm still missing about this issue?!?!

The type of handle in Control.Exception has (probably) changed since RWH was written.

Right.

latest (from ghc-6.10.1 onwards): handle :: Exception e => (e -> IO a) -> IO a -> IO a

old (upto ghc-6.8.3, I think, the so called Old.Exception):

Small typo, no dot there, the former Control.Exception still available as Control.OldException

handle :: (Exception -> IO a) -> IO a -> IO a

Best wishes

Stephen