[Haskell-beginners] type ambiguity confusion

[Stephen Tetley]

On 15 June 2010 14:57, Thomas <haskell at phirho.com> 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.

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

Best wishes

Stephen