Ambiguous type variable error msg

Question

I don\'t think it is a bug, but I am a bit puzzled as to why that doesn\'t work. A bonus question is why does it mention variable e? There is no variable e.

    P         


        

Answer 1

"Exception e" is likely from the type signature of "handle".

The documentation says:

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

In GHC 6.8 it used to be different, which would explain why I don't get that error.

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

Seems you're running into the monomorphism restriction. That "_"-Pattern must be monomorphic (which it is with ghc 6.8) or explicitly typed. A "workaround" is to put the pattern on the left hand side of a definition, where it constitutes a "simple pattern binding" as specified by the Haskell Report.

Try this:

let f _ = return "err"
handle f undefined

http://www.haskell.org/haskellwiki/Monomorphism_restriction