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

This problem shows up only in GHC 6.10; it can't be duplicated in GHC 6.8 because the type of handle is different:

: nr@homedog 620 ; ghci
GHCi, version 6.8.2: http://www.haskell.org/ghc/  :? for help
Loading package base ... linking ... done.
Prelude> :m +Control.Exception
Prelude Control.Exception>  handle (\_ -> return "err") undefined
"err"
Prelude Control.Exception> 

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OK maybe I can get this right at last. I think the problem is not the monomorphism restriction, but rather you've hit an instance of the Read/Show problem: you're offering to handle some type of exception, in the new version of `handle, there is more than one type of exception, and the type of that exception does not appear in your result. So the compiler has no way of knowing which type of exception you're trying to handle. One way to work this is to pick one. Here's some code that works:

Prelude Control.Exception> let alwaysError :: SomeException -> IO String; alwaysError = \_ -> return "err"
Prelude Control.Exception> handle alwaysError undefined
"err"

Incidentally, the example use of handle in the GHC library documentation does not compile under 6.10. I have filed a bug report.