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It seems ghc allows a syntax for types which if beyond Haskell98 even without -fglasgow-exts. Consider the following program:

destroy :: (forall a. (a -> Maybe (b,a)) -> a -> c) -> [b] -> c
destroy g xs = g listpsi xs
  where listpsi :: [a] -> Maybe (a,[a])
        listpsi [] = Nothing
        listpsi (x:xs) = Just (x,xs)

It gives the following rather cryptic error message without -fglasgow-exts:

ConcatMap.hs:7:17:
    Couldn't match expected type `.' (a rigid variable)
           against inferred type `(->)'
      `.' is bound by the type signature for `destroy'
        at ConcatMap.hs:6:20
    Probable cause: `listpsi' is applied to too few arguments
    In the first argument of `g', namely `listpsi'
    In the expression: g listpsi xs

The fact that ghc interpreted `.' as a type variable baffeled me for a moment. I would expect ghc to report a syntax error since the type is not syntactically Haskell98.

Trac metadata

Trac field	Value
Version	6.5
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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Operating system	Unknown
Architecture	Unknown