mapException type under extensible exceptions

The old type is mapException :: (Exception -> Exception) -> a -> a

e.g. you might map have mapped a !DivZeroError to a dynamic exception of some sort (gee, I don't know). Of course the "dynamic exception" will be less of a hack when the code is converted to extensible exceptions.

In any case that doesn't work with the 6.9-current signature mapException :: (Exception e) => (e -> e) -> a -> a

because !DivZeroErrors are not necessarily the same type as the exceptions you're throwing, obviously!

The old signature is closer in meaning to mapException :: (SomeException -> SomeException) -> a -> a

which is isomorphic to mapException :: (Exception e1, Exception e2) => (e1 -> e2) -> a -> a

which I think is the inferred type, and ought to be the explicit type too.

Trac metadata

Trac field	Value
Version	6.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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Operating system	Unknown
Architecture	Unknown