Consider
module Bug where
e = fmap id 1
ghci --version
The Glorious Glasgow Haskell Compilation System, version 5.02.2
says
:l Bug.hs
Compiling Bug ( Bug.hs, interpreted )
Bug.hs:5:
No instance for (Num (f a))
arising from the literal `1' at Bug.hs:5
In the second argument of `fmap', namely `1'
In the definition of `e': fmap id 1
Bug.hs:5:
Ambiguous type variable(s) `f' in the constraint `Functor f'
arising from use of `fmap' at Bug.hs:5
In the definition of `e': fmap id 1
although
:t (fmap id 1)
forall f a. (Num (f a), Functor f) => f a
Clearly, 'f' is not ambiguous.
Martin
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