Hugs -98 (Feb 2001) rejects the following program (which should be well-typed)
> foo :: ((?x :: Int) => b) -> Int -> b
> foo s z = s with ?x = z
with the message
ERROR Bug.hs:2 - Inferred type is not general enough
*** Expression : foo
*** Expected type : ((?x :: Int) => a) -> Int -> a
*** Inferred type : ((?x :: Int) => a) -> Int -> Int
It seems to unify the type of the implicit parameter with the type of
the result.
GHC accepts this program (but generates wrong code).
John Hughes
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