Iavor Diatchki writes:

It appears that Hugs (20050909) does not deal correctly with existentials and multi-parameter type classes:

...

class C a b where f :: a -> b

data T b = forall a. C a b => T a

test (T a) = f a

Hugs accepts this program, inferring the following type for 'test': Main> :t test test :: C _5 a => T b -> a Main> :t test (undefined :: T Char) test undefined :: C (T Char) a => a

The same bug shows without multi-parameter type classes: Hugs accepts data T = forall a. C a test (C x) = toInteger x Now fixed in CVS.