The restriction that a method context must constrain at least one
type variable that is quantified there has biten me:
class Member c a | c -> a where member :: Eq a => c -> a -> Bool
class Member (s a) a => Sequence s a where single :: a -> s a
instance Member [a] a where member s a = elem a s
instance Sequence [] a where single x = [x]
I cannot move the constraint to the class context because it would
force Eq a on all Sequence methods. I cannot have class Member c
(where c has kind *->*) because it is used differently in other places.
Hugs does accept the above definitions.
Could this restriction be removed in ghc please?
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