{-# LANGUAGE MultiParamTypeClasses #-} module A where class Foo a b where foo :: a -> b
instance Foo Bool Int where foo True = 1 foo False = 0
module B where import A
bar :: (Foo a b) => [a] -> [b] bar = map foo
I can load B.hs into GHCi and call bar without problems. So the import of Foo is fine. But you still get an error if you try to declare an instance of Foo in B.hs.
instance Foo Bool Integer where foo True = 1 foo False = 0
B.hs:8:0: Illegal instance declaration for `Foo Bool Integer' (Only one type can be given in an instance head. Use -XMultiParamTypeClasses if you want to allow more.) In the instance declaration for `Foo Bool Integer' Failed, modules loaded: A.
Ah, that is one definite bug waiting for a ticket, then: - inheritance of MultiParamTypeClasses is not specified - if it is inherited, the instance in B should be permitted - if it is not inherited, the context in B should not be permitted Claus