------------------------------------------ type F a = Int
class A a where foo :: A b => a (F b) ------------------------------------------
GHC - OK Hugs - Illegal type "F b" in constructor application
This time, I'd say Hugs is wrong (though eliminating that initial complaint leads back to an ambiguous and unusable method 'foo'). 4.2.2 Type Synonym Declarations, lists only class instances as exceptions for type synonyms, and 'Int' isn't illegal there.
------------------------------------------ type F a = Int
class A a where foo :: F a
instance A Bool where foo = 1
instance A Char where foo = 2
xs = [foo :: F Bool, foo :: F Char] ------------------------------------------
GHC:
M.hs:14:6: Ambiguous type variable `a' in the constraint: `A a' arising from a use of `foo' at M.hs:14:6-8 Probable fix: add a type signature that fixes these type variable(s)
M.hs:14:21: Ambiguous type variable `a1' in the constraint: `A a1' arising from a use of `foo' at M.hs:14:21-23 Probable fix: add a type signature that fixes these type variable(s)
Hugs: [1,2]
Neither seems correct? 4.3.1 Class Declarations, says: The type of the top-level class method vi is: vi :: forall u,w. (C u, cxi) =>ti The ti must mention u; .. 'foo's type, after synonym expansion, does not mention 'a'. Claus