apologies if this gets to the list twice. --- hello, there seems to be something wrong with the implementation of functional dependencies, or perhaps the expansion of type synonyms. i am using hugs (september 2003). the program:
module Test where
class C a b | a -> b where mk :: IO a eq :: a -> a -> IO Bool
type F = IO
instance C a x => C [a] (F a)
test ys = do x <- mk xs <- mk eq ys (x:xs)
causes the error: ERROR "test.lhs":18 - Constraints are not consistent with functional dependency *** Constraint : C [a] (IO a a) *** And constraint : C [a] (F a) *** For class : C a b *** Break dependency : a -> b if 'type F = IO' is replaced with 'type F a = IO a' one gets *** Constraint : C [a] (IO [a]) if i inline the definition of 'F' things seem to work. if i remove the assumption on the instance: *** Constraint : C [a] (b a) if i remove the assumption on the instance, and eta-expand the type synonym: *** Constraint : C [a] (IO (IO a)) bye iavor -- ================================================== | Iavor S. Diatchki, Ph.D. student | | Department of Computer Science and Engineering | | School of OGI at OHSU | | http://www.cse.ogi.edu/~diatchki | ==================================================