there seems to be something wrong with the implementation of functional dependencies, or perhaps the expansion of type synonyms. i am using hugs from july 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 | ==================================================
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