type synonym or fun dep problem

[Iavor Diatchki]

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               |
==================================================