Assume the following type class declarations with functional dependencies:
{-# OPTIONS -fglasgow-exts #-}
class C a b c | a b -> c where foo :: (a, b) -> c
instance C a a r => C a (b, c) r where foo (a, (b, c)) = foo (a, a)
Now, in GHCi (version 6.4),
*FDs> let bar x = foo ((x, x), (x, x)) *FDs> :t bar bar :: (C (a, a) (a, a) c) => a -> c
but GHC is also happy to accept the more general type
bar :: a -> c bar x = foo ((x, x), (x, x))
Again, in GHCi,
*FDs> let bar x = foo ((x::Int, x), (x, x)) *FDs> :t bar bar :: Int -> ()
(which by itself is bizarre), but it also accepts
bar :: Int -> c bar x = foo ((x::Int, x), (x, x))
For those who expected GHC to diverge during type checking with the above class declaration, console yourself by knowing that Hugs (Feb'05) does diverge...well, it invokes the emergency break:
Hugs.Base> :l FDs ERROR "./FDs.hs" - *** The type checker has reached the cutoff limit while trying to *** determine whether: *** C (Int,Int) (Int,Int) a *** can be deduced from: *** () *** This may indicate that the problem is undecidable. However, *** you may still try to increase the cutoff limit using the -c *** option and then try again. (The current setting is -c40)
Manuel