On March 3, 2010 21:10:56 Daniel Fischer wrote:
Well, GHC takes only the class head into account for instance selection, and
u -> (v -> w)
matches both,
a -> b -- (a == u, b == v -> w)
and
m (c -> d) -- (m == ((->) u), c == v, d == w),
... <snip> ...
are indeed conflicting, so you can't even use OverlappingInstances etc. to make it work.
Thanks very much for the explanation. I had only read 7.7 (Type families) from the GHC manual. After reading what you wrote and all of 7.6 (Class and instances declarations) a couple of times I think I've got it. As it says in 7.7.2.2.2, "[t]he instance declarations of a type family used in a single program may only overlap if the right-hand sides of the overlapping instances coincide for the overlapping types." Cheers! -Tyson