Hi Pablo,
On Mon, Jul 7, 2008 at 10:07 AM, Pablo Nogueira
GHCi infers it has type (up to renaming):
(From a1 (s (a1 x)) x, Bifunctor s, To a2 (s (a2 y)) y) => (x -> y) -> a1 x -> a2 y
But if I cut and paste the type into the code I get type errors:
Could not deduce (From a1 (s1 (a11 x)) x) ... Could not deduce (From a11 (s1 (a11 x)) x, To a21 (s1 (a21 y)) y) ... Could not deduce (From a1 (s1 (a11 x)) x) ...
I myselft don't understand why GHCi doesn't accept the type it infered as an explicit signature ... but your problem seems to be caused by a lack of functional dependencies. Redefining To and From as .. class From a c x | a -> c where from :: a x -> c x class To a c y | c -> a where to :: c y -> a y ... hushes GHCi. The question now is, of course, if the new dependencies are too restrictive for your problem.