I errorneously specified categories as

class (Eq object, Eq morphism) => Category id object morphism | id ->, id -> morphism where o :: id -> morphism -> morphism -> Maybe morphism dom, cod :: id -> morphism -> object

it should have been

class (Eq object, Eq morphism) => Category id object morphism | id -> object, id -> morphism ... ^^^^^^

- but ghci 5.02.2 does not complain. Why?

The syntax of a functional dependency is (from GHC's parser): fd : varids0 '->' varids0 varids0 : {- empty -} | varids0 tyvar so the list of tyvars on either side of the '->' can be empty. Functional dependency experts can correct me if I'm wrong, but I imagine though that 'a ->' is not a very useful functional dependency (isn't it the same as giving no functional dependency at all?), and '-> a' means that there can only ever be one instantiation for 'a' in the whole program. Cheers, Simon