> 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
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