Managing function declarations in typeclass instances (here Functor)

[ornamentist]

I'm probably missing something obvious here, but, I have a simple Set type:

 (define-type (Set :a)
    "A set of type :A elements.

     Sets are simple wrappers of lists, so they inherit all the performance
     limitations of lists. The maximum set size is system dependent."

    (Set (List :a)))

And I'd like to implement Functor for the type (just return a empty set for this example):

  (define-instance (Functor Set)

    (define (map fn set)
      "Map the function FN over SET."

      (empty-set)))

Coalton wants a constraint of (Eq :b) on the map function, which is fair enough. How do I manage type constraints in Coalton instances like this since define-instance doesn't support declare forms?

[eliaslfox]

A couple things:

Unfortunately there isn't a way to add a constraint where you want it.

The empty-set function probably doesn't need an Eq :a constraint.

I don't think sets have a valid functor instance. Map should obey (map (compose f g)) == (compose (map f) (map g)). Set's will remove elements if they compare equal. See Functor for more details.

[ornamentist]

Very true, I am pushing against the semantics of Functor, but I’m wondering if I’d have the same issue with say my own Vector or Matrix or Sequence types.

[eliaslfox]

Those other container types don't need extra constraints to map their elements.

[ornamentist]

I think I understand now why Traversable was invented (no help for Set but now I see why it's needed). Thanks again for your patient handling of Coalton discussions.