Allow additional constraints in typeclass methods

[Jason94]

This pull request allows typeclass methods to provide additional constraints on the typeclass variables within individual method definitions.

The motivating use case is developing the collections library, to allow for a hierarchy of collection types so that Dictionary/Map like types are collections of tuples and so that linear collections like Vectors/Lists have the same interface as maps of UFix's.

The following is currently compiling (on this branch that fixes a bug):

  (define-class (Collection :m :a (:m -> :a)))

  (define-class (Collection :m (Tuple :k :v) => KeyedCollection :m :k :v (:m -> :k :v)))

  (define-class (KeyedCollection :m UFix :a => LinearCollection_ :m :a (:m -> :a))))

However, some collection functions rely on an Eq instance for the contained type. Under Coalton's current limitations on method constraints, supporting these functions requires creating an additional typeclass:

  (define-class ((Eq :a) (Collection :m :a) => EqCollection :m :a (:m -> :a)))

This PR follows a discussion on the best way to approach this problem in the Discord.

---

The actual change is small. This was essentially already supported, most of what needed to be done was removing checks against this use case.

See the tests in the commit for examples of what will/won't compile.

[stylewarning]

@Jason94 Can you investigate the CCL errors? Allegro errors can be ignored.

[Jason94]

@Jason94 Can you investigate the CCL errors? Allegro errors can be ignored.

CCL & SBCL tests passing now 👍

[stylewarning]

@eliaslfox Do you see any issue removing this extra check?

[Jason94]

Found a regression. Previously this was typechecking, and now it's failing to unify. Fix and add a test case.

  (define-class (Collection :m :a (:m -> :a))
    (new-convert
     "Convert a collection of another type. If converting a LinearCollection to a LinearCollection, should preserve order."
     ((Collection :f :a) (itr:IntoIterator :f :a) => :f -> :m)))

    (define-instance (Collection (List :a) :a)
      (define (new-convert coll)
        (itr:collect! (itr:into-iter coll))))

;     Unknown instance for predicate ITR:INTOITERATOR #T25109 #T25379
;
;     (in form starting at line: 308, column: 0, position: 10581)

[Jason94]

This seems to be the minimal code needed to reproduce it the error. I'm not sure if it is because of the functional dependencies or having multiple typeclass parameters:

Compiles:

  (define-type TestType X Y)

  (define-class (A :a)
    (id-a (:a -> :a)))

  (define-class (B :b)
    (a-to-b (A :a => :a -> :b -> :b)))

  (define-instance (A TestType)
    (define id-a id))

  (define-instance (B TestType)
    (define (a-to-b _ b) b))

Doesn't compile:

  (define-type (Box :a)
    (Box :a))

  (define-class (ClassA :m :a (:m -> :a))
    (get-a (:m -> :a)))

  (define-class (ClassB :m :a (:m -> :a))
    (convert (ClassA :n :a => :n -> :m)))

  (define-instance (ClassA (Box :a) :a)
    (define (get-a (Box a)) a))

  (define-instance (ClassB (Box :a) :a)
    (define (convert bx)
      (Box (get-a bx))))
==>
Compile-time error:
  (during macroexpansion of (COALTON-TOPLEVEL
  (DEFINE-TYPE #
    ...)
  ...))
Unknown instance for predicate CLASSA #T23431 #T23442

[stylewarning]

changing my review status until a little more testing is done

[stylewarning]

@Jason94 I just rebased this; sorry I should have asked first. Is this PR ready for (re-)review?