Improve error message for bad args to operators

[jpolitz]

fun insert<T>(e :: T, l :: List<T>)
  -> List<T>:
  cases (List) l:
    | empty => [list: e]
    | link(f, r) =>
      if e < f:
        link(e, l)
      else:
        link(f, insert(e, r))
      end
  end
end

fun isort<T>(l :: List<T>)
  -> List<T>:
  cases (List) l:
    | empty => empty
    | link(f, r) =>
      insert(f, isort(r))
  end
end

With TC on yields: The type checker rejected your program because it found a T but it expected a an object type.

I think we could do better here, and make the type that's synthesized have a little more explanation than “an object type”.

[jpolitz]

A pretty specific type is synthed (https://github.com/brownplt/pyret-lang/blob/f1103c90388a6de3d3468ea48e67bf527f568d36/src/arr/compiler/type-check.arr#L1568), so the issue must be that in constraint solving the related error just gets called “an object type” – harder to track down; that string constant appears in a few places.

[blerner]

Isn't it just failing on "T isn't an object", and never getting to the field-level constraints?

[jpolitz]

Right, I think that's what I was saying – in constraint solving, it fails on “is type variable T an object type” and gives an error with that constant string.

One idea is to, when we see “S is not object” (for any type S), we search the constraint set for field constraints and report those.

That or augment whatever obj-exists is with more information to report in constraints, even up to something that says, effectively “This one was created as a constraint for lessthan” and use that to augment the error on failure.