Confusing error message with pattern matching on polymorphic type in ae's native language

[Gbury]

Consider the following problem:

type 'a list = Cons of {elt : 'a; tail : 'a list} | Nil

function length (list : 'b) : int =
  match list with
  | Nil -> 0
  | Cons(_, l') -> length(l') + 1
end

This results in the following error message:

File "tmp.ae", line 5, character 4-7:
5 |   | Nil -> 0
        ^^^
Error The constructor Nil : ∀ 'a : Type . list('a) does not belong to type w1

Which might be surprising to users. To help clarify things, it would help if:

• the polymorphic type w1 could instead by named 'b as it is in the source
• if the messages explicits that w1 is a type variable that is quantified over in the definition of length