[Caml-list] Equality between abstract type definitions

[Peter Frey <pjfrey@sympatico.ca>]

Please consider the following two definitions of map which I would have
considered equal.  

  exception Empty

  type 'a t = Cons of 'a * (int -> 'a t) * int

  let null = Cons( Obj.magic None, (fun _ -> assert false), max_int ) 

  let map (f: 'a -> 'b) (t: 'a t) : 'b t =
      if t == null then null else match t with (Cons( h, fn, p )) ->
      let rec aux h1 p1 =
      try match fn p1 with (Cons (h2,_, p2)) -> 
         (Cons(f h1, aux h2, p2 ))
      with Empty                             -> (Cons(f h1, fn, p1)) in
      aux h p

  let map : 'a 'b. ( 'a -> 'b ) -> 'a t -> 'b t = fun f t -> 
    if t == null then null else match t with (Cons( h, fn, p )) ->
    let rec aux h1 p1 =
      try match fn p1 with (Cons (h2,_, p2)) -> (Cons(f h1, aux h2, 2 ))
    with Empty                               -> (Cons(f h1, fn, p1)) in
    aux h p

The second one gives the error below; no surprise.

Error: This definition has type 'b. ('b -> 'b) -> 'b t -> 'b t
       which is less general than 'a 'b. ('a -> 'b) -> 'a t -> 'b t

Why is the first definition accepted?  

Peter Frey