On 3 August 2012 08:50, Alain Frisch <alain@frisch.fr> wrote:

> Hello,
>
> I don't know if this covers all the case, but you need to keep in mind the
> following points.  Feel free to add to the list.
>

Thanks for nice and structured answer!

I'll try to expand it with my cases, but due to lack of comprehension they
will not be so clear =)

So the setup is follows: I have a class with a type explicitly specified in
mli file. No inference on it's methods. And a function, that takes some
object and type system tries to infer what objects it can accept.

Starting from a  simple case:

let reset_env_model state v =
  let q = Queue.create () in
  Queue.iter (fun p -> v#expand_row p) q

produces an error:
    This expression has type GTree.view
       but an expression was expected of type < expand_row : 'a -> unit; ..
>
       Types for method expand_row are incompatible

because expand_row method of type GTree.view has type:
   expand_row : Gtk.tree_path -> unit;

I think, that compiler have a reason to refuse this code: function wants a
method, that can take an arbitrary value of type 'a, but I pass a method
that has a covariant type Gtk.tree_path in position left to arrow. So he
complains correctly. I agree =) I think that this case has
some correspondence with your  case #1.

But, why in the following code:

let reset_env_model state v =
  let q = Queue.create () in
  let m,_,_ = State.env_model state in
  m#foreach (fun p _ -> if v#row_expanded p then Queue.push p q; false);
  set_env_model state v;
  Queue.iter (fun p -> v#expand_row p) q

compiler cannot infer, that object p has type Gtk.tree_path, and with this
proposition imply, that the expand_row method has a type "Gtk.tree_path ->
unit"?