Date: Mon, 4 Apr 2005 17:17:40 +1200
From: Tim Smith <tim at desert.net>
To: LablGTK List <lablgtk at math.nagoya-u.ac.jp>
Subject: odd type annotation needed; why?
Message-ID: <20050404051740.GB816 at siva.hindu.god>
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Hi!  I asked this question on ocaml_beginners at  list, but no one was able
to respond.  I'm hoping someone here will know.  Thanks for any answers!

I have a simple program here:

http://www.magnesium.net/~thim/tmp/hello3.ml

If I compile it as is, it works fine.  But I had to add an odd type
annotation to my function, and I know why.  My function is:

let pack_new_button_in_box (box:GPack.box) str =
    let button = GButton.button ~label:str ~packing:box#pack () in
    ignore (button#connect#clicked ~callback:(clicked str))

I call it like:

    let box1 = GPack.hbox ~packing: window#add () in
    pack_new_button_in_box box1 "A Button";

Why do I need to force the 'box' argument to be of type GPack.box?  Why
can't OCaml sort that out on its own?

If I change it to just 'box', then I get this error:

ocamlc -verbose -I +lablgtk2 lablgtk.cma gtkInit.cmo hello3.ml -o hello3
File "hello3.ml", line 30, characters 27-31:
This expression has type GPack.box but is here used with type
  < pack : GObj.widget -> unit; .. >
Types for method pack are incompatible

Thanks,

Tim
-- 
If you're not part of the solution, you're part of the precipitate.