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Yet another question about insufficient polymorphism
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Date: -- (:)
From: Richard Jones <rich@a...>
Subject: Re: [Caml-list] Yet another question about insufficient polymorphism
On Wed, Mar 12, 2008 at 07:34:32PM +0000, Richard Jones wrote:
> I have a signature like this:
> 
> newt.mli:
> 
>   type 'a component
> 
>   val form_add_component :
>     [> `Form ] component -> [> `Component ] component -> unit
>   val form_add_components :
>     [> `Form ] component -> [> `Component ] component list -> unit
> 
> An implementation like this:
> 
> newt.ml:
> 
>   type 'a component = {
>     co : newtComponent;
>     mutable in_form : bool;
>   }

[...]

> 04_form.ml:
> 
>   let text = textbox 1 1 15 5 [WRAP; SCROLL] in
>   textbox_set_text text "This is some text in a reflowed text box.";
>   let b1 = button 5 8 "  OK  " in
>   let b2 = button 5 13 "Cancel" in
>   let form = form None None [] in
>   form_add_components form [text; b1; b2];
>                                   ^^

The answer to this, pointed out to me by milanst, is to write the type
like this:

  type _component = {
    co : newtComponent;
    mutable in_form : bool;
  }
  type 'a component = _component

which works as expected, without any "casts" needed on the call to
form_add_components.

Rich.

-- 
Richard Jones
Red Hat