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From: Peng Zang <peng.zang@gmail.com>
Reply-To: peng.zang@gmail.com
To: caml-list@yquem.inria.fr
Subject: Re: [Caml-list] class constraints
Date: Fri, 27 Mar 2009 10:22:22 -0400
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Cc: Jacques Le Normand <rathereasy@gmail.com>
References: <f74178430903270706h4be1bfa3p56913b557d5a4b95@mail.gmail.com>
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X-Spam: no; 0.00; hash:01 foo:01 'self:01 'self:01 foo:01 baz:01 baz:01 abbreviation:01 subtype:01 ocaml:01 peng:98 peng:98 2009:98 lcs:01 wrote:01 

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Perhaps you mean this?

  class type foo = object('self)
    method get_parent : 'self
  end

  class type foo2 = object('self)
    constraint foo2 = #foo
    method baz : float
    method get_parent : 'self
  end


The error you get is because the first get_parent returns a foo, while the 
second returns a foo2.  While you can write a function that takes "foo or 
foo2" or really anything that is "foo-able", but you can't write a function 
that returns "foo or foo2".  When it comes to return types you have to choose 
what to return.  At least.. that's how I understand it.

Peng


On Friday 27 March 2009 10:06:03 am Jacques Le Normand wrote:
> Dear list,
> I'm having a little problem understanding constraints in objects. Consider
> the following code:
>
> class type foo =
> object
>   method get_parent : foo
> end
>
> class type foo2 =
> object
>   constraint foo2 = #foo
>   method baz : float
>   method get_parent : foo2
> end
>
> it gives me the following error:
>
> The abbreviation foo2 expands to type < baz : float; get_parent : foo2 >
> but is used with type #foo
>
> but foo2 is clearly a subtype of foo. What am I doing wrong? I'm using
> ocaml 3.10.2
> --Jacques Le Normand
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