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Message-ID: <47E26D74.8040109@ed.ac.uk>
Date: Thu, 20 Mar 2008 13:58:12 +0000
From: Jeremy Yallop <jeremy.yallop@ed.ac.uk>
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To: Ralph Douglass <ralph@grayskies.net>
Cc: Caml List <caml-list@inria.fr>
Subject: Re: [Caml-list] unused variable error with objects
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X-Spam: no; 0.00; foo:01 denotes:01 foo:01 bindings:01 wrote:01 incompatible:01 caml-list:01 functions:01 int:01 expression:02 declaring:02 underscore:02 annotation:02 annotation:02 variables:02 

Ralph Douglass wrote:
> With labeled functions, I usually do something like ~snoo:_, but it does 
> not work in this exampled with methods:
> ---
> class foo = object
>   method bar ~(snoo : string):_ = ()
> end;;

In this case the ':_' denotes a type annotation, using the "any-type" 
expression '_'.  You're declaring the return type of the method, as 
you'll see if you change the annotation to something incompatible with 
unit, such as

    class foo = object
      method bar ~(snoo : string):int = ()
    end

> On a whim, I did the following, which surprisingly worked:
> 
> class foo = object
>   method bar ~(_snoo : string) = ()
> end;;

This works because the "unused variables" warning is disabled for 
variables which start with an underscore.  It's a useful feature for 
normal bindings but I don't think you should use it for labels, since it 
changes the interface.

> Is there a solution to this?

I think the following is what you want:

    class foo = object
      method bar ~snoo:(_:string) = ()
    end

Jeremy.