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Date: Fri, 02 May 2003 12:43:29 +0200
From: =?ISO-8859-1?Q?Rodrigo_Garc=EDa?= <rodrigo.garcia.ARROBA.epfl.ch>
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Subject: Re: Generic formal access types
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Date: 2003-05-02T12:43:29+02:00
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Well, it is just that null is a perfectly possible return value for my 
function and I did not want to raise an exception to process that case. 
In your example however (Get_Element), it makes sense to raise an 
exception. But imagine a list of access types, even if some of them have 
the value null, you might want to return that value as well (it is not 
that they are not in the list).

Anyway, I still think it would be great if I could write something like:

generic
    type Pointer is access (<>);

Then, the compiler would be able to check that I am actually 
instantiating the package with an access type (something I do not have 
in your solution) and I could use the "null" value as well.

Rodrigo

Ludovic Brenta wrote:
> Do you absolutely have to return null in one of the functions of your
> generic package?  How about raising an exception instead?  Something
> along the lines of:
> 
> generic
>    type Element (<>) is private;
> package List is
>    No_Element_Error: Exception;
>    type List is private;
>    function Get_Element (L : List) return Element;
> private
>    type List is ... ;
> end List;
> 
> package body List is
>    function Get_Element (L : List) return Element is
>    begin
>       raise No_Element_Error; -- instead of return null
>    end Get_Element;
> end List;
> 
> I'm sure you've thought about that, but I'm curious to know why it
> wouldn't work for you.
> 
> --
> Ludovic Brenta.