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From: Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: ML-like alternatives to out parameters for functions
Date: Tue, 18 Mar 2003 09:37:51 +0100
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Date: 2003-03-18T09:37:51+01:00
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On 17 Mar 2003 23:17:12 +0100, Mark Lorenzen <mark.lorenzen@ofir.dk>
wrote:

>There has lately been some discussions about Ada's missing
>capabilities for specifying out parameters for functions. Apparently
>this is a needed feature and an AI (AI-323) has been issued.
>
>An out parameter can be used when the validity of the result of
>evaluating a function is not always well-defined. For example:

[snip]

>generic
>   type Value_Type is private;
>package Optional_Value is

[snip]

>   type Optional_Value_Type (Defined : Boolean := False) is
>      record
>        case Defined is
>           when True  =>
>              Value : Value_Type;
>           when False =>
>              null;
>        end case;
>      end record;

This won't work if Value_Type is unconstrained.

The beauty of Ada's functions is that they are capable to return
objects of unknown size, discriminants, specific type. Procedures lack
this very important (especially when class-wide objects are concern)
feature.

So ether (1) functions should be allowed to have in-out parameters, or
(2) procedures should be allowed to have a result.

I prefer 2 and, maybe, a RM statement that an impure function is a
bounded error.

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de