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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Press Release - Ada 2012 Language Standard Approved by ISO
Date: Wed, 19 Dec 2012 19:52:33 -0600
Organization: Jacob Sparre Andersen Research & Innovation
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Date: 2012-12-19T19:52:33-06:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
news:7wrdmbre6jw9.qww9l0uzj6mg.dlg@40tude.net...
...
> Take an instance of Ada.Numerics.Discrete_Random and do:
>
>   subtype More_Fun_Number is Integer with
>      Dynamic_Predicate => Random = More_Fun_Number;

A predicate (or any contract) that can get different answers on successive
calls is *wrong*. Ada 2012 has a permission (11.4.2(27/3)) for
implementations to reject such expressions if they can detect it. (This
isn't required simply because we don't know of any way to characterize what
should not be allowed -- that's a work item for future versions of Ada.)

One would hope that obvious cases like this take advantage of the permission
and are rejected outright.

Functions in dynamic predicates (and all of the other contract aspects, as
well) ought to be logically pure, returning the same result for the same
parameter values.

                                         Randy.