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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: The future of Spark . Spark 2014 : a wreckage
Date: Fri, 12 Jul 2013 08:25:15 +0100
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Date: 2013-07-12T08:25:15+01:00
List-Id: <comp.lang.ada>

"Randy Brukardt" <randy@rrsoftware.com> writes:

> GNAT has a pragma Pure_Function which can be used on a single
> function. This isn't in Ada because it is just an assumption; there is
> no checks at all whether it is true. In that case, any mistake in the
> function would make a program calling it erroneous (because the
> compiler is allowed to assume the function is pure, even if it
> isn't). It can only be made safe if it is combined with a proof that
> there are no side-effects, in which case it surely should never be
> separated from that proof -- but pragmas are *always* separated from
> the thing they apply to. As such, they are never appropriate for
> setting properties of single entities.

GNAT supports Pure_Function as an aspect; so you can write

   package Pf is
      function F (X : Integer) return Integer with Pure_Function;
   end Pf;