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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Not intended for use in medical,
Date: 1997/05/16
Message-ID: <EAA6so.JAz@world.std.com>#1/1
X-Deja-AN: 241956011
References: <3.0.32.19970423164855.00746db8@mail.4dcomm.com>
 <01bc6006$c13cf880$LocalHost@xhv46.dial.pipex.com> <dewar.863605491@merv>
 <01bc6182$30e3a7c0$LocalHost@xhv46.dial.pipex.com>
Organization: The World Public Access UNIX, Brookline, MA
Newsgroups: comp.lang.ada
Date: 1997-05-16T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <01bc6182$30e3a7c0$LocalHost@xhv46.dial.pipex.com>,
Nick Roberts <Nick.Roberts@dial.pipex.com> wrote:
>Quite right. I was forgetting that as the packages
>Ada.Numerics.Float_Random and Ada.Numerics.Discrete_Random are
>grandchildren of the package Ada, which is declared as pure, they must be
>pure also, ...

There is no such rule.  Ada.Numerics.Float_Random and
Ada.Numerics.Discrete_Random are in fact *not* pure.
AARM-A.5.2(27.a) explains why.

Note that all library units are (direct or indirect) children of
Standard.  And Standard is declared Pure.  So the (nonexistent) rule you
stated above wouldn't quite work -- good thing it's not in the RM!  ;-)

>...(Is this it, or is there some other reason
>that I have missed?)

As Robert said: Try to write the code using only portable features -- I
think you'll find that there's nowhere to put the Generator data, except
in the heap.  (Actually, I suppose I should say "a heap" -- you could
create your own data structure that acts as a heap.)  No fair using
Unchecked_Conversion to convert an Address (or an access-to-constant) to
an access-to-variable type -- you could do it that way on some
compilers, but it's not formally portable.  Actually, the semantics of
GNAT's 'Unrestricted_Access are defined as Unch_Conv of a 'Address --
which of course doesn't really "define" the semantics at all, since the
semantics of such an unchecked conversion is not defined.

An 'in' parameter is a constant.  If you succeed in modifying one, you
are necessarily doing something undefined!

- Bob