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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Static_Predicate on array-types.
Date: Fri, 18 Jul 2014 18:05:27 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Shark8 <OneWingedShark@gmail.com> writes:

> Is there any particular reason why we cannot put static-predicates on an
> array-type, such as (e.g.) to ensure that some condition always holds?
>
> Example, we want an unbounded array, but to ensure the first index is
> always 1:
>     Type One_Based_Vector is Array(Positive Range <>) of Integer
>       with Static_Predicate => One_Based_Vector'First = 1;

What advantage would that have over using Dynamic_Predicate (or in GNAT,
Predicate)?

If you want to know the rationale for the predicate rules, take a look
at the AI that introduced them:

http://www.ada-auth.org/cgi-bin/cvsweb.cgi/ai05s/ai05-0153-3.txt

which I wrote.  You might be able to guess from the discussion that I
disagreed with the decision to have two aspects Static_Predicate and
Dynamic_Predicate.  I preferred to have a single aspect Predicate, which
is either static or dynamic depending on what the expression is.
Just like for "X: constant T := expression;", X is a static constant if
the expression is static.

There's a GNAT-specific aspect Predicate, which is equivalent to
Static_Predicate if that would be legal, and Dynamic_Predicate
otherwise.

- Bob