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From: "G.B." <bauhaus@notmyhomepage.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Ada 2012 Constraints (WRT an Ada IR)
Date: Wed, 21 Dec 2016 19:26:56 +0100
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Xref: news.eternal-september.org comp.lang.ada:32937
Date: 2016-12-21T19:26:56+01:00
List-Id: <comp.lang.ada>

On 21/12/2016 16:56, Dmitry A. Kazakov wrote:

> It is a continuous subset. This preserves a lot of properties like being a convex set a,b in S => (a+b)/2 in S etc.

The subtype in question seems to be this:

    subtype Z_Too is
      X range X'Range  -- 1 .. 100
    with Static_Predicate => Z_Too > 0;

I see no breaks there. But the subset of floors (of buildings) in the USA,
arguably reflects normal expectations:

    subtype ZZ is
      X range X'Range
    with Static_Predicate => ZZ /= 13;

    V1 : constant ZZ := 13;  -- compiler complains

But!  The Static_Predicate is *not* meant to be defining or implementing
a type that excludes 13!  Instead, it *should* be documenting a property
of the ZZ objects that a correct algorithm is using.


A useful curiosity for users of Float, maybe,

    type F is new Float with Static_Predicate => F /= 0.0;

as in
    function A (...) return F;

Then any checking before division, like in

    V := A (...);
    if V /= 0 then
       ... W / V ...;

is truly useless.
Will it be possible, in any normal project, to write a 0.0-excluding
private type in Ada, say, thus making Static_Predicate strictly
the equivalent of redundant, with all expectations on FPT still met?


> That would produce non-sets.

At compile time? Otherwise, how can a specification of membership by
any Boolean function "produce" anything but that decision, if it does
compute a result? Also, how can one predict, from a SW design POV or
otherwise, that those "run-time sets" of subtypes do not reflect what
the engineers had been carefully designing, and documenting that way?