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From: phil.clayton@lineone.net
Newsgroups: comp.lang.ada
Subject: Re: Quantified expressions: no support for restriction predicates
Date: Tue, 1 May 2012 04:35:23 -0700 (PDT)
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Date: 2012-05-01T04:35:23-07:00
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On Sunday, April 29, 2012 8:35:18 PM UTC+1, Robert A Duff wrote:
> phil.c...@lineone.net writes:
>=20
> > Is that actually possible for a Dynamic_Predicate?  Section 3.2.4 Subty=
pe Predicates, para 27/3 says:
> >
> >   The discrete_subtype_definition of a loop_parameter_specification
> >   shall not denote ... or any subtype to which Dynamic_Predicate
> >   specifications apply.
>=20
> Yeah, I think you're right.  The idea is that we didn't want hidden
> inefficiencies.
>=20
>     type T is new Integer with
>         Dynamic_Predicate =3D> T mod 10_000 =3D 0;
>     for X in T loop -- Illegal!
>=20
> If that were legal, it would probably be grossly inefficient.

Right, this is the key point.  Writing pre- and postconditions where many e=
lements are quantified over but immediately ignored due to a 'restriction p=
redicate' is not good style, given these conditions are to be evaluated.  I=
t would be preferable to quantify over just the elements that satisfy the r=
estriction predicate in the first place.

For a subtype with a static predicate, presumably there is no efficiency is=
sue because the restrictions placed on a static predicate enable the compil=
er to generate code that skips the elements not satisfying the predicate.

For static analysis, one would not worry about efficiency of evaluation and=
 restriction predicates are common-place. =20

Phil