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Date: Wed, 6 Jan 2016 15:48:14 -0800 (PST)
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Subject: Re: Instantiating package problems
From: Anh Vo <anhvofrcaus@gmail.com>
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Xref: news.eternal-september.org comp.lang.ada:29038
Date: 2016-01-06T15:48:14-08:00
List-Id: <comp.lang.ada>

On Wednesday, January 6, 2016 at 6:25:47 AM UTC-8, Bob Duff wrote:
> Anh Vo <anhvofrcaus@gmail.com> writes:
>=20
> > On Tuesday, January 5, 2016 at 7:31:00 PM UTC-8, Andrew Shvets wrote:
> >> On Monday, January 4, 2016 at 3:49:32 PM UTC-5, Anh Vo wrote:
> >> > On Sunday, January 3, 2016 at 4:07:30 PM UTC-8, Andrew Shvets wrote:
> >> > > Hi Bob,
> >> > >=20
> >> > > Thanks for writing back.  This is the entire example:
> >> > > http://pastebin.com/GEJT3WzL
> >> > =20
> >> > Just recommend to add a precondition to function Division as shown b=
elow. By the way, this feature is part of latest Ada (Ada 2012). In additio=
n, you should think about adding precondition to other operations such as A=
ddition where Input1 + Input2 <=3D Integer'Last and Input1 + Input2 >=3D In=
teger'Last.=20
> >> >=20
> >> >   function Division(
> >> >     Input1 : in Integer;
> >> >     Input2 : in Integer)
> >> >       return Integer
> >> >    with pre =3D> Input2 /=3D 0;
>=20
> I prefer predicates in a case like that:
>=20
>     subtype Nonzero is Integer with Predicate =3D> Nonzero /=3D 0;
>=20
> And then Input2 can be of subtype Nonzero.  (Predicate is specific to
> GNAT; use Static_Predicate if you want to conform to the Ada standard.)
>=20
> >> I'm not sure that that worked.  I added it to my ads file and after
> >> compilation and then re-running the application, this is what I got:
> >>=20
> >> ....
> >>=20
> >>  Division:        4
> >>=20
> >>=20
> >> raised CONSTRAINT_ERROR : calculator.adb:33 divide by zero
> >>=20
> >> Shouldn't the division function been prevented from running the actual=
 division operation beforehand?
> >
> > The preconditions and postconditions are controlled by pragma
> > Assertion_Policy. Thus, pragma Assertion_Policy(Check) must be added at=
 the top
> > of the package specification. Absent of this pragma is equivalent to pr=
agma
> > Assertion_Policy (Ignore). That means these conditions are ignored. By =
the way,
> > ASSERTION_ERROR exception would be raised instead of CONSTRAINT_ERROR.
>=20
> I wouldn't bother fooling about with Assertion_Policy.
> Just use the -gnata switch.

I like to stay away from compiler dependence as much as I can. Thus, I chos=
e to stay with Ada Standard in this case.
=20
> If you do use Assertion_Policy, you probably want it in a global
> configuration file, so it applies to all of your code.

I agree that this is a good advice for large number of packages.

Anh Vo