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Newsgroups: comp.lang.ada
Date: Tue, 5 Jan 2016 20:51:57 -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:29024
Date: 2016-01-05T20:51:57-08:00
List-Id: <comp.lang.ada>

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 belo=
w. By the way, this feature is part of latest Ada (Ada 2012). In addition, =
you should think about adding precondition to other operations such as Addi=
tion where Input1 + Input2 <=3D Integer'Last and Input1 + Input2 >=3D Integ=
er'Last.=20
> >=20
> >   function Division(
> >     Input1 : in Integer;
> >     Input2 : in Integer)
> >       return Integer
> >    with pre =3D> Input2 /=3D 0;
> >=20
> > Anh Vo
>=20
> I'm not sure that that worked.  I added it to my ads file and after compi=
lation 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 di=
vision operation beforehand?

The preconditions and postconditions are controlled by pragma Assertion_Pol=
icy. Thus, pragma Assertion_Policy(Check) must be added at the top of the p=
ackage specification. Absent of this pragma is equivalent to pragma Asserti=
on_Policy (Ignore). That means these conditions are ignored. By the way, AS=
SERTION_ERROR exception would be raised instead of CONSTRAINT_ERROR.