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From: =?utf-8?Q?Yannick_Duch=C3=AAne_=28Hibou57?=
 =?utf-8?Q?=29?=
 <yannick_duchene@yahoo.fr>
Newsgroups: comp.lang.ada
Subject: Re: SPARK : surprising failure with implication
Date: Thu, 03 Jun 2010 11:06:55 +0200
Organization: Ada At Home
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Date: 2010-06-03T11:06:55+02:00
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Le Thu, 03 Jun 2010 10:54:51 +0200, Yannick Duch=C3=AAne (Hibou57)  =

<yannick_duchene@yahoo.fr> a =C3=A9crit:
> I suspect it to see Result - 1, standing for the value of Result in th=
e  =

> Assert clause (and thus as the actual expression standing for Result),=
  =

> as a monolithic subexpression. If this is really what happens, then it=
  =

> would not see (Result + 1 + 1) as (Result - 1 + 1 + 1), and instead as=
  =

> ((Result - 1) + 1 + 1), then see 1 + 1 as an expression, which is  =

> simplifies to 2, getting ((Result - 1) + 2), which it could not simpli=
fy  =

> any more, as it could not see -1 + 2 as a simplifiable expression,  =

> because -1 belongs to a subexpression.

Note for readers : if you are not aware of how SPARK and Simplifier work=
s,  =

reading =E2=80=9Csee (Result + 1 + 1) as (Result - 1 + 1 + 1)=E2=80=9D, =
you may believe  =

I'm crazy ;) So here is how you should understand it : in the first  =

expression, Result stands for its actual value, in the line starting wit=
h  =

=E2=80=9C--# check=E2=80=9D. If you read carefully, you may notice betwe=
en the line  =

starting with =E2=80=9C--# assert=E2=80=9D and the one starting with =E2=
=80=9C--# check=E2=80=9D, the is a  =

line =E2=80=9CResult :=3D Result + 1=E2=80=9D. So, the previous value of=
 Result, in terms of  =

its actual one, is =E2=80=9CResult - 1=E2=80=9D. In the proofs, SPARK se=
es Result as an  =

expression which is equivalent to its actual value. This expression is  =

relative the value of Result in the line starting with =E2=80=9C--# asse=
rt=E2=80=9D. So  =

(Result + 1 + 1) is an expression in the terms of the actual value of  =

Result, as the program see it would see, and (Result - 1 + 1 + 1) is the=
  =

same expression as SPARK see it. You may also, for better understanding,=
  =

read it as (previous-value-of-Result - 1 + 1 + 1).

Hope this note helped readers to understand the likely strange things th=
ey  =

may feel to have read here ;)

-- =

There is even better than a pragma Assert: a SPARK --# check.
--# check C and WhoKnowWhat and YouKnowWho;
--# assert Ada;
--  i.e. forget about previous premises which leads to conclusion
--  and start with new conclusion as premise.