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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Beginning Ada Programming, by Andrew T. Shvets (2020)
Date: Fri, 6 Dec 2019 18:47:05 -0600
Organization: JSA Research & Innovation
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Xref: reader01.eternal-september.org comp.lang.ada:57679
Date: 2019-12-06T18:47:05-06:00
List-Id: <comp.lang.ada>

"Shark8" <onewingedshark@gmail.com> wrote in message 
news:b08cad8b-eea0-435d-8058-19321affe42c@googlegroups.com...
...
>Example:
>Function Copy( Object : Password ) return Password is (Object);
>We know that passing in parameter object, the item must be conformant to
>Password [checked on call) and so can copy and pass out the Password
>without checking its validity... MORE, given a variable X --X :
>Password:= Create_Password("ThisPassword#1!")-- we know that X must
>conform to the definition, so then Y : Password:= Copy( X ); needn't make
>any check at all.

Such an optimization is not true in the most general case, since the 
predicate might have a side effect or have its result changed by some side 
effect of another operation. In this case, of course, it could be done. 
There's a rule that implies that predicates (and other assertions) that 
change values when only assertion expressions are being evaluated can be 
rejected (and thus aren't portable), but that does not apply if some other 
operation occurs in between the evaluations (such as the call to Copy here).

                                  Randy.


                                       Randy.