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From: "Matthew Goulet" <blueherring0@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Conditional compilation of debug traces without cpp
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Date: 6 Jul 2006 22:00:47 -0700
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Date: 2006-07-06T22:00:47-07:00
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guillaume.portail@grospied.enanglais.com wrote:
> Jean-Pierre Rosen wrote:
>
> > Therefore yes, the *execution*  of the call will always involve the
> > *evaluation* of the parameters (unless the compiler can prove that this
> > evaluation has no side-effects - like a call to a function declared in a
> > pure package).
>
> Ah yes, I just read LRM95-10.2.1(18), it seems to be what I want (I
> forgot this point). But Pure is applicable only to library units. And I
> need to apply it to functions of packages :
>
> package Data is
>    type T is private
>    procedure ...
> private
>    type T is ...
> end;
>
> package Data.Image
>    subtype T_String is String(...);
>    function "+" (R : T) return T_String;
>    ...
> end;
>
>
> - Data.Image."+" has really no side effect, but it cannot be declared
> Pure. Why ? The legality of such a pragma seems statically computable
> (the body of "+" should not access 'global' variables, etc).
> - "+" is located in its own package, where UI resources are grouped, as
> to facilitate their potential elimination/counting (all are named
> *.*...*.Image).

Perhaps I misunderstand what you are saying, but declaring the packages
Pure means that the functions and procedures in them are also Pure.
So, if Data and Data.Image were Pure, so should "+", even though pragma
Pure cannot be applied to "+" directly.

There is also the new pragma Pure_Function, but I cannot recall off the
top of my head if it can be used outside of the context of a pure
package.

-Matt