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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Everything You Know Is Wrong
Date: Tue, 29 Dec 2015 16:05:09 -0600
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Date: 2015-12-29T16:05:09-06:00
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"G.B." <bauhaus@futureapps.invalid> wrote in message 
news:n5uh6q$chh$1@dont-email.me...
> On 29.12.15 18:36, Dmitry A. Kazakov wrote:
...
>> No, because initialization of T may have side effects.
>
> I assume you meant Yes, impossible. But,
> if F1 and F2 are from a Pure package, is the compiler allowed to
> ignore side effects because the programmer specified Pure? I think
> that follows.


Ada does allow compilers to ignore side-effects for Pure functions, but only 
for successive calls to the same function with the same parameters. Which is 
not enough for your example.

And you're right, it *should* follow, but Pure is broken. For parallel 
execution, we need something stronger - and it's in the hopper today. 
(Algebraic optimizations also need something stronger.) [Pure, for instance, 
allows dereferencing of pointers, which could change by some other path. And 
making it apply to an entire package screws up organization, since most 
packages have a bunch of pure functions and many other, non-pure 
operations.] One of the reasons Ada 2012 has expression functions is that if 
the compiler can see the entire function definition, it can then do these 
sorts of optimizations. (But that's a weak solution for many reasons.)

                                 Randy.