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From: Adam Beneschan <adam@irvine.com>
Newsgroups: comp.lang.ada
Subject: Re: and then... (a curiosity)
Date: Tue, 2 Sep 2008 08:06:49 -0700 (PDT)
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Date: 2008-09-02T08:06:49-07:00
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On Aug 30, 8:35 am, "Peter C. Chapin" <pcc482...@gmail.com> wrote:
> Dmitry A. Kazakov wrote:
> > I am unsure if the arguments of "and" are eager. You are a language lawyer,
> > I am not. But I used to think that the eagerness of evaluation is rather
> > unspecified, so that the compiler is free "not to care," when it optimizes
> > the code, as Jeff Carter has pointed out.
>
> I thought all function arguments must be evaluated, but that it is
> unspecified in which order this evaluation is done.

Correct.  If an argument to a subprogram contains another function
call that has side-effects, that argument must be evaluated, and the
compiler is generating incorrect code if it doesn't (which is *not*
the case with "or else").  I think that if the compiler can determine
that the only possible side effect would be to raise a predefined
exception, the compiler doesn't have to actually do any evaluation---
the code isn't required to perform computations just for the sake of
doing the computations.  I'm not sure how far the predefined-exception
thing goes, though.  If you say

   if X = null or X.ID < 0 then ...    -- "or", not "or else"!

even though the right side is supposed to be evaluated, I suppose that
a compiler could in theory generate code that doesn't bother to
evaluate it if X is null, because there would be no point to reading
that field (assuming no memory-mapped I/O is involved here!!).  I
think that behavior is allowed by the Implementation Permissions in
11.6, but of course it would be quite foolish to write code like this
and rely on the optimization.

                               -- Adam