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Newsgroups: comp.lang.ada
Subject: Re: and then... (a curiosity)
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From: Stephen Leake <stephen_leake@stephe-leake.org>
Date: Tue, 02 Sep 2008 21:24:36 -0400
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Date: 2008-09-02T21:24:36-04:00
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stefan-lucks@see-the.signature writes:

> True. But the case that you want both expressions to be evaluated is a 
> rare exception. On a *logic* level, "A and B" implies "if A is false, the 
> result is false, regardless of B". 

It can also be read "if B is false, the result is false, regardless of
 A".

Which is precisely why the Ada semantics are what they are; 

    in some unspecified order: evaluate A, evaluate B
    compute 'and' of result

> So short-circuit evaluation is a natural 

By 'natural' I suspect you really mean 'what I'm used to from
experience'.

That's valid, but it's not like it's built into the human genome, or
dicated by the laws of physics. It's more of a cultural issue, and
therefore it varies from programmer to programmer.

> and good thing for a programming language, 

Why is it "good"?

Being the same as what you are used to makes it easy. But when
comparing languages, you need more than that. 

Something like:

    Given two sets of typical programmers, one thoroughly familiar
    with language Red, which has C-like boolean operator semantics,
    and the other thoroughly familiar with language Green, which has
    Ada-like boolean operator semantics, the number of errors made in
    using boolean operators is higher in Red than Green.

or vice-versa :). I haven't seen anyone make a claim like that.

Then you need to get the sets of programmers from China, India, and
Serbia to be really valid :).

> and it is regrettable that Ada needs a more complex syntax for this.
> (Well, "A and B" also implies "if B is false, don't care about A",
> but in a programming language you can't have it both ways.)

Ah, but you can! That's what speculative execution gives you. Ada
allows for that, under user control. In C, you must use nested 'if' to
achieve control.

-- 
-- Stephe