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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada,comp.lang.fortran
Subject: Re: Shortcut logicals (was: Re: F200x )
Date: 01 Feb 2005 08:57:54 -0500
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2005-02-01T08:57:54-05:00
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Martin Krischik <martin@krischik.com> writes:

> Christoph Grein wrote:
> 
> >  > Robert A Duff wrote:
> >  >
> >  > > And the syntax rules forbid mixing "and" and "and then" -- you have
> >  > > to use
> >  > > parens.??All?of?these?("and",?"and?then",?"or",?"or?else",?"xor") are
> >  > > lower precedence than most other operators in Ada.??So?you?can write:
> >  >
> >  > Are you sure? I did not find anything in the RM to support that. I
> > checked
> >  > both the RM and AARM:
> 
> > of course Robert is sure, if not, who else - you should know, he's
> > deeply involved in Ada development.

I wouldn't say "of course" -- after all, Martin Dowie pointed out that I
got the run time semantics wrong.  ;-)

> Ok you are right,  I am a just bit weak on names.
> 
> > See RM 4.4(2..7).
> 
> For our friends from comp.lang.fortran: You can read it up at
> http://www.adaic.com/standards/95lrm/html/RM-4-4.html:
>  
> > You'll find
> > relation and relation gives an expression,
> > relation and then relation also gives an expression.
> 
> > But there is no syntax rule to combine an expression with a relation.
> > You can only combine a parenthesized expression (a primary) with a
> > relation.
> 
> Sure, you are right. But that rule means that:
> 
>  X or Y and Z
> 
> is not valid either.

Yes, that's what it means.  You have to say:

    (X or Y) and Z
or
    X or (Y and Z)

in Ada.  In maths, folks usually presume that "and" has higher
precedence than "or".  I'm pretty happy with the Ada rule, though.  In
general, I don't think programming languages should use precedence and
associativity rules beyond the ones folks know by age 9.  Many people
disagree with that, though -- they think "extra" parens cause too much
clutter, and make the code hard to read.

- Bob