Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!yale!cmcl2!lanl!jlg
From: jlg@lanl.gov (Jim Giles)
Newsgroups: comp.lang.fortran
Subject: Re: <None>
Message-ID: <58379@lanl.gov>
Date: 28 Jul 90 00:01:45 GMT
References: <3250.26b00265@mccall.com>
Organization: Los Alamos Natl Lab, Los Alamos, N.M.
Lines: 45

From article <3250.26b00265@mccall.com>, by tp@mccall.com:
> In article <58237@lanl.gov>, jlg@lanl.gov (Jim Giles) writes:
>>> [...]
>>> 	X .GT. Y .OR. L(Z)
>> [...]
>> Since this started as a comparison to C, let's remember that C is just
>> as bad here.  You can't predict a-priori whether the function L() will
>> be executed in either language.  In pretty much any language, it's a
>> bad idea to have functions with side-effects in a context like this.
> 
> Not true. In the expression "x > y || l(z)", l will only be called if x is
> not greater than y. The definition of the || (or) and && (and) operators
> explicitly state that evaluation is left to right, and the right hand side
> will only be evaluated if necessary to determine the value of the
> expression.  [...]

Please _read_ people's postings before responding.  When I say "you
can't predict a-priori whether the function L() will be executed in
either language", that's what I mean.  If I could predict whether
"X>Y" or not _before_ (ie. "a-priori") I do the if-test, I wouldn't
bother to test it!

> [... lots of garbage about C's 'advantages' because it short-circuits
>  logical expressions ...]

There are two good reasons _not_ to short-circuit logical operators:

1) Mathematically, 'and' and 'or' ('/\' and '\/') are both commutative.
   They are also both associative.  It is important to preserve these
   properties in a procgamming language.  It allows maximum parallel
   evaluation of conditionals.  It allows the use of mathematical
   principles in program verification.

2) Short-circuiting is a _separate_ operation from logical 'and' or
   'or'.  Your programming language _should_ implement separate features
   in distinct ways.

The only argument _in_favor_ of having logical operators short-circuit
is that you think a terse notation is better than an explicitly written-
out notation.  Unfortunately, both C and Fortran allow logical operators
to short-circuit.  This just means that program verification is slightly
harder and code doesn't parallelize well (actually, Fortran _allows_
you not to short-circuit, but it's not portable that way).

J. Giles