Re: Shortcut logicals (was: Re: F200x )

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"James Van Buskirk" <not_valid@comcast.net> writes:

> "James Giles" <jamesgiles@worldnet.att.net> wrote in message
> news:mXzLd.100$xR1.94@bgtnsc04-news.ops.worldnet.att.net...
> 
> > Well, I have the F2003 FCD open right now, in �7.4.3 Masked array
> > assignment - WHERE.  So, perhaps you can point out what part of
> > it resolves the issue I raised:
> 
> >    cond1 .and. cond2 .andthen. cond3 .and. cond4
> 
> > Is COND4 permitted to be evaluated or not?  I don't see that
> > WHERE tells me.  It has a concept of "pending control" which
> > I see can be applied on nested constructs, but in the above
> > expression, not of the terms actually present are part of any
> > "pending" condition.  Or, are you talking about some other
> > aspect of WHERE entirely?
> 
> OK, I have added comp.lang.ada, so hopefully someone there
> can answer the question about the effect of precedence on
> your snippet, and also discuss the ergonomics of Ada short-
> circuiting logical operators, or whatever they call them.

Sorry, I've no idea what the Fortran rules are.

In Ada, the above expression is syntactically illegal -- compile time
error.  You can write:

    (cond1 and cond2) and then (cond3 and cond4)

or:

    ((cond1 and cond2) and then cond3) and cond4)

for example.

"and" is just a function call.  If you say "X and Y" then X and Y are
both evaluated (in either order) and passed to the "and" function.  The
predefined version of "and" returns True if both are True.  But "and
then" is a short-circuit control form: if you say "X and then Y", X is
evaluated first, and if it's True, you're done.  Otherwise (X is False)
Y is evaluated.

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:

    A <= B and B <= C

which means the same as:

    (A <= B) and (B <= C)

The point of "and then" is so you can write things like:

    Y /= 0 and then X/Y < 10

where X/Y makes no sense if Y is zero.

- Bob