At 05:51 PM 04-16-2002 +0100, Piers Cawley wrote:
>Also known as constructs you wish you hadn't discovered.
>
>So, I'm reading through Finkel and I came across the following, which
>computes the greatest common divisor of a and b (recast into perl6ish
>syntax)
>
> while {
> when $a < $b { $b -= $a }
> when $b < $a { $a -= $b }
> }
>
>The idea is that the loop keeps on looping until none of the guard
>conditions match.
I believe that it also has exactly one guarded clause executed per loop, no
matter how many guard conditions are true. So the following would be legal:
while {
when $a < $b {$b -= $a; print "First clause\n"; }
when $b < $c {$c -= $b; print "Second clause\n"; }
when $c < $a {$a -= $c; print "Third clause\n"; }
}
and if it started with ($a,$b,$c) = (3, 18, 24) there is no guarantee what
order it would print in, but it would stop when ($a, $b, $c) == (3,3,3).
>So, do we want this monstrosity? I confess that I did find myself
>using something like it occasionally when I was working on the scheme
>evaluator, but it looks very weird indeed.
It's weirder when you allow multiple guard conditions to be true with no
guarantee of evaluation order. But I see no reason to disallow it.
