Re: Control flow variables

[Damian Conway]

Luke Palmer started a discussion:


I see this idiom a lot in code.  You loop through some values on a
condition, and do something only if the condition was never true.
$is_ok is a control flow variable, something I like to minimize.  Now,
there are other ways to do this:
    if (0..6 ==> grep -> $t { abs(@new[$t] - @new[$t+1]) })
    { ... }
But one would say that's not the cleanest thing in the world.
Only because you overdid the sugar. :

    if grep {abs(@new[$^t] - @new[$^t+1]) > 3} 0..6
    { ... }
is pretty clean.

But, in any case, handling exceptional cases are what exceptions are for:

    try {
        for 0..6 -> $t {
            die if abs(@new[$t] - @new[$t+1]) > 3;
        }
        CATCH {
            push @moves: [$i, $j];
        }
    }
As regards return values of control structures, I had always assumed that:

    * scalar control structures like C<if>, C<unless>, and C<given> return
      the value of the last statement in their block that they evaluated;
    * vector control structures like C<loop>, C<while>, and C<for> in a list
      context return a list of the values of the last statement each
      iteration evaluated;
    * vector control structures like C<loop>, C<while>, and C<for> in a scalar
      context return an integer indicating the number of times their block
      was iterated.
So we might also write:

    for 0..6 -> $t {
       last if abs(@new[$t] - @new[$t+1]) > 3;
    }
    < 7 and push @moves: [$i, $j];
Damian