[Melbourne-pm] given/when

[Damian Conway]

Mathew Robertson asked:

> What is the expected output of this:
> my $x = 0;
> given ($x) {
>   when (true) { print "true".$/; }
>   when (false) { print "false".$/; }
>   default { print "unknown".$/; }
> }

Expected outcome is: "true"

As explained in perlsyn:

      Most of the time, "when(EXPR)" is treated as an implicit smart match of
       $_, i.e. "$_ ~~ EXPR". (See "Smart matching in detail" for more
       information on smart matching.) But when EXPR is one of the below
       exceptional cases, it is used directly as a boolean:

       o   a subroutine or method call

Yes, it's arguably broken and/or stupid. But it's as documented.

Workaround:

    my $TRUE  = 1;
    my $FALSE = 0;
    my $x = 0;
    given ($x) {
        when ($TRUE) { print "true".$/; }
        when ($FALSE) { print "false".$/; }
        default { print "unknown".$/; }
    }

Damian