Perl 6 Deep Dive

Parameter traits

A sub taking arguments uses them in its body. By default, the sub's arguments are read-only values; it is not possible to modify the values inside the sub, as shown in the following code:

sub f($a) {
    $a = 0;
}

my $x = 10;
f($x);

This leads to the following error:

Cannot assign to a readonly variable ($a) or a value

There are a couple of ways of overcoming that, depending on how you intend to use the modified value. If the modified argument is only needed inside the sub, then create a copy of it, as shown here:

sub f($a) {
    my $b = $a;
    $b = 0;
    say "b = $b";
}

my $x = 10;
f($x); # b = 0

To avoid creating temporary variables, it is better to mark the sub's argument by appending the is copy trait as follows:

sub f($a is copy) ...