class Label

Tagged location in the source code

class Label {}

Labels are used in Perl 6 to tag loops so that you can specify the one you want to jump to with statements such as last. You can use it to jump out of loops and get to outer ones, instead of just exiting the current loop or going to the statement before.

USERS:          # the label 
for @users -> $u {
    for $u.pets -> $pet {
        # usage of a label 
        next USERS if $pet.barks;
    }
    say "None of {$u}'s pets barks";
}
say USERS.^name;        # OUTPUT: «Label␤» 

Those label are objects of type Label, as shown in the last statement. Labels can be used in any loop construct, as long as they appear right before the loop statement.

my $x = 0;
my $y = 0;
my $t = '';
A: while $x++ < 2 {
    $t ~= "A$x";
    B: while $y++ < 2 {
        $t ~= "B$y";
        redo A if $y++ == 1;
        last A
    }
}
say $t# OUTPUT: «A1B1A1A2» 

Putting them on the line before the loop or the same line is optional. Labels must follow the syntax of ordinary identifiers, although traditionally we will use the latin alphabet in uppercase so that they stand out in the source. You can use, however, other alphabets like here:

駱駝道: while True {
  say 駱駝道.name;
  last 駱駝道;
}

Methods

method name

Defined as:

method name()

Not terribly useful, returns the name of the defined label:

A: while True {
  say A.name# OUTPUT: «A» 
  last A;
}

method next

Defined as:

method next(Label:)

Begin the next iteration of the loop associated with the label.

MY-LABEL:
for 1..10 {
    next MY-LABEL if $_ < 5;
    print "$_ ";
}
 
# OUTPUT: «5 6 7 8 9 10 » 

method redo

Defined as:

method redo(Label:)

Repeat the same iteration of the loop associated with the label.

my $has-repeated = False;
 
MY-LABEL:
for 1..10 {
    print "$_ ";
    if $_ == 5 {
        LEAVE $has-repeated = True;
        redo MY-LABEL unless $has-repeated;
    }
}
 
# OUTPUT: «1 2 3 4 5 5 6 7 8 9 10 » 

method last

Defined as:

method last(Label:)

Terminate the execution of the loop associated with the label.

MY-LABEL:
for 1..10 {
    last MY-LABEL if $_ > 5;
    print "$_ ";
}
 
# OUTPUT: «1 2 3 4 5 » 

Type Graph

Type relations for Label
perl6-type-graph Label Label Any Any Label->Any Mu Mu Any->Mu

Expand above chart