Perl 6 Essentials

Subroutines

The most basic form of a subroutine is simply the sub keyword, followed by the name of the sub, followed by the block that defines the sub:

sub alert {
    print "We have normality.";
}

In a simple sub, all arguments are passed in the @_ array:

sub sum {
    my $sum;
    for @_ -> $number {
        $sum += $number;
    }
    return $sum;
}

Formal Parameters

Perl 6 subroutines can define named formal parameters. The parameter list is part of the subroutine definition, often called the “signature” of the subroutine:

sub standardize ($text, $method) {
    my $clean;
    given $method {
        when 'length' { $clean = wrap($text, 72); }
        when 'lower' { $clean = lowercase($text); }
        ...
    }
    return $clean;
}

Subroutine parameter lists are non-flattening. Any array or hash passed into a subroutine is treated as a single parameter. An array in the signature expects to be passed an actual array or arrayref, and a hash expects a hash or hashref:

sub whole (@names, %flags) {
    ...
}

# and elsewhere
whole(@array, %hash);

To get the old-style behavior where the elements of an array (or the pairs of a hash) flatten out into the parameter list, use the flattening operator in the call to the subroutine. Here, $first is bound to @array[0] and $second is bound to @array[1]:

sub flat ($first, $second) {
    ...
}

flat(*@array);

To make an array (or hash) in the parameter list slurp up all the arguments passed to it, use the flattening operator in the signature definition. These are known as variadic parameters because they can take a variable number ...