Perl 6 Deep Dive

Multi subs

The signature is an important property of a sub. It not only helps to check the types of the arguments, but Perl 6 also uses it to control the number of arguments passed. For example, declare a function for summation that takes three parameters, but call it with only two arguments:

sub add($x, $y, $z) {
    return $x + $y + $z;
}

say add(1, 2);

This program does not work. Again, signature is our friend:

===SORRY!=== Error while compiling add.pl
Calling add(Int, Int) will never work with declared signature ($x, $y, $z)
at add.pl:5

So, we see that when deciding which function to call, Perl 6 takes into account the number of the arguments as well as their types together with the name of the sub. A programmer can benefit from this feature ...