Passing functions as arguments

Functions in Perl 6 can be passed to other functions as arguments. A typical example is a sort algorithm that needs a function to compare two values. Depending on the data types, it can be different functions that know how to compare the values of that type.

Let's examine the following tiny example:

sub less($a, $b) {$a < $b}
sub more($a, $b) {$a > $b}

sub compare($a, $b, $f) {
    $f($a, $b) 
}

say compare(10, 20, &less); # True
say compare(10, 20, &more); # False

The main code calls the compare sub with three arguments—two integer numbers and a reference to one of the functions—&less or &more. An ampersand before the name tells us that a function should not be called at this point (remember that, in Perl 6, parentheses are not required when calling a function).

Inside the compare function, the third argument...