Subset types

In Perl 6, you can create your own type based on exising one. Such types are called subtypes, and there there is a subset keyword for creating it. The name subset assumes that by creating a subtype you restrict the range of possible values the variable of that subtype may agree to take.

To create a named subtype, the following syntax is used:

my subset Odd of Int where {$^n % 2};

Here, Odd is the name of the new subtype, Int is the base type, and the block following where defines the rules and restricts the values of the subtype. Finally, $^n is a placeholder block variable, which corresponds to the first argument passed to the anonumous sub, which this block defines.

Later, the just created subtype may be used in place where you can use any other regular type or class name: my Odd $value.

Having said that, the variable will be only able to accept odd integer numbers on assignments. Thus, assiging the variable to 3 is legal, which an attempt to set it to 4 will result in an exception: Type check failed in assignment to '$value'; expected 'Odd' but got 'Int'.

my Odd $value;
$value = 3;
# $value = 4; # error

As the where block assumes smartmatching, it is possible to use many things there, including regexes:

my subset PerlFamily of Str where /Perl/;
my PerlFamily $language = 'Perl 6'; # OK
#$language = 'Python'; # Failure

A subtype can use also another existing subtype for further restricting the data set. Let's continue with the previously defined Odd subtype and define its subtype, which allows number betweeen 10 and 15 only:

my subset Odd of Int where {$^n % 2};
my subset OddRange of Odd where {10 < $^n < 15};

for 1..20 -> $test {
    try {
        my OddRange $x;
        $x = $test;
        say $x;

A try block is needed here to prevent the programme from exiting on incorrect assigments.

It is also possible to use anonymous subset. Here is an example of using it in a sub signature:

sub odd($x where {$x % 2}) {
    say $x;

for 1..10 {
    try odd($^x);

The programme will only print odd numbers, as it failes on passing even numbers to the odd() sub.

Using subtypes in sub signatures can be very useful in combination with milti subs. You can split variable ranges and create separate versions of the sub for handling different cases.

multi sub getnum($x where {$x % 2 == 1}) {
    say "$x is odd";

multi sub getnum($x where {$x % 2 == 0}) {
    say "$x is even";

for 1..10 {

Note that there is no try keyword anymore here because subtypes cover the whole range of values providing to the loop body: odd numbers go to one multi sub, while even ones go to another.