Course of Raku / Advanced / Operators

User-defined operators

Operators in Raku are really just subroutines with special names. That means you can define your own, using the category names you met on the Types of Raku operators page: prefix, infix, postfix, and so on.

To declare an operator, write a sub whose name is the category, a colon, and the operator’s symbol in angle brackets. Here is a new infix operator called plus:

sub infix:<plus>($a, $b) {
    $a + $b
}

say 3 plus 4; # 7

Once defined, plus is used between its two operands, exactly like any built-in infix operator.

A postfix operator follows its operand. The factorial is a classic example — here it is as the ! postfix operator, built on the reduction meta-operator you met earlier:

sub postfix:<!>(Int $n) {
    [*] 1..$n
}

say 5!; # 120

You are not limited to letters and ASCII punctuation; an operator’s symbol can be any character. This prefix operator uses the section sign to double a number:

sub prefix:<§>($x) {
    $x * 2
}

say §5; # 10

Defining operators is a powerful tool, so use it with taste: a well-chosen operator can make code read like the problem domain, while an obscure one only puzzles the next reader.

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