Course of Raku / Advanced / Operators / Traits and pragmas 🆕

Built-in traits

A trait is applied with the is keyword right after a declaration. It runs at compile time and changes some fixed property of the thing it is attached to. Raku ships with many; you have already used a few.

By default, a subroutine’s parameters are read-only — you cannot assign to them inside the routine. The is copy trait gives you a private, modifiable copy of the argument, which you can change without affecting the caller:

sub greet($name is copy) {
    $name = "dear $name";
    say "Hello, $name!";
}

my $who = 'Anna';
greet($who); # Hello, dear Anna!
say $who;    # Anna — the caller's own variable is untouched

The is rw trait goes further: it binds the parameter to the caller’s variable, so a change made inside the routine is visible outside:

sub bump($n is rw) {
    $n++;
}

my $x = 10;
bump($x);
say $x; # 11

Without one of these traits, $n++ would be a compile-time error, because the parameter would be read-only.

Another common trait sets a default value:

my $port is default(8080);
say $port; # 8080

Here is default gives the variable a value to fall back on. Each built-in trait — is rw, is copy, is default, and more — attaches one specific, compile-time behaviour to a declaration. The next topic shows that traits are not a closed set: you can define your own.

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