Course of Raku / Advanced / Operators / Traits and pragmas 🆕 / Exercises / A writable parameter

Solution: A writable parameter

Here is a possible solution to the task.

Code

sub double($n is rw) {
    $n *= 2;
}

my $score = 21;
double($score);
say $score;

🦋 You can find the source code in the file writable-parameter.raku.

Output

42

Comments

1. is rw binds the parameter $n to the caller’s variable $score, so $n *= 2 inside double changes $score itself — which is why it prints 42.

2. Without is rw, the parameter would be read-only and $n *= 2 would be a compile-time error. Marking it is copy instead would give double a private copy to modify, leaving $score untouched at 21.

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