Course of Raku / Advanced / Containers / Special and dynamic variables 🆕

Dynamic variables

A dynamic variable has the * twigil. Unlike a lexical variable, which is visible only in the block where it is declared, a dynamic variable is found by looking outward through the call stack — through whoever called the current code, no matter where that code is defined.

my $*greeting = 'Hello';

sub greet {
    say $*greeting;
}

greet(); # Hello

The subroutine greet does not declare $*greeting and does not receive it as an argument, yet it can see it, because the variable is dynamic and greet was called from a place where it is in scope.

This example would work with a simple global scalar variable $greeting, but the nature of dynamic variables will be seen in the next example.

Because the lookup follows the call stack, a closer declaration overrides a farther one for the duration of its block:

my $*level = 'outer';

sub show {
    say $*level;
}

show(); # outer

{
    my $*level = 'inner';
    show(); # inner
}

The same show prints outer the first time and inner when called from inside the block that redeclared $*level. This is dynamic scoping, and it is how Raku’s built-in $*OUT, $*IN, and $*ERR work — you can temporarily redirect output by redeclaring $*OUT in a block, and every routine you call inside it sees the new value. Dynamic variables are ideal for context that should flow into called code without being passed as an explicit argument.

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