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closures
Combined from primary sources listed below.

In Functions§

See primary documentation in context for Closures.

All code objects in Raku are closures, which means they can reference lexical variables from an outer scope.

sub generate-sub($x) {
    my $y = 2 * $x;
    return sub { say $y };
    #      ^^^^^^^^^^^^^^  inner sub, uses $y
}
my $generated = generate-sub(21);
$generated(); # OUTPUT: «42␤»

Here, $y is a lexical variable inside generate-sub, and the inner subroutine that is returned uses it. By the time that inner sub is called, generate-sub has already exited. Yet the inner sub can still use $y, because it closed over the variable.

Another closure example is the use of map to multiply a list of numbers:

my $multiply-by = 5;
say join ', ', map { $_ * $multiply-by }, 1..5;     # OUTPUT: «5, 10, 15, 20, 25␤»

Here, the block passed to map references the variable $multiply-by from the outer scope, making the block a closure.

Languages without closures cannot easily provide higher-order functions that are as easy to use and powerful as map.