Course of Raku / Functional, concurrent, reactive, and web programming / Functional programming / Lambdas and closures

Closures

A closure is a subroutine that captures variables from the scope in which it was defined, and keeps them alive even after that scope has ended. This gives the subroutine its own private, persistent state.

The classic example is a counter:

sub make-counter {
    my $n = 0;
    return sub { ++$n };
}

my &count = make-counter;
say count(); # 1
say count(); # 2
say count(); # 3

The variable $n is declared inside make-counter. The returned subroutine refers to $n, so it closes over it: each call to count increments and returns the same $n, even though make-counter itself finished long ago.

Every call to make-counter creates a fresh $n, so separate counters are independent:

my &a = make-counter;
my &b = make-counter;
say a(); # 1
say a(); # 2
say b(); # 1

Closures let a function carry state without a global variable and without an object. They are the functional-programming way to remember something between calls.

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