Mastering Modern Perl: Writing Efficient and Maintainable Code/

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Closures

Closures

Learn about closures in Perl.

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The computer science term higher-order functions refers to functions that manipulate other functions. Every time control flow enters a function, that function gets a new environment representing that invocation’s lexical scope. That applies equally well to anonymous functions. The implication is powerful, and closures show off this power.

Creating closures

A closure is a function that uses lexical variables from an outer scope. You’ve probably already created and used closures without realizing it:

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my $filename = shift @ARGV;
sub get_filename { return $filename }

If this code seems straightforward, good! Of course the get_filename() function can see the $filename lexical. That’s how scope works!

Suppose we want to iterate over a list of items without managing the iterator ourselves. We can create a function that returns a function that, when invoked, will return the next item in the iteration:

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sub make_iterator {
    my @items = @_;
    my $count = 0;
    return sub {
        return if $count == @items;
        return $items[ $count++ ];
    }
}
my $cousins = make_iterator(qw(
    Rick Alex Kaycee Eric Corey Mandy Christine Alex
));
say $cousins->() for 1 .. 6;
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