Advanced Perl Programming

Closures

1. An anonymous subroutine that grabs lexical variables from its containing environment is a closure. Remember that it does not just take a snapshot of the value at the instant the anonymous subroutine is seen.

   # declare an anonymous subroutine, and return a reference to it.
   my $foo = 10;
   $rs = sub {
              print "Foo is $foo\n"; # Grabs $foo
         };
   &$rs();    # Call the closure through the reference

2. The closure keeps the grabbed variable’s value around even when the variable goes out of scope.

   sub init_counter {
        my $num = shift;  # lexical variable to be grabbed
        $rs = sub { print $num++," "; };
        return $rs;
   }
   $rs_counter = init_counter(10);  # $rs_counter is a ref-to-sub
   for (1..5) {&$rs_counter()};     # Prints 10 through 14