Perl Cookbook

Using Closures Instead of Objects

Problem

You want records with private state, behavior, and identity, but you don’t want to learn object-oriented programming to accomplish this.

Solution

Write a function that returns (by reference) a hash of code references. These code references are all closures created in the same scope, so when they execute, they’ll all share the same set of bindings to private variables.

Discussion

Because a closure is a binding of code and data, it can implement what might be thought of as an object.

Here’s an example that creates and returns a hash of anonymous functions. mkcounter takes an argument of a seed counter and returns a hash reference that you can use to manipulate the counter indirectly.

$c1 = mkcounter(20); 
$c2 = mkcounter(77);

printf "next c1: %d\n", $c1->{NEXT}->();  # 21 
printf "next c2: %d\n", $c2->{NEXT}->();  # 78 
printf "next c1: %d\n", $c1->{NEXT}->();  # 22 
printf "last c1: %d\n", $c1->{PREV}->();  # 21 
printf "old  c2: %d\n", $c2->{RESET}->(); # 77

The code values in the hash references in $c1 and $c2 maintain their own separate state. Here’s how to set that up:

sub mkcounter {
    my $count  = shift; 
    my $start  = $count; 
    my $bundle = { 
        "NEXT"   => sub { return ++$count  }, 
        "PREV"   => sub { return --$count  }, 
        "GET"    => sub { return $count    },
        "SET"    => sub { $count = shift   }, 
        "BUMP"   => sub { $count += shift  }, 
        "RESET"  => sub { $count = $start  },
    }; 
    $bundle->{"LAST"} = $bundle->{"PREV"}; 
    return $bundle;
}

Because the lexical variables used by the closures ...