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11.7.1. Problem

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

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 reference to a hash of code references 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 in the
$bundle hash reference are returned by the
function, they are not deallocated. The next time
mkcounter is called, the closures get a different
set of variable bindings for the same code. Because no one outside
those closures can access these two variables, this assures true
privacy.

The assignment right before the return makes both
"PREV" and "LAST" values point
to the same closure. Depending on your object-oriented background,
you might think of these as being two different messages, both
implemented using the same method.

The bundle we return is not an object in that it has no obvious
inheritance and polymorphism. (Yet.) But it certainly does have
state, behavior, and identity, as well as encapsulation.