Class::Agreement 0.02

Class::Agreement is a Perl module that add contracts to your Perl classes easily.

SYNOPSIS

package SomeClass;

use Class::Agreement;

# use base Class::Accessor or Class::MethodMaker,
# or roll your own:
sub new { ... }

invariant {
my ($self) = @_;
$self->count > 0;
};

precondition add_a_positive => sub {
my ( $self, $value ) = @_;
return ( $value >= 0 );
};
sub add_a_positive {
my ( $self, $value ) = @_;
...
}

sub choose_word {
my ( $self, $value ) = @_;
...
}
postcondition choose_word => sub {
return ( result >= 0 );
};

dependent increase_foo => sub {
my ( $self, $amount ) = @_;
my $old_foo = $self->foo;
return sub {
my ( $self, $amount ) = @_;
return ( $old_foo < $self->get_foo );
}
};
sub increase_foo {
my ( $self, $amount ) = @_;
$self->set_foo( $self->get_foo + $amount );
}

Class::Agreement is an implementation of behavioral contracts for Perl5. This module allows you to easily add pre- and postconditions to new or existing Perl classes.
This module provides contracts such as dependent contracts, contracts for higher-order functions, and informative messages when things fail.

At the time of this writing, Class::Agreement is one of only two contract implementations that blames contract-breaking components correctly. (See: "Object-oriented Programming Languages Need Well-founded Contracts" at http://citeseer.ist.psu.edu/findler01objectoriented.html.)
Using Class::Agreement lets you specify proper input and output of your functions or methods, thus strengthening your code and allowing you to spot bugs earlier.