NAME
Mic::Contracts
SYNOPSIS
# example.pl
use Mic::Contracts
'Foo' => { all => 1 }, # all contracts are run
'Bar' => { pre => 1 }, # only preconditions are run
use Foo;
use Bar;
# do stuff with Foo and BarDESCRIPTION
Allows contracts to be enabled for a given class or interface.
An Example
The following example illustrates the use of contracts, which are assertions that constrain the visible behaviour of objects.
package Example::Contracts::BoundedQueue;
use Mic::Class
interface => {
class => {
new => {
require => {
positive_int_size => sub {
my (undef, $arg) = @_;
$arg->{max_size} =~ /^\d+$/ && $arg->{max_size} > 0;
},
},
ensure => {
zero_sized => sub {
my ($obj) = @_;
$obj->size == 0;
},
}
},
},
object => {
head => {},
tail => {},
size => {},
max_size => {},
push => {
ensure => {
size_increased => sub {
my ($self, $old) = @_;
return $self->size < $self->max_size
? $self->size == $old->size + 1
: 1;
},
tail_updated => sub {
my ($self, $old, $results, $item) = @_;
$self->tail == $item;
},
}
},
pop => {
require => {
not_empty => sub {
my ($self) = @_;
$self->size > 0;
},
},
ensure => {
returns_old_head => sub {
my ($self, $old, $results) = @_;
$results->[0] == $old->head;
},
}
},
},
invariant => {
max_size_not_exceeded => sub {
my ($self) = @_;
$self->size <= $self->max_size;
},
},
},
implementation => 'Example::Contracts::Acme::BoundedQueue_v1',
;
1;Types of Contracts
Preconditions (require)
A precondition is an assertion that is run before a given method, that defines one or more conditions that must be met in order for the given method to be callable.
Preconditions are specified using the require key of a contract definition. The corresponding value is a hash of description => subroutine pairs.
Each such subroutine is a method that receives the same parameters as the method the precondition is attached to, and returns either a true or false result. If false is returned, an exception is raised indicating which precondition was violated.
Postconditions (ensure)
A postcondition is an assertion that is run after a given method, that defines one or more conditions that must be met after the given method has been called.
Postconditions are specified using the ensure key of a contract definition. The corresponding value is a hash of description => subroutine pairs.
Each such subroutine is a method that receives the following parameters: the object as it is after the method call, the object as it was before the method call, the results of the method call stored in array ref, and any parameters that were passed to the method.
The subroutine should return either a true or false result. If false is returned, an exception is raised indicating which postcondition was violated.
Invariants
An invariant is an assertion that is run before and after every method in the interface, that defines one or more conditions that must be met before and after the method has been called.
Invariants are specified using the invariant key of a interface definition. The corresponding value is a hash of description => subroutine pairs.
Each such subroutine is a method that receives the object as its only parameter, and returns either a true or false result. If false is returned, an exception is raised indicating which invariant was violated.
Enabling Contracts
Contracts are not run by default, because they can result in many additional subroutine calls.
To enable them, use Mic::Contracts, e.g. to activate contracts for the Example::Contracts::BoundedQueue class, the following can be done:
use Mic::Contracts 'Example::Contracts::BoundedQueue' => { all => 1 };This turns on preconditions, postconditions and invariants.
use Mic::Contracts 'Example::Contracts::BoundedQueue' => { pre => 1 };turns on preconditions only.
use Mic::Contracts 'Example::Contracts::BoundedQueue' => { post => 1 };turns on postconditions only.
use Mic::Contracts 'Example::Contracts::BoundedQueue' => { invariant => 1 };turns on invariants only.
See Also
Mic::Contracts are inspired by Design by Contract in Eiffel