NAME

MooX::Pression - express yourself through moo

SYNOPSIS

MyApp.pm

  use v5.18;
  use strict;
  use warnings;
  
  package MyApp {
    use MooX::Pression;
    
    class Person {
      has name   ( type => Str, required => true );
      has gender ( type => Str );
      
      factory new_man (Str $name) {
        return $class->new(name => $name, gender => 'male');
      }
      
      factory new_woman (Str $name) {
        return $class->new(name => $name, gender => 'female');
      }
      
      method greet (Person *friend, Str *greeting = "Hello") {
        printf("%s, %s!\n", $arg->greeting, $arg->friend->name);
      }
      
      coerce from Str via from_string {
        return $class->new(name => $_);
      }
    }
  }

my_script.pl

  use v5.18;
  use strict;
  use warnings;
  use MyApp;
  use MyApp::Types qw( is_Person );
  
  # Create a new MyApp::Person object.
  #
  my $alice  = MyApp->new_woman("Alice");
  is_Person($alice) or die;
  
  # The string "Bob" will be coerced to a MyApp::Person.
  #
  $alice->greet(friend => "Bob", greeting => 'Hi');

DESCRIPTION

MooX::Pression is kind of like Moops; a marrying together of Moo with Type::Tiny and some keyword declaration magic. Instead of being built on Kavorka, Parse::Keyword, Keyword::Simple and a whole heap of crack, it is built on MooX::Press, Keyword::Simple, and PPR. I'm not saying there isn't some crazy stuff going on under the hood, but it ought to be a little more maintainable.

Some of the insane features of Moops have been dialled back, and others have been amped up.

It's more opinionated about API design and usage than Moops is, but in most cases, it should be fairly easy to port Moops code to MooX::Pression.

MooX::Pression requires Perl 5.18.0 or above. It may work on Perl 5.14.x and Perl 5.16.x partly, but there are likely to be issues.

MooX::Press is a less magic version of MooX::Pression and only requires Perl 5.8.8 or above.

Important Concepts

The Factory Package and Prefix

MooX::Pression assumes that all the classes and roles you are building with it will be defined under the same namespace prefix. For example "MyApp::Person" and "MyApp::Company" are both defined under the common prefix of "MyApp".

It also assumes there will be a factory package that can be used to build new instances of your class. Rather than creating a new person object with MyApp::Person->new(), you should create a new person object with MyApp->new_person(). Calling MyApp::Person->new() directly is only encouraged from within the "MyApp::Person" class itself, and from within the factory. Everywhere else, you should call MyApp->new_person() instead.

By default, the factory package and the prefix are the same: they are the caller that you imported MooX::Pression into. But they can be set to whatever:

  use MooX::Pression (
    prefix          => 'MyApp::Objects',
    factory_package => 'MyApp::Makers',
  );

MooX::Pression assumes that you are defining all the classes and roles within this namespace prefix in a single Perl module file. This Perl module file would normally be named based on the prefix, so in the example above, it would be "MyApp/Objects.pm" and in the example from the SYNOPSIS, it would be "MyApp.pm".

Of course, there is nothing to stop you from having multiple prefixes for different logical parts of a larger codebase, but MooX::Pression assumes that if it's been set up for a prefix, it owns that prefix and everything under it, and it's all defined in the same Perl module.

Each object defined by MooX::Pression will have a FACTORY method, so you can do:

  $person_object->FACTORY

And it will return the string "MyApp". This allows for stuff like:

  class Person {
    method give_birth {
      return $self->FACTORY->new_person();
    }
  }

The Type Library

While building your classes and objects, MooX::Pression will also build type constraints for each of them. So for the "MyApp::Person" class above, it also builds a Person type constraint. This can be used in Moo/Moose attribute definitions like:

  use MyApp;
  use MyApp::Types qw( Person );
  
  use Moose;
  has boss => (is => 'rw', isa => Person);

And just anywhere a type constraint may be used generally. You should know this stuff by now.

Note that we had to use MyApp before we could use MyApp::Types. This is because there isn't a physical "MyApp/Types.pm" file on disk; it is defined entirely by "MyApp.pm".

Your type library will be the same as your namespace prefix, with "::Types" added at the end. But you can change that:

  use MooX::Pression (
    prefix          => 'MyApp::Objects',
    factory_package => 'MyApp::Makers',
    type_library    => 'MyApp::TypeLibrary',
  );

It can sometimes be helpful to pre-warn MooX::Pression about the types you're going to define before you define them, just so it is able to allow them as barewords in some places...

  use MooX::Pression (
    prefix          => 'MyApp::Objects',
    factory_package => 'MyApp::Makers',
    type_library    => 'MyApp::TypeLibrary',
    declare         => [qw( Person Company )],
  );

Keywords

`class`

Define a very basic class:

  class Person;

Define a more complicated class:

  class Person {
    ...;
  }

Note that for the class keyword without a block, it does not act like the package keyword by changing the "ambient" package. It just defines a totally empty class with no methods or attributes.

The prefix will automatically be added to the class name, so if the prefix is MyApp, the above will create a class called MyApp::Person. It will also create a factory method MyApp->new_person. (The name is generated by stripping the prefix from the class name, replacing any "::" with an underscore, lowercasing, and prefixing it with "new_".) And it will create a type called Person in the type library. (Same rules to generate the name apart from lowercasing and adding "new_".)

Classes can be given more complex names:

  class Person::Neanderthal {
    ...;
  }

Will create "MyApp::Person::Neanderthal" class, a factory method called MyApp->new_person_neanderthal, and a Person_Neanderthal type.

It is possible to create a class without the prefix:

  class ::Person {
    ...;
  }

The class name will now be "Person" instead of "MyApp::Person"!

Nested classes

class blocks can be nested. This establishes an inheritance heirarchy.

  class Animal {
    has name;
    class Mammal {
      class Primate {
        class Monkey;
        class Gorilla;
        class Human {
          class Superhuman;
        }
      }
    }
    class Bird;
    class Fish {
      class Shark;
    }
  }
  
  my $superman = MyApp->new_superhuman( name => 'Kal El' );

See also extends as an alternative way of declaring inheritance.

It is possible to prefix a class name with a plus sign:

  package MyApp {
    use MooX::Pression;
    class Person {
      has name;
      class +Employee {
        has job_title;
      }
    }
  }

Now the employee class will be named MyApp::Person::Employee instead of the usual MyApp::Employee.

Classes can be declared as abstract:

  package MyApp {
    use MooX::Pression;
    abstract class Animal {
      class Cat;
      class Dog;
    }
  }

For abstract classes, there is no constructor or factory, so you cannot create an Animal instance directly; but you can create instances of the subclasses. It is usually better to use roles than abstract classes, but sometimes the abstract class makes more intuitive sense.

`role`

Define a very basic role:

  role Person;

Define a more complicated role:

  role Person {
    ...;
  }

This is just the same as class but defines a role instead of a class.

Roles cannot be nested within each other, nor can roles be nested in classes, nor classes in roles.

`interface`

An interface is a lightweight role. It cannot define attributes, methods, multimethods, or method modifiers, but otherwise functions as a role. (It may have requires statements and define constants.)

  package MyApp;
  use MooX::Pression;
  
  interface Serializer {
    requires serialize;
  }
  
  interface Deserializer {
    requires deserialize;
  }
  
  class MyJSON {
    with Serializer, Deserialize;
    method serialize   ($value) { ... } 
    method deserialize ($value) { ... } 
  }
  
  my $obj = MyApp->new_myjson;
  $obj->does('MyApp::Serializer');   # true

`toolkit`

Use a different toolkit instead of Moo.

  # use Mouse
  class Foo {
    toolkit Mouse;
  }
  
  # use Moose
  # use MooseX::Aliases
  # use MooseX::StrictConstructor
  class Bar {
    toolkit Moose ( Aliases, StrictConstructor );
  }

You can of course specify you want to use Moo:

  class Baz {
    toolkit Moo;
  }

Not all MooseX/MouseX/MooX packages will work, but *X::StrictConstructor will.

It is possible to set a default toolkit when you import MooX::Pression.

  use MooX::Pression (
    ...,
    toolkit => 'Moose',
  );

  use MooX::Pression (
    ...,
    toolkit => 'Mouse',
  );

`extends`

Defines a parent class. Only for use within class blocks.

  class Person {
    extends Animal;
  }

This works:

  class Person {
    extends ::Animal;   # no prefix
  }

`with`

Composes roles.

  class Person {
    with Employable, Consumer;
  }
  
  role Consumer;
  
  role Worker;
  
  role Payable;
  
  role Employable {
    with Worker, Payable;
  }

Because roles are processed before classes, you can compose roles into classes where the role is defined later in the file. But if you compose one role into another, you must define them in a sensible order.

It is possible to compose a role that does not exist by adding a question mark to the end of it:

  class Person {
    with Employable, Consumer?;
  }
  
  role Employable {
    with Worker?, Payable?;
  }

This is equivalent to declaring an empty role.

`begin`

This code gets run early on in the definition of a class or role.

  class Person {
    begin {
      say "Defining $package";
    }
  }

At the time the code gets run, none of the class's attributes or methods will be defined yet.

The lexical variables $package and $kind are defined within the block. $kind will be either 'class' or 'role'.

It is possible to define a global chunk of code to run too:

  use MooX::Pression (
    ...,
    begin => sub {
      my ($package, $kind) = @_;
      ...;
    },
  );

Per-package begin overrides the global begin.

Unlike Perl's BEGIN keyword, a package can only have one begin.

If class definitions are nested, begin blocks will be inherited by child classes. If a parent class is specified via extends, begin blocks will not be inherited.

`end`

This code gets run late in the definition of a class or role.

  class Person {
    end {
      say "Finished defining $package";
    }
  }

The lexical variables $package and $kind are defined within the block. $kind will be either 'class' or 'role'.

It is possible to define a global chunk of code to run too:

  use MooX::Pression (
    ...,
    end => sub {
      my ($package, $kind) = @_;
      ...;
    },
  );

Per-package end overrides the global end.

Unlike Perl's END keyword, a package can only have one end.

If class definitions are nested, end blocks will be inherited by child classes. If a parent class is specified via extends, end blocks will not be inherited.

`has`

  class Person {
    has name;
    has age;
  }
  
  my $bob = MyApp->new_person(name => "Bob", age => 21);

Moo-style attribute specifications may be given:

  class Person {
    has name ( is => rw, type => Str, required => true );
    has age  ( is => rw, type => Int );
  }

Note there is no fat comma after the attribute name! It is a bareword.

Use a plus sign before an attribute name to modify an attribute defined in a parent class.

  class Animal {
    has name ( type => Str, required => false );
    
    class Person {
      has +name ( required => true );
    }
  }

rw, rwp, ro, lazy, true, and false are allowed as barewords for readability, but is is optional, and defaults to rw.

Note type instead of isa. Any type constraints from Types::Standard, Types::Common::Numeric, and Types::Common::String will be avaiable as barewords. Also, any pre-declared types can be used as barewords. It's possible to quote types as strings, in which case you don't need to have pre-declared them.

  class Person {
    has name   ( type => Str, required => true );
    has age    ( type => Int );
    has spouse ( type => 'Person' );
    has kids   (
      is      => lazy,
      type    => 'ArrayRef[Person]',
      builder => sub { [] },
    );
  }

Note that when type is a string, MooX::Pression will consult your type library to figure out what it means.

It is also possible to use isa => 'SomeClass' or does => 'SomeRole' to force strings to be treated as class names or role names instead of type names.

  class Person {
    has name   ( type => Str, required => true );
    has age    ( type => Int );
    has spouse ( isa  => 'Person' );
    has pet    ( isa  => '::Animal' );   # no prefix
  }

For enumerations, you can define them like this:

  class Person {
    ...;
    has status ( enum => ['alive', 'dead', 'undead'] );
  }

MooX::Pression integrates support for MooX::Enumeration (and MooseX::Enumeration, but MouseX::Enumeration doesn't exist).

  class Person {
    ...;
    has status (
      enum    => ['alive', 'dead', 'undead'],
      default => 'alive',
      handles => 1,
    );
  }
  
  my $bob = MyApp->new_person;
  if ( $bob->is_alive ) {
    ...;
  }

handles => 1 creates methods named is_alive, is_dead, and is_undead, and handles => 2 creates methods named status_is_alive, status_is_dead, and status_is_undead.

Checking $bob->status eq 'alvie' is prone to typos, but $bob->status_is_alvie will cause a runtime error because the method is not defined.

MooX::Pression also integrates support for Sub::HandlesVia allowing you to delegate certain methods to unblessed references and non-reference values. For example:

  class Person {
    has age (
      type         => 'Int',
      default      => 0,
      handles_via  => 'Counter',
      handles      => {
        birthday => 'inc',   # increment age
      },
    );
    after birthday {
      if ($self->age < 30) {
        say "yay!";
      }
      else {
        say "urgh!";
      }
    }
  }

A trailing ! indicates a required attribute.

  class Person {
    has name!;
  }

It is possible to give a default using an equals sign.

  class WidgetCollection {
    has name = "Widgets";
    has count (type => Num) = 0;
  }

Note that the default comes after the spec, so in cases where the spec is long, it may be clearer to express the default inside the spec:

  class WidgetCollection {
    has name = "Widgets";
    has count (
      type     => Num,
      lazy     => true,
      required => false,
      default  => 0,
    );
  }

Defaults given this way will be eager (non-lazy), but can be made lazy using the spec:

  class WidgetCollection {
    has name = "Widgets";
    has count (is => lazy) = 0;
  }

Defaults can use the $self object:

  class WidgetCollection {
    has name         = "Widgets";
    has display_name = $self->name;
  }

Any default that includes $self will automatically be lazy, but can be made eager using the spec. (It is almost certainly a bad idea to do so though.)

  class WidgetCollection {
    has name = "Widgets";
    has display_name ( lazy => false ) = $self->name;
  }

If you need to decide an attribute name on-the-fly, you can replace the name with a block that returns the name as a string.

  class Employee {
    extends Person;
    has {
      $ENV{LOCALE} eq 'GB'
        ? 'national_insurance_no'
        : 'social_security_no'
    } (type => Str)
  }
  
  my $bob = Employee->new(
    name               => 'Bob',
    social_security_no => 1234,
  );

This can be used to define a bunch of types from a list.

  class Person {
    my @attrs = qw( name age );
    for my $attr (@attrs) {
      has {$attr} ( required => true );
    }
  }

You can think of the syntax as being kind of like print.

  print BAREWORD_FILEHANDLE @strings;
  print { block_returning_filehandle(); } @strings;

The names of attributes can start with an asterisk:

  has *foo;

This adds no extra meaning, but is supported for consistency with the syntax of named parameters in method signatures. (Depending on your text editor, it may also improve syntax highlighting.)

`constant`

  class Person {
    extends Animal;
    constant latin_name = 'Homo sapiens';
  }

MyApp::Person->latin_name, MyApp::Person::latin_name, and $person_object->latin_name will return 'Homo sapiens'.

`method`

  class Person {
    has spouse;
    
    method marry {
      my ($self, $partner) = @_;
      $self->spouse($partner);
      $partner->spouse($self);
      return $self;
    }
  }

sub { ... } will not work as a way to define methods within the class. Use method { ... } instead.

The variables $self and $class will be automatically defined within all methods. $self is set to $_[0] (though the invocant is not shifted off @_). $class is set to ref($self)||$self. If the method is called as a class method, both $self and $class will be the same thing: the full class name as a string. If the method is called as an object method, $self is the object and $class is its class.

Like with has, you may use a block that returns a string instead of a bareword name for the method.

  method {"ma"."rry"} {
    ...;
  }

MooX::Pression supports method signatures for named arguments and positional arguments. A mixture of named and positional arguments is allowed, with some limitations. For anything more complicates, you should define the method with no signature at all, and unpack @_ within the body of the method.

Signatures for Named Arguments

  class Person {
    has spouse;
    
    method marry ( Person *partner, Object *date = DateTime->now ) {
      $self->spouse( $arg->partner );
      $arg->partner->spouse( $self );
      return $self;
    }
  }

The syntax for each named argument is:

  Type *name = default

The type is a type name, which will be parsed using Type::Parser. (So it can include the ~, |, and &, operators, and can include parameters in [ ] brackets. Type::Parser can handle whitespace in the type, but not comments.

Alternatively, you can provide a block which returns a type name as a string or returns a blessed Type::Tiny object. For very complex types, where you're expressing additional coercions or value constraints, this is probably what you want.

The asterisk indicates that the argument is named, not positional.

The name may be followed by a question mark to indicate an optional argument.

  method marry ( Person *partner, Object *date? ) {
    ...;
  }

Or it may be followed by an equals sign to set a default value.

Comments may be included in the signature, but not in the middle of a type constraint.

  method marry (
    # comment here is okay
    Person
    # comment here is fine too
    *partner
    # and here
  ) { ... }

  method marry (
    Person # comment here is not okay!
           | Horse
    *partner
  ) { ... }

As with signature-free methods, $self and $class wll be defined for you in the body of the method. However, when a signature has been used $self is shifted off @_.

Also within the body of the method, a variable called $arg is provided. This is a hashref of the named arguments. So you can access the partner argument in the above example like this:

  $arg->{partner}

But because $arg is blessed, you can also do:

  $arg->partner

The latter style is encouraged as it looks neater, plus it helps catch typos. ($ars->{pratner} for example!) However, accessing it as a plain hashref is supported and shouldn't be considered to be breaking encapsulation.

For optional arguments you can check:

  exists($arg->{date})

Or:

  $arg->has_date

For types which have a coercion defined, the value will be automatically coerced.

Methods with named arguments can be called with a hash or hashref.

  $alice->marry(  partner => $bob  );      # okay
  $alice->marry({ partner => $bob });      # also okay

Signatures for Positional Arguments

  method marry ( Person $partner, Object $date? ) {
    $self->spouse( $partner );
    $partner->spouse( $self );
    return $self;
  }

The dollar sign is used instead of an asterisk to indicate a positional argument.

As with named arguments, $self is automatically shifted off @_ and $class exists. Unlike named arguments, there is no $arg variable, and instead a scalar variable is defined for each named argument.

Optional arguments and defaults are supported in the same way as named arguments.

It is possible to include a slurpy hash or array at the end of the list of positional arguments.

  method marry ( $partner, $date, @vows ) {
    ...;
  }

If you need to perform a type check on the slurpy parameter, you should pretend it is a hashref or arrayref.

  method marry ( $partner, $date, ArrayRef[Str] @vows ) {
    ...;
  }

Signatures with Mixed Arguments

Since MooX::Pression 0.200, you may mix named and positional arguments with the following limitations:

Positional arguments must appear at the beginning and/or end of the list. They cannot be surrounded by named arguments.
Positional arguments cannot be optional and cannot have a default. They must be required. (Named arguments can be optional and have defaults.)
No slurpies!

  method print_html ($tag, Str $text, *htmlver?, *xml?, $fh) {
  
    confess "update your HTML" if $arg->htmlver < 5;
    
    if (length $text) {
      print $fh "<tag>$text</tag>";
    }
    elsif ($arg->xml) {
      print $fh "<tag />";
    }
    else {
      print $fh "<tag></tag>";
    }
  }
  
  $obj->print_html('h1', 'Hello World', { xml => true }, \*STDOUT);
  $obj->print_html('h1', 'Hello World',   xml => true  , \*STDOUT);
  $obj->print_html('h1', 'Hello World',                  \*STDOUT);

Mixed signatures are basically implemented like named signatures, but prior to interpreting @_ as a hash, some parameters are spliced off the head and tail. We need to know how many elements to splice off each end, so that is why there are restrictions on slurpies and optional parameters.

Empty Signatures

There is a difference between the following two methods:

  method foo {
    ...;
  }
  
  method foo () {
    ...;
  }

In the first, you have not provided a signature and are expected to deal with @_ in the body of the method. In the second, there is a signature, but it is a signature showing that the method expects no arguments (other than the invocant of course).

Optimizing Methods

For a slight compiled-time penalty, you can improve the speed which methods run at using the :optimize attribute:

  method foo :optimize (...) {
    ...;
  }

Optimized methods must not close over any lexical (my or our) variables; they can only access the variables declared in their, signature, $self, $class, @_, and globals.

Anonymous Methods

It is possible to use method without a name to return an anonymous method (coderef):

  use MooX::Pression prefix => 'MyApp';
  
  class MyClass {
    method get_method ($foo) {
      method ($bar) {
        return $foo . $bar;
      }
    }
  }
  
  my $obj   = MyApp->new_myclass;
  my $anon  = $obj->get_method("FOO");
  say ref($anon);                       # CODE
  say $obj->$anon("BAR");               # FOOBAR

Note that while $anon is a coderef, it is still a method, and still expects to be passed an object as $self.

Due to limitations with Keyword::Simple, keywords are always complete statements, so method ... has an implicit semicolon before and after it. This means that this won't work:

  my $x = method { ... };

Because it gets treated as:

  my $x = ;
  method { ... };

A workaround is to wrap it in a do { ... } block.

  my $x = do { method { ... } };

Multimethods

Multi methods should Just Work [tm] if you prefix them with the keyword multi

  use MooX::Pression prefix => 'MyApp';
  
  class Widget {
    multi method foo :alias(quux) (Any $x) {
      say "Buzz";
    }
    multi method foo (HashRef $h) {
      say "Fizz";
    }
  }
  
  my $thing = MyApp->new_widget;
  $thing->foo( {} );       # Fizz
  $thing->foo( 42 );       # Buzz
  
  $thing->quux( {} );      # Buzz

This feature requires MooX::Press 0.035 and Sub::MultiMethod to be installed.

`requires`

Indicates that a role requires classes to fulfil certain methods.

  role Payable {
    requires account;
    requires deposit (Num $amount);
  }
  
  class Employee {
    extends Person;
    with Payable;
    has account!;
    method deposit (Num $amount) {
      ...;
    }
  }

Required methods have an optional signature; this is usually ignored, but if Devel::StrictMode determines that strict behaviour is being used, the signature will be applied to the method via an around modifier.

Or to put it another way, this:

  role Payable {
    requires account;
    requires deposit (Num $amount);
  }

Is a shorthand for this:

  role Payable {
    requires account;
    requires deposit;
    
    use Devel::StrictMode 'STRICT';
    if (STRICT) {
     around deposit (Num $amount) {
       $self->$next(@_);
     }
    }
  }

`before`

  before marry {
    say "Speak now or forever hold your peace!";
  }

As with method, $self and $class are defined.

As with method, you can provide a signature:

  before marry ( Person $partner, Object $date? ) {
    say "Speak now or forever hold your peace!";
  }

Note that this will result in the argument types being checked/coerced twice; once by the before method modifier and once by the method itself. Sometimes this may be desirable, but at other times your before method modifier might not care about the types of the arguments, so can omit checking them.

  before marry ( $partner, $date? ) {
    say "Speak now or forever hold your peace!";
  }

The :optimize attribute is supported for before.

`after`

There's not much to say about after. It's just like before.

  after marry {
    say "You may kiss the bride!";
  }
  
  after marry ( Person $partner, Object $date? ) {
    say "You may kiss the bride!";
  }
  
  after marry ( $partner, $date? ) {
    say "You may kiss the bride!";
  }

The :optimize attribute is supported for after.

`around`

The around method modifier is somewhat more interesting.

  around marry ( Person $partner, Object $date? ) {
    say "Speak now or forever hold your peace!";
    my $return = $self->$next(@_);
    say "You may kiss the bride!";
    return $return;
  }

The $next variable holds a coderef pointing to the "original" method that is being modified. This gives your method modifier the ability to munge the arguments seen by the "original" method, and munge any return values. (I say "original" in quote marks because it may not really be the original method but another wrapper!)

$next and $self are both shifted off @_.

If you use the signature-free version then $next and $self are not shifted off @_ for you, but the variables are still defined.

  around marry {
    say "Speak now or forever hold your peace!";
    my $return = $self->$next($_[2], $_[3]);
    say "You may kiss the bride!";
    return $return;
  }

The :optimize attribute is supported for around.

Note that SUPER:: won't work as expected in MooX::Pression, so around should be used instead.

`factory`

The factory keyword is used to define alternative constructors for your class.

  class Person {
    has name   ( type => Str, required => true );
    has gender ( type => Str );
    
    factory new_man (Str $name) {
      return $class->new(name => $name, gender => 'male');
    }
    
    factory new_woman (Str $name) {
      return $class->new(name => $name, gender => 'female');
    }
  }

But here's the twist. These methods are defined within the factory package, not within the class.

So you can call:

  MyApp->new_man("Bob")             # yes

But not:

  MyApp::Person->new_man("Bob")     # no

Note that if your class defines any factory methods like this, then the default factory method (in this case MyApp->new_person will no longer be automatically created. But you can create the default one easily:

  class Person {
    has name   ( type => Str, required => true );
    has gender ( type => Str );
    
    factory new_man (Str $name) { ... }
    factory new_woman (Str $name) { ... }
    factory new_person;   # no method signature or body!
  }

Within a factory method body, the variable $class is defined, just like normal methods, but $self is not defined. There is also a variable $factory which is a string containing the factory package name. This is because you sometimes need to create more than just one object in a factory method.

  class Wheel;
  
  class Car {
    has wheels = [];
    
    factory new_three_wheeler () {
      return $class->new(
        wheels => [
          $factory->new_wheel,
          $factory->new_wheel,
          $factory->new_wheel,
        ]
      );
    }
    
    factory new_four_wheeler () {
      return $class->new(
        wheels => [
          $factory->new_wheel,
          $factory->new_wheel,
          $factory->new_wheel,
          $factory->new_wheel,
        ]
      );
    }
  }

As with method and the method modifiers, if you provide a signature, $factory and $class will be shifted off @_. If you don't provide a signature, the variables will be defined, but not shifted off @_.

An alternative way to provide additional constructors is with method and then use factory to proxy them.

  class Person {
    has name   ( type => Str, required => true );
    has gender ( type => Str );
    
    method new_guy (Str $name) { ... }
    method new_gal (Str $name) { ... }
    
    factory new_person;
    factory new_man via new_guy;
    factory new_woman via new_gal;
  }

Now MyApp->new_man will call MyApp::Person->new_guy.

factory new_person with no via or method body is basically like saying via new.

The :optimize attribute is supported for factory.

Implementing a singleton

Factories make it pretty easy to implement a singleton.

  class AppConfig {
    ...;
    
    factory get_appconfig () {
      state $config = $class->new();
    }
  }

Now MyApp->get_appconfig will always return the same AppConfig object. Because any explicit use of the factory keyword in a class definition suppresses the automatic creation of a factory method for the class, there will be no MyApp->new_appconfig method for creating new objects of that class.

(People can still manually call MyApp::AppConfig->new to get a new AppConfig object, but remember MooX::Pression discourages calling constructors directly, and encourages you to use the factory package for instantiating objects!)

`type_name`

  class Homo::Sapiens {
    type_name Human;
  }

The class will still be called MyApp::Homo::Sapiens but the type in the type library will be called Human instead of Homo_Sapiens.

`coerce`

  class Person {
    has name   ( type => Str, required => true );
    has gender ( type => Str );
    
    coerce from Str via from_string {
      $class->new(name => $_);
    }
  }
  
  class Company {
    has owner ( type => 'Person', required => true );
  }
  
  my $acme = MyApp->new_company( owner => "Bob" );

Note that the company owner is supposed to be a person object, not a string, but the Person class knows how create a person object from a string.

Coercions are automatically enabled in a lot of places for types that have a coercion. For example, types in signatures, and types in attribute definitions.

Note that the coercion body doesn't allow signatures, and the value being coerced will be found in $_. If you want to have signatures, you can define a coercion as a normal method first:

  class Person {
    has name   ( type => Str, required => true );
    has gender ( type => Str );
    
    method from_string ( Str $name ) {
      $class->new(name => $name);
    }
    
    coerce from Str via from_string;
  }

In both cases, a MyApp::Person->from_string method is generated which can be called to manually coerce a string into a person object.

They keyword from is technically optional, but does make the statement more readable.

  coerce Str via from_string {      # this works
    $class->new(name => $_);
  }

The :optimize attribute is not currently supported for coerce.

`overload`

  class Collection {
    has items = [];
    overload '@{}' => sub { shift->list };
  }

The list passed to overload is passed to overload with no other processing.

`version`

  class Person {
    version 1.0;
  }

This just sets $MyApp::Person::VERSION.

You can set a default version for all packages like this:

  use MooX::Pression (
    ...,
    version => 1.0,
  );

If class definitions are nested, version will be inherited by child classes. If a parent class is specified via extends, version will not be inherited.

`authority`

  class Person {
    authority 'cpan:TOBYINK';
  }

This just sets $MyApp::Person::AUTHORITY.

It is used to indicate who is the maintainer of the package.

  use MooX::Pression (
    ...,
    version   => 1.0,
    authority => 'cpan:TOBYINK',
  );

If class definitions are nested, authority will be inherited by child classes. If a parent class is specified via extends, authority will not be inherited.

Helper Subs

Earlier it was stated that sub cannot be used to define methods in classes and roles. This is true, but that doesn't mean that it has no use.

  package MyApp {
    use MooX::Pression;
    
    sub helper_function { ... }
    
    class Foo {
      method foo {
        ...;
        helper_function(...);
        ...;
      }
    }
    
    class Bar {
      sub other_helper { ... }
      method bar {
        ...;
        helper_function(...);
        other_helper(...);
        ...;
      }
    }
  }

The subs defined by sub end up in the "MyApp" package, not "MyApp::Foo" or "MyApp::Bar". They can be called by any of the classes and roles defined in MyApp. This makes them suitable for helper subs like logging, List::Util/Scalar::Util sorts of functions, and so on.

  package MyApp {
    use MooX::Pression;
    
    use List::Util qw( any all first reduce );
    # the above functions are now available within
    # all of MyApp's classes and roles, but won't
    # pollute any of their namespaces.
    
    use namespace::clean;
    # And now they won't even pollute MyApp's namespace.
    # Though I'm pretty sure this will also stop them
    # from working in any methods that used ":optimize".
    
    class Foo { ... } 
    role Bar { ... } 
    role Baz { ... } 
  }

sub is also usually your best option for those tiny little coderefs that need to be defined here and there:

  has foo (
    is       => lazy,
    type     => ArrayRef[Str],
    builder  => sub {  []  },
  );

Though consider using Sub::Quote if you're using Moo.

Utilities

MooX::Pression also exports constants true and false into your namespace. These show clearer boolean intent in code than using 1 and 0.

MooX::Pression exports rw, ro, rwp, and lazy constants which make your attribute specs a little cleaner looking.

MooX::Pression exports blessed from Scalar::Util because that can be handy to have, and confess from Carp. MooX::Pression's copy of confess is super-powered and runs its arguments through sprintf.

  before vote {
    if ($self->age < 18) {
      confess("Can't vote, only %d", $self->age);
    }
  }

MooX::Pression turns on strict, warnings, and the following modern Perl features:

  # Perl 5.14 and Perl 5.16
  say state unicode_strings
  
  # Perl 5.18 or above
  say state unicode_strings unicode_eval evalbytes current_sub fc

If you're wondering why not other features, it's because I didn't want to enable any that are currently classed as experimental, nor any that require a version of Perl above 5.18. The switch feature is mostly seen as a failed experiment these days, and lexical_subs cannot be called as methods so are less useful in object-oriented code.

You can, of course, turn on extra features yourself.

  package MyApp {
    use MooX::Pression;
    use feature qw( lexical_subs postderef );
    
    ...;
  }

(The current_sub feature is unlikely to work fully unless you have :optimize switched on for that method, or the method does not include a signature. For non-optimized methods with a signature, a wrapper is installed that handles checks, coercions, and defaults. __SUB__ will point to the "inner" sub, minus the wrapper.)

MooX::Pression exports Syntax::Keyword::Try for you. Useful to have.

And last but not least, it exports all the types, is_* functions, and assert_* functions from Types::Standard, Types::Common::String, and Types::Common::Numeric.

Anonymous Classes and Roles

Anonymous classes

It is possible to make anonymous classes:

  my $class  = do { class; };
  my $object = $class->new;

The do { ... } block is necessary because of a limitation in Keyword::Simple, where any keywords it defines must be complete statements.

Anonymous classes can have methods and attributes and so on:

  my $class = do { class {
    has foo (type => Int);
    has bar (type => Int);
  }};
  
  my $object = $class->new(foo => 1, bar => 2);

Anonymous classes do not implicitly inherit from their parent like named nested classes do. Named classes nested inside anonymous classes do not implicitly inherit from the anonymous class.

Having one anonymous class inherit from another can be done though:

  my $base     = do { class; }
  my $derived  = do { class {
    extends {"::$k1"};
  }};

This works because extends accepts a block which returns a string for the package name, and the string needs to begin with "::" to avoid the auto prefix mechanism.

Anonymous roles

Anonymous roles work in much the same way.

Parameterizable Classes and Roles

Parameterizable classes

  package MyApp {
    use MooX::Pression;
    
    class Animal {
      has name;
    }
    
    class Species ( Str $common_name, Str $binomial ) {
      extends Animal;
      constant common_name  = $common_name;
      constant binomial     = $binomial;
    }
    
    class Dog {
      extends Species('dog', 'Canis familiaris');
      method bark () {
        say "woof!";
      }
    }
  }

Here, "MyApp::Species" isn't a class in the usual sense; you cannot create instances of it. It's like a template for generating classes. Then "MyApp::Dog" generates a class from the template and inherits from that.

  my $Cat = MyApp->generate_species('cat', 'Felis catus');
  my $mog = $Cat->new(name => 'Mog');
  
  $mog->isa('MyApp::Animal');         # true
  $mog->isa('MyApp::Species');        # false!!!
  $mog->isa($Cat);                    # true

Because there are never any instances of "MyApp::Species", it doesn't make sense to have a Species type constraint. Instead there are SpeciesClass and SpeciesInstance type constraints.

  use MyApp::Types -is;
  
  my $lassie = MyApp->new_dog;
  
  is_Animal( $lassie );               # true
  is_Dog( $lassie );                  # true
  is_SpeciesInstance( $lassie );      # true
  is_SpeciesClass( ref($lassie) );    # true

Subclasses cannot be nested inside parameterizable classes, but parameterizable classes can be nested inside regular classes, in which case the classes they generate will inherit from the outer class.

  package MyApp {
    use MooX::Pression;
    
    class Animal {
      has name;
      class Species ( Str $common_name, Str $binomial ) {
        constant common_name  = $common_name;
        constant binomial     = $binomial;
      }
    }
    
    class Dog {
      extends Species('dog', 'Canis familiaris');
      method bark () {
        say "woof!";
      }
    }
  }

Anonymous parameterizable classes are possible:

  my $generator = do { class ($footype, $bartype) {
    has foo (type => $footype);
    has bar (type => $bartype);
  };
  
  my $class = $generator->generate_package(Int, Num);
  
  my $object = $class->new(foo => 42, bar => 4.2);

Parameterizable roles

Often it makes more sense to parameterize roles than classes.

  package MyApp {
    use MooX::Pression;
    
    class Animal {
      has name;
    }
    
    role Species ( Str $common_name, Str $binomial ) {
      constant common_name  = $common_name;
      constant binomial     = $binomial;
    }
    
    class Dog {
      extends Animal;
      with Species('dog', 'Canis familiaris'), GoodBoi?;
      method bark () {
        say "woof!";
      }
    }
  }

Anonymous parameterizable roles are possible.

MooX::Pression vs Moops

MooX::Pression has fewer dependencies than Moops, and crucially doesn't rely on Package::Keyword and Devel::CallParser which have... issues. MooX::Pression uses Damian Conway's excellent PPR to handle most parsing needs, so parsing should be more predictable.

Moops is faster though.

Here are a few key syntax and feature differences.

Declaring a class

Moops:

  class Foo::Bar 1.0 extends Foo with Bar {
    ...;
  }

MooX::Pression:

  class Foo::Bar {
    version 1.0;
    extends Foo;
    with Bar;
  }

Moops and MooX::Pression use different logic for determining whether a class name is "absolute" or "relative". In Moops, classes containing a "::" are seen as absolute class names; in MooX::Pression, only classes starting with "::" are taken to be absolute; all others are given the prefix.

Moops:

  package MyApp {
    use Moops;
    class Foo {
      class Bar {
        class Baz {
          # Nesting class blocks establishes a naming
          # heirarchy so this is MyApp::Foo::Bar::Baz!
        }
      }
    }
  }

MooX::Pression:

  package MyApp {
    use MooX::Pression;
    class Foo {
      class Bar {
        class Baz {
          # This is only MyApp::Baz, but nesting
          # establishes an @ISA chain instead.
        }
      }
    }
  }

How namespacing works

Moops:

  use feature 'say';
  package MyApp {
    use Moops;
    use List::Util qw(uniq);
    class Foo {
      say __PACKAGE__;         # MyApp::Foo
      say for uniq(1,2,1,3);   # ERROR!
      sub foo { ... }          # MyApp::Foo::foo()
    }
  }

MooX::Pression:

  use feature 'say';
  package MyApp {
    use MooX::Pression;
    use List::Util qw(uniq);
    class Foo {
      say __PACKAGE__;         # MyApp
      say for uniq(1,2,1,3);   # this works fine
      sub foo { ... }          # MyApp::foo()
    }
  }

This is why you can't use sub to define methods in MooX::Pression. You need to use the method keyword. In MooX::Pression, all the code in the class definition block is still executing in the parent package's namespace!

Multimethods

Moops/Kavorka multimethods are faster, but MooX::Pression is smarter at picking the best candidate to dispatch to, and intelligently selecting candidates across inheritance hierarchies and role compositions.

Other crazy Kavorka features

Kavorka allows you to mark certain parameters as read-only or aliases, allows you to specify multiple names for named parameters, allows you to rename the invocant, allows you to give methods and parameters attributes, allows you to specify a method's return type, etc, etc.

MooX::Pression's method keyword is unlikely to ever offer as many features as that. It is unlikely to offer many more features than it currently offers.

If you need fine-grained control over how @_ is handled, just don't use a signature and unpack @_ inside your method body however you need to.

Lexical accessors

Moops automatically imported lexical_has from Lexical::Accessor into each class. MooX::Pression does not, but thanks to how namespacing works, it only needs to be imported once if you want to use it.

  package MyApp {
    use MooX::Pression;
    use Lexical::Accessor;
    
    class Foo {
      my $identifier = lexical_has identifier => (
        is      => rw,
        isa     => Int,
        default => sub { 0 },
      );
      
      method some_method () {
        $self->$identifier( 123 );    # set identifier
        ...;
        return $self->$identifier;    # get identifier
      }
    }
  }

Lexical accessors give you true private object attributes.

Factories

MooX::Pression puts an emphasis on having a factory package for instantiating objects. Moops didn't have anything similar.

`augment` and `override`

These are Moose method modifiers that are not implemented by Moo. Moops allows you to use these in Moose and Mouse classes, but not Moo classes. MooX::Pression simply doesn't support them.

Type Libraries

Moops allowed you to declare multiple type libraries, define type constraints in each, and specify for each class and role which type libraries you want it to use.

MooX::Pression automatically creates a single type library for all your classes and roles within a module to use, and automatically populates it with the types it thinks you might want.

If you need to use other type constraints:

  package MyApp {
    use MooX::Pression;
    # Just import types into the factory package!
    use Types::Path::Tiny qw( Path );
    
    class DataSource {
      has file => ( type => Path );
      
      method set_file ( Path $file ) {
        $self->file( $file );
      }
    }
  }
  
  my $ds = MyApp->new_datasource;
  $ds->set_file('blah.txt');      # coerce Str to Path
  print $ds->file->slurp_utf8;

Constants

Moops:

  class Foo {
    define PI = 3.2;
  }

MooX::Pression:

  class Foo {
    constant PI = 3.2;
  }

Parameterizable classes and roles

These were always on my todo list for Moops; I doubt they'll ever be done. They work nicely in MooX::Pression though.

BUGS

Please report any bugs to http://rt.cpan.org/Dist/Display.html?Queue=MooX-Pression.

TODO

Plugin system

MooX::Pression can often load MooX/MouseX/MooseX plugins and work fine with them, but some things won't work, like plugins that rely on being able to wrap has. So it would be nice to have a plugin system that extensions can hook into.

If you're interested in extending MooX::Pression, file a bug report about it and let's have a conversation about the best way for that to happen.

AUTHOR

Toby Inkster <tobyink@cpan.org>.

COPYRIGHT AND LICENCE

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.

To install MooX::Pression, copy and paste the appropriate command in to your terminal.

cpanm

cpanm MooX::Pression

CPAN shell

perl -MCPAN -e shell
install MooX::Pression

For more information on module installation, please visit the detailed CPAN module installation guide.

	Global
`s`	Focus search bar
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NAME

SYNOPSIS

DESCRIPTION

Important Concepts

The Factory Package and Prefix

The Type Library

Keywords

class

Nested classes

role

interface

toolkit

extends

with

begin

end

has

constant

method

Signatures for Named Arguments

Signatures for Positional Arguments

Signatures with Mixed Arguments

Empty Signatures

Optimizing Methods

Anonymous Methods

Multimethods

requires

before

after

around

factory

Implementing a singleton

type_name

coerce

overload

version

authority

Helper Subs

Utilities

Anonymous Classes and Roles

Anonymous classes

Anonymous roles

Parameterizable Classes and Roles

Parameterizable classes

Parameterizable roles

MooX::Pression vs Moops

Declaring a class

How namespacing works

Multimethods

Other crazy Kavorka features

Lexical accessors

Factories

augment and override

Type Libraries

Constants

Parameterizable classes and roles

BUGS

TODO

Plugin system

SEE ALSO

AUTHOR

COPYRIGHT AND LICENCE

DISCLAIMER OF WARRANTIES

Module Install Instructions

`class`

`role`

`interface`

`toolkit`

`extends`

`with`

`begin`

`end`

`has`

`constant`

`method`

`requires`

`before`

`after`

`around`

`factory`

`type_name`

`coerce`

`overload`

`version`

`authority`

`augment` and `override`