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VEESH KEEDI LUCAS

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Author image Paul Evans

NAME

Object::Pad - a simple syntax for lexical slot-based objects

SYNOPSIS

   use Object::Pad;

   class Point {
      has $x = 0;
      has $y = 0;

      method BUILD {
        ($x, $y) = @_;
      }

      method move($dX, $dY) {
         $x += $dX;
         $y += $dY;
      }

      method describe {
         print "A point at ($x, $y)\n";
      }
   }

   Point->new(5,10)->describe;

DESCRIPTION

WARNING This is an experimental proof-of-concept. Please don't actually use this in production unless you are crazy :)

This module provides a simple syntax for creating object classes, which uses private variables that look like lexicals as object member fields.

Automatic Construction

Classes are automatically provided with a constructor method, called new, which helps create the object instances.

By default, this constructor will invoke the BUILD method of every component class, passing the list of arguments the constructor was invoked with. Each class should perform its required setup behaviour in a method called BUILD, but does not need to chain to the SUPER class first; this is handled automatically.

   $self->BUILD( @_ )

If the class provides a BUILDARGS class method, that is used to mangle the list of arguments before the BUILD methods are called. Note this must be a class method not an instance method (and so implemented using sub). It should perform any SUPER chaining as may be required.

   @args = $class->BUILDARGS( @_ )
   $self->BUILD( @args )

If the class provides a BUILDALL method, that is used to implement the setup behaviour instead. It is passed the entire parameters list from the new method. It should perform any SUPER chaining as may be required.

   $self->BUILDALL( @_ )

KEYWORDS

class

   class Name {
      ...
   }

   class Name;

Behaves similarly to the package keyword, but provides a package that defines a new class. Such a class provides an automatic constructor method called new.

As with package, an optional block may be provided. If so, the contents of that block define the new class and the preceding package continues afterwards. If not, it sets the class as the package context of following keywords and definitions.

As with package, an optional version declaration may be given. If so, this sets the value of the package's $VERSION variable.

   class Name VERSION { ... }

   class Name VERSION;

A single superclass is supported by the keyword extends

   class Name extends BASECLASS {
      ...
   }

   class Name extends BASECLASS VERSION {
      ...
   }

If a package providing the superclass does not exist, an attempt is made to load it by code equivalent to

   require Animal ();

and thus it must either already exist, or be locatable via the usual @INC mechanisms.

The superclass must either be implemented by Object::Pad, or be some class whose instances are blessed hash references. For more detail on this latter case see "SUBCLASSING CLASSIC PERL CLASSES".

An optional version check can also be supplied; it performs the equivalent of

   BaseClass->VERSION( $ver )

has

   has $var;
   has $var = CONST;
   has @var;
   has %var;

Declares that the instances of the class have a member field of the given name. This member field (called a "slot") will be accessible as a lexical variable within any method declarations in the class.

Array and hash members are permitted and behave as expected; you do not need to store references to anonymous arrays or hashes.

Member fields are private to a class. They are not visible to users of the class, nor to subclasses. In order to provide access to them a class may wish to use "method" to create an accessor.

A scalar slot may provide a expression that gives an initialisation value, which will be assigned into the slot of every instance during the constructor before BUILDALL is invoked. For ease-of-implementation reasons this expression must currently be a compiletime constant, but it is hoped that a future version will relax this restriction and allow runtime-computed values.

method

   method NAME {
      ...
   }

   method NAME (SIGNATURE) {
      ...
   }

   method NAME :attrs... {
      ...
   }

Declares a new named method. This behaves similarly to the sub keyword, except that within the body of the method all of the member fields ("slots") are also accessible. In addition, the method body will have a lexical called $self which contains the invocant object directly; it will already have been shifted from the @_ array.

The signatures feature is automatically enabled for method declarations. In this case the signature does not have to account for the invocant instance; that is handled directly.

   method m($one, $two) {
      say "$self invokes method on one=$one two=$two";
   }

   ...
   $obj->m(1, 2);

A list of attributes may be supplied as for sub. The most useful of these is :lvalue, allowing easy creation of read-write accessors for slots.

   class Counter {
      has $count;

      method count :lvalue { $count }
   }

   my $c = Counter->new;
   $c->count++;

Every method automatically gets the :method attribute applied, which suppresses warnings about ambiguous calls resolved to core functions if the name of a method matches a core function.

IMPLIED PRAGMATA

In order to encourage users to write clean, modern code, the body of the class block acts as if the following pragmata are in effect:

   use strict;
   use warnings;
   no indirect ':fatal';
   use feature 'signatures';

This list may be extended in subsequent versions to add further restrictions and should not be considered exhaustive.

Further additions will only be ones that remove "discouraged" or deprecated language features with the overall goal of enforcing a more clean modern style within the body. As long as you write code that is in a clean, modern style (and I fully accept that this wording is vague and subjective) you should not find any new restrictions to be majorly problematic. Either the code will continue to run unaffected, or you may have to make some small alterations to bring it into a conforming style.

SUBCLASSING CLASSIC PERL CLASSES

There are a number of details specific to the case of deriving an Object::Pad class from an existing classic Perl class that is not implemented using Object::Pad.

Storage of Instance Data

Instances will store their data in a key called "Object::Pad/slots", which is fairly unlikely to clash with existing storage on the instance. The exact format of the value stored here is not specified and may change between module versions, though it can be relied on to be well-behaved as some kind of perl data structure for purposes of modules like Data::Dumper or serialisation into things like YAML or JSON.

Object State During Methods Invoked By Superclass Constructor

It is common in classic Perl OO style to invoke methods on $self during the constructor. This is supported here since Object::Pad version 0.19. Note however that any methods invoked by the superclass constructor may not see the object in a fully consistent state. (This fact is not specific to using Object::Pad and would happen in classic Perl OO as well). The slot initialisers will have been invoked but the BUILD methods will not.

For example; in the following

   package ClassicPerlBaseClass {
      sub new {
         my $self = bless {}, shift;
         say "Value seen by superconstructor is ", $self->get_value;
         return $self;
      }
      sub get_value { return "A" }
   }

   class DerivedClass extends ClassicPerlBaseClass {
      has $_value = "B";
      method BUILD {
         $_value = "C";
      }
      method get_value { return $_value }
   }

   my $obj = DerivedClass->new;
   say "Value seen by user is ", $obj->get_value;

Until the ClassicPerlBaseClass::new superconstructor has returned the BUILD method will not have been invoked. The $_value slot will still exist, but its value will be B during the superconstructor. After the superconstructor, the BUILD methods are invoked before the completed object is returned to the user. The result will therefore be:

   Value seen by superconstructor is B
   Value seen by user is C

STYLE SUGGESTIONS

While in no way required, the following suggestions of code style should be noted in order to establish a set of best practices, and encourage consistency of code which uses this module.

File Layout

Begin the file with a use Object::Pad line; ideally including a minimum-required version. This should be followed by the toplevel class declaration for the file. As it is at toplevel there is no need to use the block notation; it can be a unit class.

There is no need to use strict or apply other usual pragmata; these will be implied by the class keyword.

   use Object::Pad 0.16;

   class My::Classname 1.23;

   # other use statements

   # has, methods, etc.. can go here

Slot Names

Slot names should follow similar rules to regular lexical variables in code - lowercase, name components separated by underscores. For tiny examples such as "dumb record" structures this may be sufficient.

   class Tag {
      has $name;  method name  :lvalue { $name }
      has $value; method value :lvalue { $value }
   }

In larger examples with lots of non-trivial method bodies, it can get confusing to remember where the slot variables come from (because we no longer have the $self->{ ... } visual clue). In these cases it is suggested to prefix the slot names with a leading underscore, to make them more visually distinct.

   class Spudger {
      has $_grapefruit;

      ...

      method mangle {
         $_grapefruit->peel; # The leading underscore reminds us this is a slot
      }
   }

WITH OTHER MODULES

Syntax::Keyword::Dynamically

A cross-module integration test asserts that dynamically works correctly on object instance slots:

   use Object::Pad;
   use Syntax::Keyword::Dynamically;

   class Container {
      has $value = 1;

      method example {
         dynamically $value = 2;
         ,..
         # value is restored to 1 on return from this method
      }
   }

Future::AsyncAwait

As of Future::AsyncAwait version 0.38 and Object::Pad version 0.15, both modules now use XS::Parse::Sublike to parse blocks of code. Because of this the two modules can operate together and allow class methods to be written as async subs which await expressions:

   use Future::AsyncAwait;
   use Object::Pad;

   class Example
   {
      async method perform($block)
      {
         say "$self is performing code";
         await $block->();
         say "code finished";
      }
   }

These three modules combine; there is additionally a cross-module test to ensure that object instance slots can be dynamically set during a suspended async method.

DESIGN TODOs

The following points are details about the design of pad slot-based object systems in general:

  • Is multiple inheritence actually required, if role composition is implemented including giving roles the ability to use private slots?

  • Consider the visibility of superclass slots to subclasses. Do subclasses even need to be able to see their superclass's slots, or are accessor methods always appropriate?

    Concrete example: The $self->{split_at} access that Tickit::Widget::HSplit makes of its parent class Tickit::Widget::LinearSplit.

IMPLEMENTATION TODOs

These points are more about this particular module's implementation:

  • Implement roles, including required method checking and the ability to have private slots.

  • Consider multiple inheritence of subclassing, if that is still considered useful after adding roles.

  • Some extensions of the has syntax:

    Non-constant default expressions

       has $var = EXPR;

    A way to request generated accessors - ro or rw.

  • Work out why no indirect doesn't appear to work properly before perl 5.20.

  • Work out why we don't get a Subroutine new redefined at ... warning if we

      sub new { ... }
  • The local modifier does not work on slot variables, because they appear to be regular lexicals to the parser at that point. A workaround is to use Syntax::Keyword::Dynamically instead:

       use Syntax::Keyword::Dynamically;
    
       has $loglevel;
    
       method quietly {
          dynamically $loglevel = LOG_ERROR;
          ...
       }

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>