NAME

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

SYNOPSIS

use Object::Pad;

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

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

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

Point->new(x => 5, y => 10)->describe;

DESCRIPTION

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

WARNING This module is still experimental. The parts that currently exist do seem to work reliably but much of the design is still evolving, and many features and have yet to be implemented. I don't yet guarantee I won't have to change existing details in order to continue its development. Feel free to try it out in experimental or newly-developed code, but don't complain if a later version is incompatible with your current code and you'll have to change it.

That all said, please do get in contact if you find the module overall useful. The more feedback you provide in terms of what features you are using, what you find works, and what doesn't, will help the ongoing development and hopefully eventual stability of the design. See the "FEEDBACK" section.

Automatic Construction

Classes are automatically provided with a constructor method, called new, which helps create the object instances. This may respond to passed arguments, automatically assigning values of slots, and invoking other blocks of code provided by the class. It proceeds in the following stages:

The BUILDARGS phase

If the class provides a BUILDARGS class method, that is used to mangle the list of arguments before the BUILD blocks 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( @_ )

Slot assignment

If any slot in the class has the :param attribute, then the constructor will expect to receive its argmuents in an even-sized list of name/value pairs. This applies even to slots inherited from the parent class or applied roles. It is therefore a good idea to shape the parameters to the constructor in this way in roles, and in classes if you intend your class to be extended.

The constructor will also check for required parameters (these are all the parameters for slots that do not have default initialisation expressions). If any of these are missing an exception is thrown.

The BUILD phase

As part of the construction process, the BUILD block of every component class will be invoked, passing in the list of arguments the constructor was invoked with. Each class should perform its required setup behaviour, but does not need to chain to the SUPER class first; this is handled automatically.

The ADJUST phase

Finally, before the new object is returned from the constructor, the ADJUST block of every component class is invoked. This happens after the slots are assigned their initial values and the BUILD blocks have been run.

KEYWORDS

class

class Name :ATTRS... {
   ...
}

class Name :ATTRS...;

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 isa

Since version 0.41.

class Name isa BASECLASS {
   ...
}

class Name isa BASECLASS BASEVER {
   ...
}

(prior to version 0.41 this was called extends, which is currently recognised as a compatibility synonym).

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 may or may not itself be implemented by Object::Pad, but if it is not then see "SUBCLASSING CLASSIC PERL CLASSES" for further detail on the semantics of how this operates.

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

BaseClass->VERSION( $ver )

One or more roles can be composed into the class by the keyword does

Since version 0.41.

class Name does ROLE, ROLE,... {
   ...
}

(prior to version 0.41 this was called implements, which is currently recognised as a compatibility synonym).

An optional list of attributes may be supplied in similar syntax as for subs or lexical variables. (These are annotations about the class itself; the concept should not be confused with per-object-instance data, which here is called "slots"). The following class attributes are supported:

:repr(TYPE)

Sets the representation type for instances of this class. Must be one of the following values:

:repr(native)

The native representation. This is an opaque representation type whose contents are not specified. It only works for classes whose entire inheritence hierarchy is built only from classes based on Object::Pad.

:repr(HASH)

The representation will be a blessed hash reference. The instance data will be stored in an array referenced by a key called Object::Pad/slots, which is fairly unlikely to clash with existing storage on the instance. No other keys will be used; they are available for implementions and subclasses to use. 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.

This representation type may be useful when converting existing classes into using Object::Pad where there may be existing subclasses of it that presume a blessed hash for their own use.

:repr(magic)

The representation will use MAGIC to apply the instance data in a way that is invisible at the Perl level, and shouldn't get in the way of other things the instance is doing even in XS modules.

This representation type is the only one that will work for subclassing existing classes that do not use blessed hashes.

:repr(autoselect), :repr(default)

Since version 0.23.

This representation will select one of the representations above depending on what is best for the situation. Classes not derived from a non-Object::Pad base class will pick native, and classes derived from non-Object::Pad bases will pick either the HASH or magic forms depending on whether the instance is a blessed hash reference or some other kind.

This achieves the best combination of DWIM while still allowing the common forms of hash reference to be inspected by Data::Dumper, etc. This is the default representation type, and does not have to be specifically requested.

:strict(params)

Since version 0.43.

Can only be applied to classes that contain no BUILD blocks. If set, then the constructor will complain about any unrecognised named arguments passed to it (i.e. names that do not correspond to the :param of any defined slot).

Since BUILD blocks can inspect the arguments arbitrarily, the presence of any such block means the constructor cannot determine which named arguments are not recognised.

This attribute is a temporary stepping-stone for compatibility with existing code. It is recommended to enable this whenever possible, as a later version of this module will likely perform this behaviour unconditionally whenever no BUILD blocks are present. How to handle other named arguments that are not simply assigned to slots remains an unanswered design question.

role

role Name :ATTRS... {
   ...
}

role Name :ATTRS...;

Since version 0.32.

Similar to class, but provides a package that defines a new role. A role acts simliar to a class in some respects, and differently in others.

Like a class, a role can have a version, and named methods.

role Name VERSION {
   method a { ... }
   method b { ... }
}

A role does not provide a constructor, and instances cannot directly be constructed. A role cannot extend a class.

A role can declare that it requires methods of given names from any class that implements the role.

role Name {
   requires METHOD;
}

A role can provide instance slots. These are visible to any BUILD blocks or methods provided by that role.

Since version 0.33.

role Name {
   has $slot;

   BUILD { $slot = "a value" }

   method slot { return $slot }
}

The following role attributes are supported:

:compat(invokable)

Since version 0.35.

Enables a form of backward-compatibility behaviour useful for gradually upgrading existing code from classical Perl inheritance or mixins into using roles.

Normally, methods of a role cannot be directly invoked and the role must be applied to an Object::Pad-based class in order to be used. This however presents a problem when gradually upgrading existing code that already uses techniques like roles, multiple inheritance or mixins when that code may be split across multiple distributions, or for some other reason cannot be upgraded all at once. Methods within a role that has the :compat(invokable) attribute applied to it may be directly invoked on any object instance. This allows the creation of a role that can still provide code for existing classes written in classical Perl that has not yet been rewritten to use Object::Pad.

The tradeoff is that a :compat(invokable) role may not create slot data using the "has" keyword. Whatever behaviours the role wishes to perform must be provided only by calling other methods on $self, or perhaps by making assumptions about the representation type of instances.

It should be stressed again: This option is only intended for gradual upgrade of existing classical Perl code into using Object::Pad. When all existing code is using Object::Pad then this attribute can be removed from the role.

has

has $var;
has $var = EXPR;
has @var;
has %var;

has $var :ATTR ATTR...;

Declares that the instances of the class or role 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 or role. They are not visible to users of the class, nor to subclasses, nor to any class that a role is applied to. In order to provide access to them a class may wish to use "method" to create an accessor, or use the attributes such as ":reader" to get one generated.

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 the BUILD blocks are invoked. Since version 0.29 this expression does not have to be a compiletime constant, though it is evaluated exactly once, at runtime, after the class definition has been parsed. It is not evaluated individually for every object instance of that class.

The following slot attributes are supported:

:reader, :reader(NAME)

Since version 0.27.

Generates a reader method to return the current value of the slot. Currently these are only permitted for scalar slots. If no name is given, the name of the slot is used. A single prefix character _ will be removed if present.

has $slot :reader;

# equivalent to
has $slot;  method slot { return $slot }

:writer, :writer(NAME)

Since version 0.27.

Generates a writer method to set a new value of the slot from its first argument. Currently these are only permitted for scalar slots. If no name is given, the name of the slot is used prefixed by set_. A single prefix character _ will be removed if present.

has $slot :writer;

# equivalent to
has $slot;  method set_slot { $slot = shift; return $self }

Since version 0.28 a generated writer method will return the object invocant itself, allowing a chaining style.

$obj->set_x("x")
   ->set_y("y")
   ->set_z("z");

:mutator, :mutator(NAME)

Since version 0.27.

Generates an lvalue mutator method to return or set the value of the slot. These are only permitted for scalar slots. If no name is given, the name of the slot is used. A single prefix character _ will be removed if present.

has $slot :mutator;

# equivalent to
has $slot;  method slot :lvalue { $slot }

Since version 0.28 all of these generated accessor methods will include argument checking similar to that used by subroutine signatures, to ensure the correct number of arguments are passed - usually zero, but exactly one in the case of a :writer method.

:weak

Since version 0.44.

Generated code which sets the value of this slot will weaken it if it contains a reference. This applies to within the constructor if :param is given, and to a :writer accessor method. Note that this only applies to automatically generated code; not normal code written in regular method bodies. If you assign into the slot variable you must remember to call Scalar::Util::weaken yourself.

:param, :param(NAME)

Since version 0.41.

Sets this slot to be initialised automatically in the generated constructor. This is only permitted on scalar slots. If no name is given, the name of the slot is used. A single prefix character _ will be removed if present.

Any slot that has :param but does not have a default initialisation expression becomes a required argument to the constructor. Attempting to invoke the constructor without a named argument for this will throw an exception. In order to make a parameter optional, make sure to give it a default expression - even if that expression is undef:

has $x :param;          # this is required
has $z :param = undef;  # this is optional

Values for slots are assigned by the constructor before any BUILD blocks are invoked.

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 (but see also the :reader, :writer and :mutator slot attributes).

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.

The following additional attributes are recognised by Object::Pad directly:

:override

Since version 0.29.

Marks that this method expects to override another of the same name from a superclass. It is an error at compiletime if the superclass does not provide such a method.

BUILD

BUILD {
   ...
}

BUILD (SIGNATURE) {
   ...
}

Since version 0.27.

Declares the builder block for this component class. A builder block may use subroutine signature syntax, as for methods, to assist in unpacking its arguments. A build block is not a subroutine and thus is not permitted to use subroutine attributes (for example :lvalue).

Note that a BUILD block is a named phaser block and not a method. Attempts to create a method named BUILD (i.e. with syntax method BUILD {...}) will fail with a compiletime error, to avoid this confusion.

ADJUST

ADJUST {
   ...
}

Since version 0.43.

Declares an adjust block for this component class. This block of code runs within the constructor, after any BUILD blocks and automatic slot value assignment. It can make any final adjustments to the instance (such as initialising slots from calculated values). No additional parameters are passed.

An adjust block is not a subroutine and thus is not permitted to use subroutine attributes. Note that an ADJUST block is a named phaser block and not a method; it does not use the sub or method keyword.

requires

requires NAME;

Declares that this role requires a method of the given name from any class that implements it. It is an error at compiletime if the implementing class does not provide such a method.

CREPT FEATURES

While not strictly part of being an object system, this module has nevertheless gained a number of behaviours by feature creep, as they have been found useful.

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';  # or  no feature 'indirect' on perl 5.32 onwards
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.

Yield True

A class statement or block will yield a true boolean value. This means that it can be used directly inside a .pm file, avoiding the need to explicitly yield a true value from the end of it.

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 pick either the :repr(HASH) or :repr(magic) storage type.

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 blocks 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 isa ClassicPerlBaseClass {
   has $_value = "B";
   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 block 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 blocks 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.

$VERSION declaration

While it would be nice for CPAN and other toolchain modules to parse the embedded version declarations in class statements, the current state at time of writing (June 2020) is that none of them actually do. As such, it will still be necessary to make a once-per-file $VERSION declaration in syntax those modules can parse.

Further note that these modules will also not parse the class declaration, so you will have to duplicate this with a package declaration as well as a class keyword. This does involve repeating the package name, so is slightly undesirable.

It is hoped that eventually upstream toolchain modules will be adapted to accept the class syntax as being sufficient to declare a package and set its version.

File Layout

Begin the file with a use Object::Pad line; ideally including a minimum-required version. This should be followed by the toplevel package and class declarations 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;

package My::Classname 1.23;
class My::Classname;

# 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  :mutator;
   has $value :mutator;
}

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:

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

FEEDBACK

The following resources are useful forms of providing feedback, especially in the form of reports of what you find good or bad about the module, requests for new features, questions on best practice, etc...

The RT queue at https://rt.cpan.org/Dist/Display.html?Name=Object-Pad.
The #cor IRC channel on irc.perl.org.

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>

To install Object::Pad, copy and paste the appropriate command in to your terminal.

cpanm

cpanm Object::Pad

CPAN shell

perl -MCPAN -e shell
install Object::Pad

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

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NAME

SYNOPSIS

DESCRIPTION

Automatic Construction

The BUILDARGS phase

Slot assignment

The BUILD phase

The ADJUST phase

KEYWORDS

class

:repr(TYPE)

:strict(params)

role

:compat(invokable)

has

:reader, :reader(NAME)

:writer, :writer(NAME)

:mutator, :mutator(NAME)

:weak

:param, :param(NAME)

method

:override

BUILD

ADJUST

requires

CREPT FEATURES

Implied Pragmata

Yield True

SUBCLASSING CLASSIC PERL CLASSES

Storage of Instance Data

Object State During Methods Invoked By Superclass Constructor

STYLE SUGGESTIONS

$VERSION declaration

File Layout

Slot Names

WITH OTHER MODULES

Syntax::Keyword::Dynamically

Future::AsyncAwait

DESIGN TODOs

IMPLEMENTATION TODOs

FEEDBACK

SPONSORS

AUTHOR

Module Install Instructions