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


       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;


    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 very 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.

    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.

    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( @_ )

    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.



       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


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


    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.


    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.


    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.


       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

    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:


    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 $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.


  :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.

  :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


       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;

    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


    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 {

    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.


       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.


    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

 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.


    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

 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


    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.

    See also


 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



    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


    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.


    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


    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;


    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,

      * The RT queue at

      * The #cor IRC channel on


    Paul Evans <>