NAME

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

SYNOPSIS

    On perl version 5.26 onwards:

       use v5.26;
       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;

    Or, for older perls that lack signatures:

       use Object::Pad;
    
       class Point {
          has $x :param = 0;
          has $y :param = 0;
    
          method move {
             my ($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.

    While most of this module has evolved into a stable state in practice,
    parts remain experimental because the design is still evolving, and
    many features and ideas have yet to 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.

 Experimental Features

    Since version 0.63.

    Some of the features of this module are currently marked as
    experimental. They will provoke warnings in the experimental category,
    unless silenced.

    You can silence this with no warnings 'experimental' but then that will
    silence every experimental warning, which may hide others
    unintentionally. For a more fine-grained approach you can instead use
    the import line for this module to only silence the module's warnings
    selectively:

       use Object::Pad ':experimental(init_expr)';
    
       use Object::Pad ':experimental(mop)';
    
       use Object::Pad ':experimental(custom_field_attr)';
    
       use Object::Pad ':experimental';  # all of the above

    Since version 0.64.

    Multiple experimental features can be enabled at once by giving
    multiple names in the parens, separated by spaces:

       use Object::Pad ':experimental(init_expr mop)';

 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 fields, 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( @_ )

  Field assignment

    If any field 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 fields 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 fields 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

    Next, the ADJUST block of every component class is invoked. This
    happens after the fields are assigned their initial values and the
    BUILD blocks have been run.

  The strict-checking phase

    Finally, before the object is returned, if the ":strict(params)" class
    attribute is present, then the constructor will throw an exception if
    there are any remaining named arguments left over after assigning them
    to fields as per :param declarations, and running any ADJUST blocks.

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. Both extends and isa keywords
    are now deprecated, in favour of the ":isa" attribute which is
    preferred because it follows a more standard grammar without this
    special-case.

    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. Both implements and does
    keywords are now deprecated, in favour of the ":does" attribute which
    is preferred because it follows a more standard grammar without this
    special-case.

    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 "fields").

    Whitespace is permitted within the value and is automatically trimmed,
    but as standard Perl parsing rules, no space is permitted between the
    attribute's name and the open parenthesis of its value:

       :attr( value here )     # is permitted
       :attr (value here)      # not permitted

    The following class attributes are supported:

  :isa

       :isa(CLASS)
    
       :isa(CLASS CLASSVER)

    Since version 0.57.

    Declares a superclass that this class extends. At most one superclass
    is supported.

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

       require CLASS ();

    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 )

  :does

       :does(ROLE)
    
       :does(ROLE ROLEVER)

    Since version 0.57.

    Composes a role into the class; optionally requiring a version check on
    the role package. This is a newer form of the implements and does
    keywords and should be preferred for new code.

    Multiple roles can be composed by using multiple :does attributes, one
    per role.

    The package will be loaded in a similar way to how the ":isa" attribute
    is handled.

  :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
    inheritance 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 field and left unconsumed by any ADJUST block).

    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.

 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 similar 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 fields. These are visible to any BUILD
    blocks or methods provided by that role.

    Since version 0.33.

       role Name {
          field $f;
    
          BUILD { $f = "a value"; }
    
          method field { return $f; }
       }

    Since version 0.57 a role can declare that it provides another role:

       role Name :does(OTHERROLE) { ... }
       role Name :does(OTHERROLE OTHERVER) { ... }

    This will include all of the methods from the included role.
    Effectively this means that applying the "outer" role to a class will
    imply applying the other role as well.

    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 field
    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.

 field

       field $var;
       field @var;
       field %var;
    
       field $var :ATTR ATTR...;
    
       field $var { BLOCK }

    Since version 0.66.

    Declares that the instances of the class or role have a member field of
    the given name. This member field 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 inherited by subclasses nor 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.

    The following field attributes are supported:

  :reader, :reader(NAME)

    Since version 0.27.

    Generates a reader method to return the current value of the field. If
    no name is given, the name of the field is used. A single prefix
    character _ will be removed if present.

       field $x :reader;
    
       # equivalent to
       field $x;  method x { return $x }

    Since version 0.55 these are permitted on any field type, but prior
    versions only allowed them on scalar fields. The reader method behaves
    identically to how a lexical variable would behave in the same context;
    namely returning a list of values from an array or key/value pairs from
    a hash when in list context, or the number of items or keys when in
    scalar context.

       field @items :reader;
    
       foreach my $item ( $obj->items ) { ... }   # iterates the list of items
    
       my $count = $obj->items;                   # yields count of items

  :writer, :writer(NAME)

    Since version 0.27.

    Generates a writer method to set a new value of the field from its
    arguments. If no name is given, the name of the field is used prefixed
    by set_. A single prefix character _ will be removed if present.

       field $x :writer;
    
       # equivalent to
       field $x;  method set_x { $x = 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");

    Since version 0.55 these are permitted on any field type, but prior
    versions only allowed them on scalar fields. On arrays or hashes, the
    writer method takes a list of values to be assigned into the field,
    completely replacing any values previously there.

  :mutator, :mutator(NAME)

    Since version 0.27.

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

       field $x :mutator;
    
       # equivalent to
       field $x;  method x :lvalue { $x }

    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.

  :accessor, :accessor(NAME)

    Since version 0.53.

    Generates a combined reader-writer accessor method to set or return the
    value of the field. These are only permitted for scalar fields. If no
    name is given, the name of the field is used. A prefix character _ will
    be removed if present.

    This method takes either zero or one additional arguments. If an
    argument is passed, the value of the field is set from this argument
    (even if it is undef). If no argument is passed (i.e. scalar @_ is
    false) then the field is not modified. In either case, the value of the
    field is then returned.

       field $x :accessor;
    
       # equivalent to
       field $x;
    
       method field {
          $x = shift if @_;
          return $x;
       }

  :weak

    Since version 0.44.

    Generated code which sets the value of this field 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 field variable you must remember
    to call Scalar::Util::weaken (or builtin::weaken on Perl 5.36 or above)
    yourself.

  :param, :param(NAME)

    Since version 0.41.

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

    Any field that has :param but does not have a default initialisation
    expression or block 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

    Any field that has a :param and an initialisation block will only run
    the code in the block if required by the constructor. If a named
    parameter is passed to the constructor for this field, then its code
    block will not be executed.

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

  Field Initialiser Blocks

    Since version 0.54 a deferred statement block is also permitted, on any
    field variable type. This permits code to be executed as part of the
    instance constructor, rather than running just once when the class is
    set up. Code in a field initialisation block is roughly equivalent to
    being placed in a BUILD or ADJUST block.

    This feature should be considered experimental, and will emit warnings
    to that effect. They can be silenced with

       use Object::Pad qw( :experimental(init_expr) );

    Control flow that attempts to leave a field initialiser block is not
    permitted. This includes any return expression, any next/last/redo
    outside of a loop, with a dynamically-calculated label expression, or
    with a label that it doesn't appear in. goto statements are also
    currently forbidden, though known-safe ones may be permitted in future.

    Loop control expressions that are known at compiletime to affect a loop
    that they appear within are permitted.

       field $x { foreach(@list) { next; } }       # this is fine
    
       field $x { LOOP: while(1) { last LOOP; } }  # this is fine too

 has

       has $var;
       has @var;
       has %var;
    
       has $var = EXPR;
    
       has $var { BLOCK }

    An older version of the "field" keyword.

    This generally behaves like field, except that inline expressions are
    also permitted.

    A scalar field may provide a expression that gives an initialisation
    value, which will be assigned into the field 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. Since version 0.54 this is also permitted on array and
    hash fields.

 method

       method NAME {
          ...
       }
    
       method NAME (SIGNATURE) {
          ...
       }
    
       method NAME :ATTRS... {
          ...
       }
    
       method NAME;

    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 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.

    If the method has no body and is given simply as a name, this declares
    a required method for a role. Such a method must be provided by any
    class that implements the role. It will be a compiletime error to
    combine the role with a class that does not provide this.

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

       class Counter {
          field $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.

  :common

    Since version 0.62.

    Marks that this method is a class-common method, instead of a regular
    instance method. A class-common method may be invoked on class names
    instead of instances. Within the method body there is a lexical $class
    available, rather than $self. Because it is not associated with a
    particular object instance, a class-common method cannot see instance
    fields.

 method (lexical)

       method $var { ... }
    
       method $var :ATTRS... (SIGNATURE) { ... }

    Since version 0.59.

    Declares a new lexical method. Lexical methods are not visible via the
    package namespace, but instead are stored directly in a lexical
    variable (with the same scoping rules as regular my variables). These
    can be invoked by subsequent method code in the same block by using
    $self->$var(...) method call syntax.

       class WithPrivate {
          field $var;
    
          # Lexical methods can still see instance fields as normal
          method $inc_var { $var++; say "Var was incremented"; }
          method $dec_var { $var--; say "Var was decremented"; }
    
          method bump {
             $self->$inc_var;
             say "In the middle";
             $self->$dec_var;
          }
       }
    
       my $obj = WithPrivate->new;
    
       $obj->bump;
    
       # Neither $inc_var nor $dec_var are visible here

    This effectively provides the ability to define private methods, as
    they are inaccessible from outside the block that defines the class. In
    addition, there is no chance of a name collision because lexical
    variables in different scopes are independent, even if they share the
    same name. This is particularly useful in roles, to create internal
    helper methods without letting those methods be visible to callers, or
    risking their names colliding with other named methods defined on the
    consuming class.

 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 {
          ...
       }
    
       ADJUST ( $params ) {    # on perl 5.26 onwards
          ...
       }
    
       ADJUST {
          my $params = shift;
          ...
       }

    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 field
    value assignment. It can make any final adjustments to the instance
    (such as initialising fields from calculated values).

    Since version 0.66 it receives a reference to the hash containing the
    current constructor parameters. This hash will not contain any
    constructor parameters already consumed by ":param" declarations on any
    fields, but only the leftovers once those are processed.

    The code in the block should delete from this hash any parameters it
    wishes to consume. Once all the ADJUST blocks have run, any remaining
    keys in the hash will be considered errors, subject to the
    ":strict(params)" check.

    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.

 ADJUSTPARAMS

    Since version 0.51.

    A synonym for ADJUST.

    Prior to version 0.66, the ADJUSTPARAMS keyword created a different
    kind of adjust block that receives a reference to the parameters hash.
    Since version 0.66, regular ADJUST blocks also receive this, so the two
    keywords are now synonyms.

 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.

    This form of declaring a required method is now vaguely discouraged, in
    favour of the bodyless method form described above.

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 field 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 field 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.

    See also

      * https://github.com/Perl-Toolchain-Gang/Module-Metadata/issues/33

 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

 Field Names

    Field 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 {
          field $name  :mutator;
          field $value :mutator;
       }

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

       class Spudger {
          field $_grapefruit;
    
          ...
    
          method mangle {
             $_grapefruit->peel; # The leading underscore reminds us this is a field
          }
       }

WITH OTHER MODULES

 Syntax::Keyword::Dynamically

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

       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 fields can be dynamically set during a
    suspended async method.

 Devel::MAT

    When using Devel::MAT to help analyse or debug memory issues with
    programs that use Object::Pad, you will likely want to additionally
    install the module Devel::MAT::Tool::Object::Pad. This will provide new
    commands and extend existing ones to better assist with analysing
    details related to Object::Pad classes and instances of them.

       pmat> fields 0x55d7c173d4b8
       The field AV ARRAY(3)=NativeClass at 0x55d7c173d4b8
       Ix Field   Value
       0  $sfield SCALAR(UV) at 0x55d7c173d938 = 123
       ...
    
       pmat> identify 0x55d7c17606d8
       REF() at 0x55d7c17606d8 is:
       └─the %hfield field of ARRAY(3)=NativeClass at 0x55d7c173d4b8, which is:
       ...

DESIGN TODOs

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

      * Is multiple inheritance actually required, if role composition is
      implemented including giving roles the ability to use private fields?

      * Consider the visibility of superclass fields to subclasses. Do
      subclasses even need to be able to see their superclass's fields, 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 inheritance 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 field 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;
      
         field $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.

SPONSORS

    With thanks to the following sponsors, who have helped me be able to
    spend time working on this module and other perl features.

      * Oetiker+Partner AG https://www.oetiker.ch/en/

      * Deriv http://deriv.com

      * Perl-Verein Schweiz https://www.perl-workshop.ch/

    Additional details may be found at
    https://github.com/Ovid/Cor/wiki/Sponsors.

AUTHOR

    Paul Evans <leonerd@leonerd.org.uk>