Object::Pad(3pm)

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 {
      field $x :param = 0;
      field $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 {
      field $x :param = 0;
      field $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.

  Relation to "use feature 'class'"
Since its original creation, this module has evolved to become the test-bed for refining the design of what a good class-based object system would look like for Perl. While useful in its own right, it also acts as a preview of what Perl's "use feature 'class'" syntax will likely become. Being a CPAN module, this can be iterated much faster than core Perl, which only gets one major release per year. Getting the design right in this module first therefore helps to reduce the amount of development time needed in core Perl.

Where features overlap, both this module and the "class" feature added to recent Perl versions should largely agree on syntax and semantics. This module will likely always be ahead of core Perl in terms of exact abilties because it is acting as a test-bed for these new ideas. However, where both implementations support the same idea, they should in general use the same syntax and semantics to provide it.

If you are interested in using this object system in new code in a way that will be forward-compatible with the feature added directly in newer Perl versions, while also supporting versions of Perl before this feature was added, you may wish to use instead the module Feature::Compat::Class. That module enables the core Perl feature on the latest version of Perl, or uses "Object::Pad" to fill in the missing syntax and features on older Perl versions.

  Experimental Features
Since version 0.63.

While most of this module has evolved into a stable state, recently-added parts remain experimental because the design is still evolving, and many features and ideas have yet to implemented. Of these experimental features, feel free to try them out in newly-developed code, but don't complain if a later version is incompatible with your current code and you'll have to change it. Once individual features are stable and no longer declared experimental, they should have the same stability guarantees as any other long-term supported CPAN module, or core Perl itself.

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 - both within this CPAN module and as the wider experiment to provide it as a core Perl feature. See the "FEEDBACK" section.

Features of this module that are currently marked as experimental 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(mop)';

   use Object::Pad ':experimental(custom_field_attr)';

   use Object::Pad ':experimental(composed_adjust)';

   use Object::Pad ':experimental(inherit_field)';

   use Object::Pad ':experimental(apply_phaser)';

   use Object::Pad ':experimental(lexical_class)';

   use Object::Pad ':experimental(:all)';  # 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(mop custom_field_attr)';

Since version 0.810 attempting to request all of the experiments at once by using an empty :experimental() is currently accepted, but yields a warning. This may be removed in future.

  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;

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.

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.

:abstract

   :abstract

Since version 0.817.

Declares that this class is abstract. An abstract class is permitted to define required methods (i.e. named methods without a body definition).

Instances may not be created of this class type directly. Instead, a subclass of this class must be derived that provides method bodies for any of the named required methods. Any subclass of an abstract class must either provide bodies for all remaining required methods, or themselves be declared ":abstract" as well.

This provides a way in which a generic type of class can be created, that is intended for multiple different specialisations to further define different kinds of behaviour by providing bodies for those named methods. Being abstract means that instances cannot be created of this as-yet-incomplete class type.

: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".

   :repr(keys)

Since version 0.803.

The representation will be a blessed hash reference. The instance data will be stored in individual keys of the hash, named after the class and the field variable name, separated by a "/" symbol. Objects in this representation should behave predictably with data printing modules like Data::Dumper or serialisation via "YAML" or "JSON".

These two hash-based representation types 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(pvobj)

Since version 0.804.

The representation will be the "SVt_PVOBJ" type newly added to Perl, which offers more efficient storage for object instances. This is only available on Perl version 5.38.0 onwards.

This is also newly-added and may not be fully tested and reliable yet. Once it has more real-world testing and has proven reliable it may become the default instance representation on versions of Perl where it is available.

   :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.

  class (anon)
   my $class = class :ATTRS... { ... };

Since version 0.809.

If a "class" keyword is not followed by a package name, it creates an anonymous class expression. This is an expression that yields a value suitable to use as a constructor invocant for creating instances of that class, without specifying what its package name will actually be.

This is useful for creating small one-off instances inline in expressions, such as in unit tests. Since it still accepts the usual attributes and inner body statements, it can be useful for creating one-off instances of roles, with required methods being applied.

   my $testobj = (class {
      apply Role::Under::Test;
      method required { return "a useful value"; }
   })->new;

Due to limitations on how classes work in Perl, anonymous classes are still backed by long-lived named classes in the global symbol table, unlike true anonymous functions which can go out of scope and be reclaimed once no references to them remain in existence. This means that anonymous classes will retain references to any variables captured within them, even if the class expression itself goes out of scope and any instances created by it no longer remain.

  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 "ADJUST" blocks or methods provided by that role.

Since version 0.33.

   role Name {
      field $f;

      ADJUST { $f = "a value"; }

      method f { 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 "field" 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.

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

Since version 0.819; experimental

If a "class" keyword is preceded by "my" it creates a class that has lexical visiblity, rather than being available globally via a fully-qualified name in the symbol table. This is useful for creating internal helper classes within modules, such as for returning complex structures, or simply to use internally without being visible to the caller. As the class itself is only visible lexically, callers in other scopes are unable to see it to create new instances of it.

   class My::Module;

   my class HelperStructure
   {
      field $name :param :reader;
      field $value :param :reader;
   }

   method get_thing ()
   {
      return HelperStructure->new(
         name  => "the name",
         value => "the value",
      );
   }

Note: the above is technically a lie. Current versions of perl do not support truely anonymous packages to be used as classes, so even these "lexical" classes are in fact named on the symbol table. However, they are given a name that is syntactically-impossible for regular code to create or use, and a const-returning lexical function is created with the correct lexical name, which returns the true name of the class, so that the "->new" method can still be called on it.

As a result of this limitation, there are a number of operations that lexical classes do not currently support - such as subclassing. It is hoped that later versions of either this module, or perl itself, will be able to expand on these abilities. For now, this feature remains an experimental best-effort basis.

Due to the way this lexical function works, it can stand in for the class name when used with the "->isa" method. Note carefully here, that the lexical class is used directly as a bareword and not quoted.

   if( $obj->isa( HelperStructure ) ) { ... }

Additionally, as the right-hand side operand of the "isa" operator (in Perl version 5.32 or later) does not need quoting, it can be used there directly:

   if( $obj isa HelperStructure ) { ... }

  inherit
   inherit Classname;
   inherit Classname VER;

   inherit Classname LIST...;
   inherit Classname VER LIST...;

Since version 0.807.

Declares a superclass that this class extends. At most one superclass is supported. If present, this declaration must come before any methods or fields are declared, or any roles applied. (Other compile-time declarations such as "use" statements that import utility functions or other behaviours may be permitted before this, however, provided that they do not interact with the class structure in any way).

This is a newer form of the ":isa" attribute intended to be more flexible if import arguments or other features are added at a later time.

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

   require Classname;

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

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

   Classname->VERSION( $ver )

Experimentally since version 0.807, an optional list of arguments can also be provided, in similar syntax to those in a "use" statement. Currently this list of arguments must be names of fields to be inherited. Only fields in the base class that are annotated with the ":inheritable" attribute may be inherited. Once a field is inherited, methods and other expressions in the class body can use that field identically to any fields defined by that class itself.

   class Class1 {
      field $x :inheritable = 123;
   }

   class Class2 {
      inherit Class1 '$x';
      field $y = 456;
      method describe { say "Class2(x=$x,y=$y)" }
   }

   Class2->new->describe;

  apply
   apply Rolename;

   apply Rolename VER;

Since version 0.807.

Composes a role into the class; optionally requiring a version check on the role package. This is a newer form of the ":does" attribute intended to be more flexible if import arguments or other features are added at a later time.

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

"apply" statements can be freely mixed with other statements inside the body of the class. In particular, an "apply" statement that adds fields or methods may appear before or after the class has defined some of its own. It is not required that they appear first.

  field
   field $var;
   field @var;
   field %var;

   field $var :ATTR ATTR...;

   field $var = EXPR;

   field $var //= EXPR;
   field $var ||= EXPR;

   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 and "ADJUST" blocks 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 x {
      $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":

   field $x :param;          # this is required
   field $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.

:inheritable

Experimentally since version 0.807 fields may be optionally inherited when deriving a subclass from another. Not every field is allowed to be inherited. This attribute marks a field as being available for subclasses to inherit.

Field Initialiser Expressions

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.

Since version 0.73 this may also be written as a plain expression introduced by an equals symbol ("="). This is equivalent to using a block. Note carefully: the equals symbol is part of the "field" syntax; it is not simply a runtime assignment operator that happens once at the time the class is declared. Just like the block form describe above, the expression is evaluated during the constructor of every instance.

Since version 0.74 this expression may also be written using a defined-or or logical-or assignment operator ("//=" or "||="). In these case, the default expression will be evaluated and assigned if the caller did not pass a value to the constructor at all, or if the value passed was undef (for "//=") or false (for "||="). For most scalar parameters, where "undef" is not a valid value, you probably wanted to use "//=" to assign defaults.

   class Action {
      field $timeout :param //= 20;
      ...
   }

   # The default of 20 will apply here too
   my $act = Action->new( timeout => $opts{timeout} );

Note that $self is specifically not visible during an initialiser expression. This is because the object is not yet fully constructed, so it would be dangerous to allow access to it while in this state. However, the "__CLASS__" keyword is available, so initialiser expressions can make use of class-based dispatch to invoke class-level methods to help provide values.

Field initialier expressions were originally experimental, but since version 0.800 no longer emit experimental warnings.

Since version 0.806 fields already declared in a class are visible during the initialisation expression of later fields, and their assigned value can be used here. If the earlier field had a ":param" declaration, it will have been assigned from the value passed to the constructor. Note however that all "ADJUST" blocks happen after field initialisation expressions, so any modified values set in such blocks will not be visible at this time.

Control flow that attempts to leave a field initialiser expression or 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
Since version 0.813 this keyword is no longer recognised.

It used to be an earlier version of what is now the "field" keyword.

   has $var;
   has @var;
   has %var;

   has $var = EXPR;

   has $var { BLOCK }

Because of the one-shot immediate nature of these initialisation expressions (and a bunch of other reasons), the keyword was removed.

If you need to evaluate an expression exactly once during the class declaration and assign its now-constant value to every instace, store it in a regular "my" variable instead:

   my $default_var = EXPR;
   field $var = $default_var;

  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 or abstract class. Such a method must be provided by any class that wishes to be non-abstract. It will be a compiletime error to apply the role to a class, or derive from an abstract 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 or abstract base classes, 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.

  my method
   my method NAME { ... }

Since version 0.814 lexical method declarations are supported using the "my" keyword prefix. These become available as lexical functions, rather than being stored in the class package. As a result, they are not available by named method resolution, package "->can" lookup, or via the MOP. These are a convenient alternative to the syntax given above, where the method is stored anonymously via a lexical variable.

Since lexical methods are not visible to named method resolution, they must be invoked by function-call syntax, remembering to pass in the invocant as the first argument:

   my method inner { ... }

   method outer {
      inner($self, @args);
   }

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

An adjust block is not a subroutine and thus is not permitted to use subroutine attributes (except see below). Note that an "ADJUST" block is a named phaser block and not a method; it does not use the "sub" or "method" keyword. But, like with "method", the member fields are accessible within the code body, as is the special $self lexical.

Currently, an "ADJUST" block receives a reference to the hash containing the current constructor arguments, as per "ADJUSTPARAMS" (see below). This was added in version 0.66 but will be removed again as it conflicts with the more flexible and generally nicer named-parameter "ADJUST :params" syntax (see below). Such uses should be considered deprecated. A warning will be printed to indicate this whenever an "ADJUST" block uses a signature. This warning can be quieted by using "ADJUSTPARAMS" instead. Additionally, a warning may be printed on code that attempts to access the params hashref via the @_ array.

Since version 0.801 in a future version of this module, "ADJUST" blocks may be implemented as true blocks and will not permit out-of-block control flow. At present, they are implemented as one full CV per block, but a warning is emitted if out-of-block control flow is attempted.

   ADJUST {
      return;
   }

   Using return to leave an ADJUST block is discouraged and will be removed
   in a later version at FILE line LINE.

Since version 0.805 an experimental feature can be enabled that puts all the "ADJUST" blocks into a single CV, rather than creating one CV for every block. This is currently being tested for stability, and may become the default behaviour in a future version. For now it must be requested specially:

   use Object::Pad ':experimental(composed_adjust)';

  ADJUST :params
   ADJUST :params ( :$var1, :$var2, ... ) {
      ...
   }

   ADJUST :params ( :$var1, :$var2, ..., %varN ) {
      ...
   }

Since version 0.70; non-experimental since version 0.805.

An "ADJUST" block can marked with a ":params" attribute, meaning that it consumes additional constructor parameters by assigning them into lexical variables.

Before the block itself, a list of lexical variables are introduced, inside parentheses. The name of each one is preceded by a colon, and consumes a constructor parameter of the same name. These parameters are considered "consumed" for the purposes of a :strict(params) check.

A named parameter may be provided with default expression, which is evaluated if no matching named argument is provided to the constructor. As with fields, if a named parameter has no defaulting expression it becomes a required argument to the constructor; an exception is thrown by the constructor if it absent.

For example,

   ADJUST :params ( :$x, :$y = "default", :$z ) { ... }

Note here that "x" and "z" are required parameters for the constructor of a class containing this block, but "y" is an optional parameter whose value will be filled in by the expression if not provided. Because these parameters are named and not positional, there is no ordering constraint; required and optional parameters can be freely mixed.

Optional parameters can also use the "//=" and "||=" operators to provide a default expression. In these cases, the default will be applied if the caller did not provide the named argument at all, or if the provided value was not defined (for "//=") or not true (for "||=").

   ADJUST :params ( :$name //= "unnamed" ) { ... }

Like with subroutine signature parameters, every declared named parameter is visible to the defaulting expression of all the later ones. This permits values to be calculated based on other ones. For example,

   ADJUST :params ( :$thing = undef, :$things = [ $thing ] ) {
      # Here, @$things is a list of values
   }

This permits the caller to pass a list of values via an array reference in the "things" parameter, or a single value in "thing".

The final element may be a regular hash variable. This requests that all remaining named parameters are made available inside it. The code in the block should "delete" from this hash any parameters it wishes to consume, as with the earlier case above.

It is unspecified whether named fields or parameters for subclasses yet to be processed are visible to hashes of earlier superclasses. In the current implementation they are, but code should not rely on this fact.

Note also that there must be a space between the ":params" attribute and the parentheses holding the named parameters. If this space is not present, perl will parse the parentheses as if they are the value to the :params() attribute, and this will fail to parse as intended. As with other attributes and subroutine signatures, this whitespace is significant.

(This notation is borrowed from a plan to add named parameter support to perl's subroutine signature syntax).

  ADJUSTPARAMS
Since version 0.51.

   ADJUSTPARAMS ( $params ) {    # on perl 5.26 onwards
      ...
   }

   ADJUST {
      my $params = shift;
      ...
   }

A variant of an "ADJUST" block that 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.

  APPLY
Experimental. Since version 0.815.

   APPLY ( $class_mop ) {        # on perl 5.26 onwards
      ...
   }

   APPLY {
      my $class_mop = shift;
      ...
   }

Only valid within a "role" definition. Declares a block of code that will be run at compile-time whenever the role is applied to a class. Each time it is applied to a new class, the code will be invoked. It receives as an argument a Object::Pad::MOP::Class instance representing the class to which the role is currently being applied.

The eventual intent is that the presence of any of these phaser blocks will replace the current implicit behaviour of applying a role, though currently they run in addition to it. This is part of an ongoing experiment whose details will change over time.

  __CLASS__
   my $classname = __CLASS__;

Since version 0.72.

Only valid within the body (or signature) of a "method", an "ADJUST" block, or the initialising expression of a "field". Yields the class name of the instance that the method, block or expression is invoked on.

This is similar to the core perl "__PACKAGE__" constant, except that it cares about the dynamic class of the actual instance, not the static class the code belongs to. When invoked by a subclass instance that inherited code from its superclass it yields the name of the class of the instance regardless of which class defined the code.

For example,

   class BaseClass {
      ADJUST { say "Constructing an instance of " . __CLASS__; }
   }

   class DerivedClass :isa(BaseClass) { }

   my $obj = DerivedClass->new;

Will produce the following output

   Constructing an instance of DerivedClass

This is particularly useful in field initialisers for invoking (constant) methods on the invoking class to provide default values for fields. This way a subclass could provide a different value.

   class Timer {
      use constant DEFAULT_DURATION => 60;
      field $duration = __CLASS__->DEFAULT_DURATION;
   }

   class ThreeMinuteTimer :isa(Timer) {
      use constant DEFAULT_DURATION => 3 * 60;
   }

  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
The following behaviour is likely to be removed in a later version of this module.

In order to encourage users to write clean, modern code, the body of the "class" block currently 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 behaviour was designed early around the original "line-0" version of the Perl 7 plan, which has subsequently been found to be a bad design and abandoned. That leaves this module in an unfortunate situation, because its behaviour here does not match the plans for core perl; where the recently-added "class" keyword does none of this, although the "method" keyword always behaves as if signatures were enabled anyway.

It is eventually planned that this behaviour will be removed from "Object::Pad" entirely (except for enabling the "signatures" feature). While that won't in itself break any existing code, it would mean that code which previously ran with the protection of "strict" and "warnings" would now not be. A satisfactory solution to this problem has not yet been found, but until then it is suggested that code using this module remembers to explicitly enable this set of pragmata before using the "class" keyword.

A handy way to do this is to use the "use VERSION" syntax; v5.36 or later will already perform all of the pragmata listed above.

   use v5.36;

If you import this module with a module version number of 0.800 or higher it will enable a warning if you forget to enable "strict" and "warnings" before using the "class" or "roll" keywords:

   use Object::Pad 0.800;

   class X { ... }

   class keyword enabled 'use strict' but this will be removed in a later version at FILE line 3.
   class keyword enabled 'use warnings' but this will be removed in a later version at FILE line 3.

  Yield True
The following behaviour is likely to be removed in a later version of this module.

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.

As with the implied pragmata above, this behaviour has also been found to be a bad design and will likely be removed soon. For now it is suggested not to rely on it and instead either use the new "module_true" feature already part of the "use v5.38" pragma, or on older perls simply remember to put an explicit true value at the end of the file.

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" and "ADJUST" 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) {
      field $_value = "B";
      ADJUST {
         $_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 "ADJUST" 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" and "ADJUST" 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

   # field, 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 {
      field $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>