NAME
Object::Pad - a simple syntax for lexical slot-based objects
SYNOPSIS
use Object::Pad;
class Point {
has $x = 0;
has $y = 0;
BUILD {
($x, $y) = @_;
}
method move ($dX, $dY) {
$x += $dX;
$y += $dY;
}
method describe {
print "A point at ($x, $y)\n";
}
}
Point->new(5,10)->describe;
DESCRIPTION
WARNING This is an experimental proof-of-concept. Please don't actually
use this in production unless you are crazy :)
This module provides a simple syntax for creating object classes, which
uses private variables that look like lexicals as object member fields.
Automatic Construction
Classes are automatically provided with a constructor method, called
new, which helps create the object instances.
As part of the construction process, the BUILD block of every component
class will be invoked, passing in the list of arguments the constructor
was invoked with. Each class should perform its required setup
behaviour, but does not need to chain to the SUPER class first; this is
handled automatically.
If the class provides a BUILDARGS class method, that is used to mangle
the list of arguments before the BUILD blocks are called. Note this
must be a class method not an instance method (and so implemented using
sub). It should perform any SUPER chaining as may be required.
@args = $class->BUILDARGS( @_ )
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 extends
class Name extends BASECLASS {
...
}
class Name extends BASECLASS BASEVER {
...
}
If a package providing the superclass does not exist, an attempt is
made to load it by code equivalent to
require Animal ();
and thus it must either already exist, or be locatable via the usual
@INC mechanisms.
The superclass may or may not itself be implemented by Object::Pad, but
if it is not then see "SUBCLASSING CLASSIC PERL CLASSES" for further
detail on the semantics of how this operates.
An optional version check can also be supplied; it performs the
equivalent of
BaseClass->VERSION( $ver )
An optional list of attributes may be supplied in similar syntax as for
subs or lexical variables. (These are annotations about the class
itself; the concept should not be confused with per-object-instance
data, which here is called "slots").
The following class attributes are supported:
:repr(TYPE)
Sets the representation type for instances of this class. Must be one
of the following values:
:repr(native)
The native representation. This is an opaque representation type whose
contents are not specified. It only works for classes whose entire
inheritence hierarchy is built only from classes based on Object::Pad.
:repr(HASH)
The representation will be a blessed hash reference. The instance data
will be stored in an array referenced by a key called
Object::Pad/slots, which is fairly unlikely to clash with existing
storage on the instance. No other keys will be used; they are available
for implementions and subclasses to use. The exact format of the value
stored here is not specified and may change between module versions,
though it can be relied on to be well-behaved as some kind of perl data
structure for purposes of modules like Data::Dumper or serialisation
into things like YAML or JSON.
This representation type may be useful when converting existing classes
into using Object::Pad where there may be existing subclasses of it
that presume a blessed hash for their own use.
:repr(magic)
The representation will use MAGIC to apply the instance data in a way
that is invisible at the Perl level, and shouldn't get in the way of
other things the instance is doing even in XS modules.
This representation type is the only one that will work for subclassing
existing classes that do not use blessed hashes.
:repr(autoselect), :repr(default)
Since version 0.23.
This representation will select one of the representations above
depending on what is best for the situation. Classes not derived from a
non-Object::Pad base class will pick native, and classes derived from
non-Object::Pad bases will pick either the HASH or magic forms
depending on whether the instance is a blessed hash reference or some
other kind.
This achieves the best combination of DWIM while still allowing the
common forms of hash reference to be inspected by Data::Dumper, etc.
This is the default representation type, and does not have to be
specifically requested.
has
has $var;
has $var = EXPR;
has @var;
has %var;
has $var :ATTR ATTR...;
Declares that the instances of the class have a member field of the
given name. This member field (called a "slot") will be accessible as a
lexical variable within any method declarations in the class.
Array and hash members are permitted and behave as expected; you do not
need to store references to anonymous arrays or hashes.
Member fields are private to a class. They are not visible to users of
the class, nor to subclasses. In order to provide access to them a
class may wish to use "method" to create an accessor.
A scalar slot may provide a expression that gives an initialisation
value, which will be assigned into the slot of every instance during
the constructor before the BUILD blocks are invoked. Note that this
expression is evaluated exactly once, at runtime, after the class
definition has been parsed. It is not evaluated individually for every
object instance of that class.
The following slot attributes are supported:
:reader, :reader(NAME)
Generates a reader method to return the current value of the slot.
Currently these are only permitted for scalar slots. If no name is
given, the name of the slot is used. A single prefix character _ will
be removed if present.
has $slot :reader;
# equivalent to
has $slot; method slot { return $slot }
:writer, :writer(NAME)
Generates a writer method to set a new value of the slot from its first
argument. Currently these are only permitted for scalar slots. If no
name is given, the name of the slot is used prefixed by set_. A single
prefix character _ will be removed if present.
has $slot :writer;
# equivalent to
has $slot; method set_slot { $slot = shift }
:mutator, :mutator(NAME)
Generates an lvalue mutator method to return or set the value of the
slot. These are only permitted for scalar slots. If no name is given,
the name of the slot is used. A single prefix character _ will be
removed if present.
has $slot :mutator;
# equivalent to
has $slot; method slot :lvalue { $slot }
method
method NAME {
...
}
method NAME (SIGNATURE) {
...
}
method NAME :ATTRS... {
...
}
Declares a new named method. This behaves similarly to the sub keyword,
except that within the body of the method all of the member fields
("slots") are also accessible. In addition, the method body will have a
lexical called $self which contains the invocant object directly; it
will already have been shifted from the @_ array.
The signatures feature is automatically enabled for method
declarations. In this case the signature does not have to account for
the invocant instance; that is handled directly.
method m ($one, $two) {
say "$self invokes method on one=$one two=$two";
}
...
$obj->m(1, 2);
A list of attributes may be supplied as for sub. The most useful of
these is :lvalue, allowing easy creation of read-write accessors for
slots (but see also the :reader, :writer and :mutator slot attributes).
class Counter {
has $count;
method count :lvalue { $count }
}
my $c = Counter->new;
$c->count++;
Every method automatically gets the :method attribute applied, which
suppresses warnings about ambiguous calls resolved to core functions if
the name of a method matches a core function.
The following additional attributes are recognised by Object::Pad
directly:
:override
Marks that this method expects to override another of the same name
from a superclass. It is an error at compiletime if the superclass does
not provide such a method.
BUILD
BUILD {
...
}
BUILD (SIGNATURE) {
...
}
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.
Currently this is just a synonym for method BUILD ..., though in a
later version this may no longer be the case. A build block is not
permitted to use subroutine attributes.
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.
SUBCLASSING CLASSIC PERL CLASSES
There are a number of details specific to the case of deriving an
Object::Pad class from an existing classic Perl class that is not
implemented using Object::Pad.
Storage of Instance Data
Instances will pick either the :repr(HASH) or :repr(magic) storage
type.
Object State During Methods Invoked By Superclass Constructor
It is common in classic Perl OO style to invoke methods on $self during
the constructor. This is supported here since Object::Pad version 0.19.
Note however that any methods invoked by the superclass constructor may
not see the object in a fully consistent state. (This fact is not
specific to using Object::Pad and would happen in classic Perl OO as
well). The slot initialisers will have been invoked but the BUILD
blocks will not.
For example; in the following
package ClassicPerlBaseClass {
sub new {
my $self = bless {}, shift;
say "Value seen by superconstructor is ", $self->get_value;
return $self;
}
sub get_value { return "A" }
}
class DerivedClass extends ClassicPerlBaseClass {
has $_value = "B";
BUILD {
$_value = "C";
}
method get_value { return $_value }
}
my $obj = DerivedClass->new;
say "Value seen by user is ", $obj->get_value;
Until the ClassicPerlBaseClass::new superconstructor has returned the
BUILD block will not have been invoked. The $_value slot will still
exist, but its value will be B during the superconstructor. After the
superconstructor, the BUILD blocks are invoked before the completed
object is returned to the user. The result will therefore be:
Value seen by superconstructor is B
Value seen by user is C
STYLE SUGGESTIONS
While in no way required, the following suggestions of code style
should be noted in order to establish a set of best practices, and
encourage consistency of code which uses this module.
$VERSION declaration
While it would be nice for CPAN and other toolchain modules to parse
the embedded version declarations in class statements, the current
state at time of writing (June 2020) is that none of them actually do.
As such, it will still be necessary to make a once-per-file $VERSION
declaration in syntax those modules can parse.
Further note that these modules will also not parse the class
declaration, so you will have to duplicate this with a package
declaration as well as a class keyword. This does involve repeating the
package name, so is slightly undesirable.
It is hoped that eventually upstream toolchain modules will be adapted
to accept the class syntax as being sufficient to declare a package and
set its version.
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
Slot Names
Slot names should follow similar rules to regular lexical variables in
code - lowercase, name components separated by underscores. For tiny
examples such as "dumb record" structures this may be sufficient.
class Tag {
has $name :mutator;
has $value :mutator;
}
In larger examples with lots of non-trivial method bodies, it can get
confusing to remember where the slot variables come from (because we no
longer have the $self->{ ... } visual clue). In these cases it is
suggested to prefix the slot names with a leading underscore, to make
them more visually distinct.
class Spudger {
has $_grapefruit;
...
method mangle {
$_grapefruit->peel; # The leading underscore reminds us this is a slot
}
}
WITH OTHER MODULES
Syntax::Keyword::Dynamically
A cross-module integration test asserts that dynamically works
correctly on object instance slots:
use Object::Pad;
use Syntax::Keyword::Dynamically;
class Container {
has $value = 1;
method example {
dynamically $value = 2;
,..
# value is restored to 1 on return from this method
}
}
Future::AsyncAwait
As of Future::AsyncAwait version 0.38 and Object::Pad version 0.15,
both modules now use XS::Parse::Sublike to parse blocks of code.
Because of this the two modules can operate together and allow class
methods to be written as async subs which await expressions:
use Future::AsyncAwait;
use Object::Pad;
class Example
{
async method perform ($block)
{
say "$self is performing code";
await $block->();
say "code finished";
}
}
These three modules combine; there is additionally a cross-module test
to ensure that object instance slots can be dynamically set during a
suspended async method.
DESIGN TODOs
The following points are details about the design of pad slot-based
object systems in general:
* Is multiple inheritence actually required, if role composition is
implemented including giving roles the ability to use private slots?
* Consider the visibility of superclass slots to subclasses. Do
subclasses even need to be able to see their superclass's slots, or
are accessor methods always appropriate?
Concrete example: The $self->{split_at} access that
Tickit::Widget::HSplit makes of its parent class
Tickit::Widget::LinearSplit.
IMPLEMENTATION TODOs
These points are more about this particular module's implementation:
* Implement roles, including required method checking and the ability
to have private slots.
* Consider multiple inheritence of subclassing, if that is still
considered useful after adding roles.
* Work out why no indirect doesn't appear to work properly before
perl 5.20.
* Work out why we don't get a Subroutine new redefined at ... warning
if we
sub new { ... }
* The local modifier does not work on slot variables, because they
appear to be regular lexicals to the parser at that point. A
workaround is to use Syntax::Keyword::Dynamically instead:
use Syntax::Keyword::Dynamically;
has $loglevel;
method quietly {
dynamically $loglevel = LOG_ERROR;
...
}
AUTHOR
Paul Evans <leonerd@leonerd.org.uk>