path: root/doc/misc/eieio.texi

\input texinfo
@setfilename ../../info/eieio.info
@set TITLE Enhanced Implementation of Emacs Interpreted Objects
@set AUTHOR Eric M. Ludlam
@settitle @value{TITLE}
@include docstyle.texi

@c *************************************************************************
@c @ Header
@c *************************************************************************

@copying
This manual documents EIEIO, an object framework for Emacs Lisp.

Copyright @copyright{} 2007--2021 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with the Front-Cover Texts being ``A GNU Manual,''
and with the Back-Cover Texts as in (a) below.  A copy of the license
is included in the section entitled ``GNU Free Documentation License.''

(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and
modify this GNU manual.''
@end quotation
@end copying

@dircategory Emacs misc features
@direntry
* EIEIO: (eieio).               An objects system for Emacs Lisp.
@end direntry

@titlepage
@center @titlefont{@value{TITLE}}
@sp 4
@center by @value{AUTHOR}
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@macro eieio{}
@i{EIEIO}
@end macro

@node Top
@top EIEIO

@eieio{} (``Enhanced Implementation of Emacs Interpreted Objects'')
provides an Object Oriented layer for Emacs Lisp, following the basic
concepts of the Common Lisp Object System (CLOS).  It provides a
framework for writing object-oriented applications in Emacs.

@ifnottex
@insertcopying
@end ifnottex

@menu
* Quick Start::           Quick start for EIEIO.
* Introduction::          Why use @eieio{}?  Basic overview, samples list.
* Building Classes::      How to write new class structures.
* Making New Objects::    How to construct new objects.
* Accessing Slots::       How to access a slot.
* Writing Methods::       How to write a method.
* Method Invocation::     How methods are invoked.
* Predicates::            Class-p, Object-p, etc-p.
* Association Lists::     List of objects as association lists.
* Customizing::           Customizing objects.
* Introspection::         Looking inside a class.
* Base Classes::          Additional classes you can inherit from.
* Browsing::              Browsing your class lists.
* Class Values::          Displaying information about a class or object.
* Default Superclass::    The root superclasses.
* Signals::               When you make errors.
* Naming Conventions::    Name your objects in an Emacs friendly way.
* CLOS compatibility::    What are the differences?
* Wish List::             Things about EIEIO that could be improved.
* GNU Free Documentation License::  The license for this documentation.
* Function Index::
@end menu

@node Quick Start
@chapter Quick Start

@eieio{} provides an Object Oriented layer for Emacs Lisp.  You can
use @eieio{} to create classes, methods for those classes, and
instances of classes.

Here is a simple example of a class named @code{person}, containing
three slots named @code{name}, @code{birthday}, and @code{phone}:

@example
(defclass person () ; No superclasses
  ((name :initarg :name
         :initform ""
         :type string
         :custom string
         :documentation "The name of a person.")
   (birthday :initarg :birthday
             :initform "Jan 1, 1970"
             :custom string
             :type string
             :documentation "The person's birthday.")
   (phone :initarg :phone
          :initform ""
          :documentation "Phone number."))
  "A class for tracking people I know.")
@end example

Each class can have methods, which are defined like this:

@example
(cl-defmethod call-person ((pers person) &optional scriptname)
  "Dial the phone for the person PERS.
Execute the program SCRIPTNAME to dial the phone."
  (message "Dialing the phone for %s"  (oref pers name))
  (shell-command (concat (or scriptname "dialphone.sh")
                         " "
                         (oref pers phone))))
@end example

@noindent
In this example, the first argument to @code{call-person} is a list,
of the form (@var{varname} @var{classname}).  @var{varname} is the
name of the variable used for the first argument; @var{classname} is
the name of the class that is expected as the first argument for this
method.

@eieio{} dispatches methods based on the type of the first argument.
You can have multiple methods with the same name for different classes
of object.  When the @code{call-person} method is called, the first
argument is examined to determine the class of that argument, and the
method matching the input type is then executed.

Once the behavior of a class is defined, you can create a new
object of type @code{person}.  Objects are created by calling the
constructor.  The constructor is a function with the same name as your
class which returns a new instance of that class.  Here is an example:

@example
(setq pers (person :name "Eric" :birthday "June" :phone "555-5555"))
@end example

@noindent
For backward compatibility reasons, the first argument can be a string (a name
given to this instance).  Each instance used to be given a name, so different
instances could be easily distinguished when debugging.

It can be a bit repetitive to also have a :name slot.  To avoid doing
this, it is sometimes handy to use the base class @code{eieio-named}.
@xref{eieio-named}.

Calling methods on an object is a lot like calling any function.  The
first argument should be an object of a class which has had this
method defined for it.  In this example it would look like this:

@example
(call-person pers)
@end example

@noindent
or

@example
(call-person pers "my-call-script")
@end example

In these examples, @eieio{} automatically examines the class of
@code{pers}, and ensures that the method defined above is called.  If
@code{pers} is some other class lacking a @code{call-person} method, or
some other data type, Emacs signals a @code{cl-no-applicable-method}
error.  @ref{Signals}.

@node Introduction
@chapter Introduction

First off, please note that this manual cannot serve as a complete
introduction to object oriented programming and generic functions in
LISP@.  Although EIEIO is not a complete implementation of the Common
Lisp Object System (CLOS) and also differs from it in several aspects,
it follows the same basic concepts.  Therefore, it is highly
recommended to learn those from a textbook or tutorial first,
especially if you only know OOP from languages like C++ or Java.  If
on the other hand you are already familiar with CLOS, you should be
aware that @eieio{} does not implement the full CLOS specification and
also differs in some other aspects which are mentioned below (also
@pxref{CLOS compatibility}).

@eieio{} supports the following features:

@enumerate
@item
A structured framework for the creation of basic classes with attributes
and methods using singular inheritance similar to CLOS.
@item
Type checking, and slot unbinding.
@item
Method definitions similar to CLOS.
@item
Simple and complex class browsers.
@item
Edebug support for methods.
@item
Imenu updates.
@item
Byte compilation support of methods.
@item
Help system extensions for classes and methods.
@item
Several base classes for interesting tasks.
@item
Simple test suite.
@item
Public and private classifications for slots (extensions to CLOS)
@item
Customization support in a class (extension to CLOS)
@end enumerate

Due to restrictions in the Emacs Lisp language, CLOS cannot be
completely supported, and a few functions have been added in place of
setf.  Here are some important CLOS features that @eieio{} presently
lacks:

@table @asis

@item Method dispatch
EIEO does not support method dispatch for built-in types and multiple
arguments types.  In other words, method dispatch only looks at the
first argument, and this one must be an @eieio{} type.

@item Support for metaclasses
There is just one default metaclass, @code{eieio-default-superclass},
and you cannot define your own.  The @code{:metaclass} tag in
@code{defclass} is ignored.  Also, functions like @code{find-class}, which
should return instances of the metaclass, behave differently in
@eieio{} in that they return symbols or plain structures instead.

@item EQL specialization
EIEIO does not support it.

@item @code{:around} method tag
This CLOS method tag is non-functional.

@item :default-initargs in @code{defclass}
Each slot can have an @code{:initform} tag, so this is not really necessary.

@item Mock object initializers
Each class contains a mock object used for fast initialization of
instantiated objects.  Using functions with side effects on object slot
values can potentially cause modifications in the mock object.  @eieio{}
should use a deep copy but currently does not.

@end table

@node Building Classes
@chapter Building Classes

A @dfn{class} is a definition for organizing data and methods
together.  An @eieio{} class has structures similar to the classes
found in other object-oriented (OO) languages.

To create a new class, use the @code{defclass} macro:

@defmac defclass class-name superclass-list slot-list &rest options-and-doc

Create a new class named @var{class-name}.  The class is represented
by a symbol with the name @var{class-name}.  @eieio{} stores the structure of
the class as a symbol property of @var{class-name} (@pxref{Symbol
Components,,,elisp,GNU Emacs Lisp Reference Manual}).

When defining a class, @eieio{} overwrites any preexisting variable or
function bindings for the symbol @var{class-name}, which may lead to
undesired consequences.  Before naming a new class, you should check
for name conflicts.  To help avoid cross-package conflicts you should
choose a name with the same prefix you chose for the rest of your
package's functions and variables (@pxref{Coding
Conventions,,,elisp,GNU Emacs Lisp Reference Manual}).

The @var{class-name} symbol's variable documentation string is a
modified version of the doc string found in @var{options-and-doc}.
Each time a method is defined, the symbol's documentation string is
updated to include the method's documentation as well.

The parent classes for @var{class-name} is @var{superclass-list}.
Each element of @var{superclass-list} must be a class.  These classes
are the parents of the class being created.  Every slot that appears
in each parent class is replicated in the new class.

If two parents share the same slot name, the parent which appears in
the @var{superclass-list} first sets the tags for that slot.  If the
new class has a slot with the same name as the parent, the new slot
overrides the parent's slot.

When overriding a slot, some slot attributes cannot be overridden
because they break basic OO rules.  You cannot override @code{:type}
or @code{:protection}.
@end defmac

@noindent
Whenever defclass is used to create a new class, a predicate is
created for it, named @code{@var{CLASS-NAME}-p}:

@defun CLASS-NAME-p object
Return non-@code{nil} if and only if @var{OBJECT} is of the class
@var{CLASS-NAME}.
@end defun

@defvar eieio-error-unsupported-class-tags
If non-@code{nil}, @code{defclass} signals an error if a tag in a slot
specifier is unsupported.

This option is here to support programs written with older versions of
@eieio{}, which did not produce such errors.
@end defvar

@menu
* Inheritance::         How to specify parents classes.
* Slot Options::        How to specify features of a slot.
* Class Options::       How to specify features for this class.
@end menu

@node Inheritance
@section Inheritance

@dfn{Inheritance} is a basic feature of an object-oriented language.
In @eieio{}, a defined class specifies the super classes from which it
inherits by using the second argument to @code{defclass}.  Here is an
example:

@example
(defclass my-baseclass ()
   ((slot-A :initarg :slot-A)
    (slot-B :initarg :slot-B))
  "My Baseclass.")
@end example

@noindent
To subclass from @code{my-baseclass}, we specify it in the superclass
list:

@example
(defclass my-subclass (my-baseclass)
   ((specific-slot-A :initarg specific-slot-A)
    )
   "My subclass of my-baseclass")
@end example

@indent
Instances of @code{my-subclass} will inherit @code{slot-A} and
@code{slot-B}, in addition to having @code{specific-slot-A} from the
declaration of @code{my-subclass}.

@eieio{} also supports multiple inheritance.  Suppose we define a
second baseclass, perhaps an ``interface'' class, like this:

@example
(defclass my-interface ()
   ((interface-slot :initarg :interface-slot))
   "An interface to special behavior."
   :abstract t)
@end example

@noindent
The interface class defines a special @code{interface-slot}, and also
specifies itself as abstract.  Abstract classes cannot be
instantiated.  It is not required to make interfaces abstract, but it
is a good programming practice.

We can now modify our definition of @code{my-subclass} to use this
interface class, together with our original base class:

@example
(defclass my-subclass (my-baseclass my-interface)
   ((specific-slot-A :initarg specific-slot-A)
    )
   "My subclass of my-baseclass")
@end example

@noindent
With this, @code{my-subclass} also has @code{interface-slot}.

If @code{my-baseclass} and @code{my-interface} had slots with the same
name, then the superclass showing up in the list first defines the
slot attributes.

Inheritance in @eieio{} is more than just combining different slots.
It is also important in method invocation.  @ref{Methods}.

If a method is called on an instance of @code{my-subclass}, and that
method only has an implementation on @code{my-baseclass}, or perhaps
@code{my-interface}, then the implementation for the baseclass is
called.

If there is a method implementation for @code{my-subclass}, and
another in @code{my-baseclass}, the implementation for
@code{my-subclass} can call up to the superclass as well.

@node Slot Options
@section Slot Options

The @var{slot-list} argument to @code{defclass} is a list of elements
where each element defines one slot.  Each slot is a list of the form

@example
  (SLOT-NAME :TAG1 ATTRIB-VALUE1
             :TAG2 ATTRIB-VALUE2
             :TAGN ATTRIB-VALUEN)
@end example

@noindent
where @var{SLOT-NAME} is a symbol that will be used to refer to the
slot.  @var{:TAG} is a symbol that describes a feature to be set
on the slot.  @var{ATTRIB-VALUE} is a lisp expression that will be
used for @var{:TAG}.

Valid tags are:

@table @code
@item :initarg
A symbol that can be used in the argument list of the constructor to
specify a value for this slot of the new instance being created.

A good symbol to use for initarg is one that starts with a colon @code{:}.

The slot specified like this:
@example
  (myslot :initarg :myslot)
@end example
could then be initialized to the number 1 like this:
@example
  (myobject :myslot 1)
@end example

@xref{Making New Objects}.

@item :initform
An expression used as the default value for this slot.

If @code{:initform} is left out, that slot defaults to being unbound.
It is an error to reference an unbound slot, so if you need
slots to always be in a bound state, you should always use an
@code{:initform} specifier.

Use @code{slot-boundp} to test if a slot is unbound
(@pxref{Predicates}).  Use @code{slot-makeunbound} to set a slot to
being unbound after giving it a value (@pxref{Accessing Slots}).

The value passed to initform used to be automatically quoted.  Thus,
@example
:initform (1 2 3)
@end example
will use the list as a value.  This is incompatible with CLOS (which would
signal an error since 1 is not a valid function) and will likely change in the
future, so better quote your initforms if they're just values.

@item :type
An unquoted type specifier used to validate data set into this slot.
@xref{Type Predicates,,,cl,Common Lisp Extensions}.
Here are some examples:
 @table @code
 @item symbol
 A symbol.
 @item number
 A number type
 @item my-class-name
 An object of your class type.
 @item (or null symbol)
 A symbol, or @code{nil}.
 @end table

@item :allocation
Either :class or :instance (defaults to :instance) used to
specify how data is stored.  Slots stored per instance have unique
values for each object.  Slots stored per class have shared values for
each object.  If one object changes a :class allocated slot, then all
objects for that class gain the new value.

@item :documentation
Documentation detailing the use of this slot.  This documentation is
exposed when the user describes a class, and during customization of an
object.

@item :accessor
Name of a generic function which can be used to fetch the value of this slot.
You can call this function later on your object and retrieve the value
of the slot.

This option is in the CLOS spec, but is not fully compliant in @eieio{}.

@item :writer
Name of a generic function which will write this slot.

This option is in the CLOS spec, but is not fully compliant in @eieio{}.

@item :reader
Name of a generic function which will read this slot.

This option is in the CLOS spec, but is not fully compliant in @eieio{}.

@item :custom
A custom :type specifier used when editing an object of this type.
See documentation for @code{defcustom} for details.  This specifier is
equivalent to the :type spec of a @code{defcustom} call.

This option is specific to Emacs, and is not in the CLOS spec.

@item :label
When customizing an object, the value of :label will be used instead
of the slot name.  This enables better descriptions of the data than
would usually be afforded.

This option is specific to Emacs, and is not in the CLOS spec.

@item :group
Similar to @code{defcustom}'s :group command, this organizes different
slots in an object into groups.  When customizing an object, only the
slots belonging to a specific group need be worked with, simplifying the
size of the display.

This option is specific to Emacs, and is not in the CLOS spec.

@item :printer
This routine takes a symbol which is a function name.  The function
should accept one argument.  The argument is the value from the slot
to be printed.  The function in @code{object-write} will write the
slot value out to a printable form on @code{standard-output}.

The output format MUST be something that could in turn be interpreted
with @code{read} such that the object can be brought back in from the
output stream.  Thus, if you wanted to output a symbol, you would need
to quote the symbol.  If you wanted to run a function on load, you
can output the code to do the construction of the value.

@item :protection
This is an old option that is not supported any more.

When using a slot referencing function such as @code{slot-value}, and
the value behind @var{slot} is private or protected, then the current
scope of operation must be within a method of the calling object.

This protection is not enforced by the code any more, so it's only useful
as documentation.

Valid values are:

@table @code
@item :public
Access this slot from any scope.
@item :protected
Access this slot only from methods of the same class or a child class.
@item :private
Access this slot only from methods of the same class.
@end table

This option is specific to Emacs, and is not in the CLOS spec.

@end table

@node Class Options
@section Class Options

In the @var{options-and-doc} arguments to @code{defclass}, the
following class options may be specified:

@table @code
@item :documentation
A documentation string for this class.

If an Emacs-style documentation string is also provided, then this
option is ignored.  An Emacs-style documentation string is not
prefixed by the @code{:documentation} tag, and appears after the list
of slots, and before the options.

@item :allow-nil-initform
If this option is non-@code{nil}, and the @code{:initform} is @code{nil}, but
the @code{:type} is specifies something such as @code{string} then allow
this to pass.  The default is to have this option be off.  This is
implemented as an alternative to unbound slots.

This option is specific to Emacs, and is not in the CLOS spec.

@item :abstract
A class which is @code{:abstract} cannot be instantiated, and instead
is used to define an interface which subclasses should implement.

This option is specific to Emacs, and is not in the CLOS spec.

@item :custom-groups
This is a list of groups that can be customized within this class.  This
slot is auto-generated when a class is created and need not be
specified.  It can be retrieved with the @code{class-option} command,
however, to see what groups are available.

This option is specific to Emacs, and is not in the CLOS spec.

@item :method-invocation-order
This controls the order in which method resolution occurs for
methods in cases of multiple inheritance.  The order
affects which method is called first in a tree, and if
@code{cl-call-next-method} is used, it controls the order in which the
stack of methods are run.

Valid values are:

@table @code
@item :breadth-first
Search for methods in the class hierarchy in breadth first order.
This is the default.
@item :depth-first
Search for methods in the class hierarchy in a depth first order.
@item :c3
Searches for methods in a linearized way that most closely matches
what CLOS does when a monotonic class structure is defined.
@end table

@xref{Method Invocation}, for more on method invocation order.

@item :metaclass
Unsupported CLOS option.  Enables the use of a different base class other
than @code{standard-class}.

@item :default-initargs
Unsupported CLOS option.  Specifies a list of initargs to be used when
creating new objects.  As far as I can tell, this duplicates the
function of @code{:initform}.
@end table

@xref{CLOS compatibility}, for more details on CLOS tags versus
@eieio{}-specific tags.

@node Making New Objects
@chapter Making New Objects

Suppose we have defined a simple class, such as:

@example
(defclass my-class ()
   ( ) "Doc String")
@end example

@noindent
It is now possible to create objects of that class type.

Calling @code{defclass} has defined two new functions.  One is the
constructor @var{my-class}, and the other is the predicate,
@var{my-class}-p.

@defun my-class object-name &rest slots

This creates and returns a new object.  This object is not assigned to
anything, and will be garbage collected if not saved.  This object
will be given the string name @var{object-name}.  There can be
multiple objects of the same name, but the name slot provides a handy
way to keep track of your objects.  @var{slots} is just all the slots
you wish to preset.  Any slot set as such @emph{will not} get its
default value, and any side effects from a slot's @code{:initform}
that may be a function will not occur.

An example pair would appear simply as @code{:value 1}.  Of course you
can do any valid Lispy thing you want with it, such as
@code{:value (if (boundp 'special-symbol) special-symbol nil)}

Example of creating an object from a class:

@example
(my-class :value 3 :reference nil)
@end example

@end defun

To create an object from a class symbol, use @code{make-instance}.

@defun make-instance class &rest initargs
@anchor{make-instance}
Make a new instance of @var{class} based on @var{initargs}.
@var{class} is a class symbol.  For example:

@example
  (make-instance 'foo)
@end example

  @var{initargs} is a property list with keywords based on the @code{:initarg}
for each slot.  For example:

@example
  (make-instance @code{'foo} @code{:slot1} value1 @code{:slotN} valueN)
@end example

@end defun

@node Accessing Slots
@chapter Accessing Slots

There are several ways to access slot values in an object.  The naming
and argument-order conventions are similar to those used for
referencing vectors (@pxref{Vectors,,,elisp,GNU Emacs Lisp Reference
Manual}).

@defmac oref obj slot
@anchor{oref}
This macro retrieves the value stored in @var{obj} in the named
@var{slot}.  Slot names are determined by @code{defclass} which
creates the slot.

This is a generalized variable that can be used with @code{setf} to
modify the value stored in @var{slot}.  @xref{Generalized
Variables,,,elisp,GNU Emacs Lisp Reference Manual}.
@end defmac

@defmac oref-default class slot
@anchor{oref-default}
This macro returns the value of the class-allocated @var{slot} from
@var{class}.

This is a generalized variable that can be used with @code{setf} to
modify the value stored in @var{slot}.  @xref{Generalized
Variables,,,elisp,GNU Emacs Lisp Reference Manual}.
@end defmac

@defmac oset object slot value
This macro sets the value behind @var{slot} to @var{value} in
@var{object}.  It returns @var{value}.
@end defmac

@defmac oset-default class slot value
This macro sets the value for the class-allocated @var{slot} in @var{class} to
@var{value}.

For example, if a user wanted all @code{data-objects} (@pxref{Building
Classes}) to inform a special object of his own devising when they
changed, this can be arranged by simply executing this bit of code:

@example
(oset-default data-object reference (list my-special-object))
@end example
@end defmac

The following accessors are defined by CLOS to reference or modify
slot values, and use the previously mentioned set/ref routines.

@defun slot-value object slot
@anchor{slot-value}
This function retrieves the value of @var{slot} from @var{object}.
Unlike @code{oref}, the symbol for @var{slot} must be quoted.

This is a generalized variable that can be used with @code{setf} to
modify the value stored in @var{slot}.  @xref{Generalized
Variables,,,elisp,GNU Emacs Lisp Reference Manual}.
@end defun

@defun set-slot-value object slot value
@anchor{set-slot-value}
This function sets the value of @var{slot} from @var{object}.  Unlike
@code{oset}, the symbol for @var{slot} must be quoted.

This is not a CLOS function, but is the obsolete setter for
@code{slot-value} used by the @code{setf} macro.  It is therefore
recommended to use @w{@code{(setf (slot-value @var{object} @var{slot})
@var{value})}} instead.
@end defun

@defun slot-makeunbound object slot
This function unbinds @var{slot} in @var{object}.  Referencing an
unbound slot can signal an error.
@end defun

@defun object-add-to-list object slot item &optional append
@anchor{object-add-to-list}
In OBJECT's @var{slot}, add @var{item} to the list of elements.
Optional argument @var{append} indicates we need to append to the list.
If @var{item} already exists in the list in @var{slot}, then it is not added.
Comparison is done with @dfn{equal} through the @dfn{member} function call.
If @var{slot} is unbound, bind it to the list containing @var{item}.
@end defun

@defun object-remove-from-list object slot item
@anchor{object-remove-from-list}
In OBJECT's @var{slot}, remove occurrences of @var{item}.
Deletion is done with @dfn{delete}, which deletes by side effect
and comparisons are done with @dfn{equal}.
If @var{slot} is unbound, do nothing.
@end defun

@defun with-slots spec-list object &rest body
@anchor{with-slots}
Bind @var{spec-list} lexically to slot values in @var{object}, and execute @var{body}.
This establishes a lexical environment for referring to the slots in
the instance named by the given slot-names as though they were
variables.  Within such a context the value of the slot can be
specified by using its slot name, as if it were a lexically bound
variable.  Both @code{setf} and @code{setq} can be used to set the value of the
slot.

@var{spec-list} is of a form similar to @dfn{let}.  For example:

@example
  ((VAR1 SLOT1)
    SLOT2
    SLOTN
   (VARN+1 SLOTN+1))
@end example

Where each @var{var} is the local variable given to the associated
@var{slot}.  A slot specified without a variable name is given a
variable name of the same name as the slot.

@example
(defclass myclass () (x :initform 1))
(setq mc (make-instance 'myclass))
(with-slots (x) mc x)                      => 1
(with-slots ((something x)) mc something)  => 1
@end example
@end defun

@node Writing Methods
@chapter Writing Methods

Writing a method in @eieio{} is similar to writing a function.  The
differences are that there are some extra options and there can be
multiple definitions under the same function symbol.

Where a method defines an implementation for a particular data type, a
@dfn{generic method} accepts any argument, but contains no code.  It
is used to provide the dispatching to the defined methods.  A generic
method has no body, and is merely a symbol upon which methods are
attached.  It also provides the base documentation for what methods
with that name do.

@menu
* Generics::
* Methods::
* Static Methods::
@end menu

@node Generics
@section Generics

Each @eieio{} method has one corresponding generic.  This generic
provides a function binding and the base documentation for the method
symbol (@pxref{Symbol Components,,,elisp,GNU Emacs Lisp Reference
Manual}).

@defmac cl-defgeneric method arglist [doc-string]
This macro turns the (unquoted) symbol @var{method} into a function.
@var{arglist} is the default list of arguments to use (not implemented
yet).  @var{doc-string} is the documentation used for this symbol.

A generic function acts as a placeholder for methods.  There is no
need to call @code{cl-defgeneric} yourself, as @code{cl-defmethod} will call
it if necessary.  Currently the argument list is unused.

@code{cl-defgeneric} signals an error if you attempt to turn an existing
Emacs Lisp function into a generic function.

You can also create a generic method with @code{cl-defmethod}
(@pxref{Methods}).  When a method is created and there is no generic
method in place with that name, then a new generic will be created,
and the new method will use it.
@end defmac

In CLOS, a generic call also be used to provide an argument list and
dispatch precedence for all the arguments.  In @eieio{}, dispatching
only occurs for the first argument, so the @var{arglist} is not used.

@node Methods
@section Methods

A method is a function that is executed if the arguments passed
to it matches the method's specializers.  Different @eieio{} classes may
share the same method names.

Methods are created with the @code{cl-defmethod} macro, which is similar
to @code{defun}.

@defmac cl-defmethod method [:before | :around | :after ] arglist [doc-string] forms

@var{method} is the name of the function to create.

@code{:before}, @code{:around}, and @code{:after} specify execution order
(i.e., when this form is called).  If none of these symbols are present, the
method is said to be a @emph{primary}.

@var{arglist} is the list of arguments to this method.  The mandatory arguments
in this list may have a type specializer (see the example below) which means
that the method will only apply when those arguments match the given type
specializer.  An argument with no type specializer means that the method
applies regardless of its value.

@var{doc-string} is the documentation attached to the implementation.
All method doc-strings are incorporated into the generic method's
function documentation.

@var{forms} is the body of the function.

@end defmac

@noindent
In the following example, we create a method @code{mymethod} for the
@code{classname} class:

@example
(cl-defmethod mymethod ((obj classname) secondarg)
  "Doc string" )
@end example

@noindent
This method only executes if the @var{obj} argument passed to it is an
@eieio{} object of class @code{classname}.

A method with no type specializer is a @dfn{default method}.  If a given
class has no implementation, then the default method is called when
that method is used on a given object of that class.

Only one method per combination of specializers and qualifiers (@code{:before},
@code{:around}, or @code{:after}) is kept.  If two @code{cl-defmethod}s appear
with the same specializers and the same qualifiers, then the second
implementation replaces the first.

When a method is called on an object, but there is no method specified
for that object, but there is a method specified for object's parent
class, the parent class's method is called.  If there is a method
defined for both, only the child's method is called.  A child method
may call a parent's method using @code{cl-call-next-method}, described
below.

If multiple methods and default methods are defined for the same
method and class, they are executed in this order:

@enumerate
@item :around methods
The most specific @code{:around} method is called first, which may invoke the
less specific ones via @code{cl-call-next-method}.  If it doesn't invoke
@code{cl-call-next-method}, then no other methods will be executed.  When there
are no more @code{:around} methods to call, falls through to run the other
(non-@code{:around}) methods.
@item :before methods
Called in sequence from most specific to least specific.
@item primary methods
The most specific method is called, which may invoke the less specific
ones via @code{cl-call-next-method}.
@item :after methods
Called in sequence from least specific to most specific.
@end enumerate

If no methods exist, Emacs signals a @code{cl-no-applicable-method} error.
@xref{Signals}.  If methods exist but none of them are primary, Emacs
signals a @code{cl-no-primary-method} error.  @xref{Signals}.

@defun cl-call-next-method &rest replacement-args
@anchor{cl-call-next-method}

This function calls the superclass method from a subclass method.
This is the ``next method'' specified in the current method list.

If @var{replacement-args} is non-@code{nil}, then use them instead of the
arguments originally provided to the method.

Can only be used from within the lexical body of a primary or around method.
@end defun

@defun cl-next-method-p
@anchor{cl-next-method-p}
Non-@code{nil} if there is a next method.

Can only be used from within the lexical body of a primary or around method.
@end defun

@node Static Methods
@section Static Methods

Static methods do not depend on an object instance, but instead
operate on a class.  You can create a static method by using
the @code{subclass} specializer with @code{cl-defmethod}:

@example
(cl-defmethod make-instance ((class (subclass mychild)) &rest args)
  (let ((new (cl-call-next-method)))
    (push new all-my-children)
    new))
@end example

The first argument of a static method will be a class rather than an
object.  Use the functions @code{oref-default} or @code{oset-default} which
will work on a class.

A class's @code{make-instance} method is defined as a static
method.

@b{Note:} The @code{subclass} specializer is unique to @eieio{}.

@c TODO - Write some more about static methods here

@node Method Invocation
@chapter Method Invocation

When classes are defined, you can specify the
@code{:method-invocation-order}.  This is a feature specific to EIEIO.

This controls the order in which method resolution occurs for
methods in cases of multiple inheritance.  The order
affects which method is called first in a tree, and if
@code{cl-call-next-method} is used, it controls the order in which the
stack of methods are run.

The original EIEIO order turned out to be broken for multiple
inheritance, but some programs depended on it.  As such this option
was added when the default invocation order was fixed to something
that made more sense in that case.

Valid values are:

@table @code
@item :breadth-first
Search for methods in the class hierarchy in breadth first order.
This is the default.
@item :depth-first
Search for methods in the class hierarchy in a depth first order.
@item :c3
Searches for methods in a linearized way that most closely matches
what CLOS does when a monotonic class structure is defined.

This is derived from the Dylan language documents by
Kim Barrett et al.: A Monotonic Superclass Linearization for Dylan
Retrieved from: https://doi.org/10.1145/236338.236343
@end table

@node Predicates
@chapter Predicates and Utilities

Now that we know how to create classes, access slots, and define
methods, it might be useful to verify that everything is doing ok.  To
help with this a plethora of predicates have been created.

@defun find-class symbol &optional errorp
@anchor{find-class}
Return the class that @var{symbol} represents.
If there is no class, @code{nil} is returned if @var{errorp} is @code{nil}.
If @var{errorp} is non-@code{nil}, @code{wrong-argument-type} is signaled.
@end defun

@defun class-p class
@anchor{class-p}
Return @code{t} if @var{class} is a valid class object.
@var{class} is a symbol.
@end defun

@defun slot-exists-p object-or-class slot
@anchor{slot-exists-p}
Non-@code{nil} if @var{object-or-class} has @var{slot}.
@end defun

@defun slot-boundp object slot
@anchor{slot-boundp}
Non-@code{nil} if OBJECT's @var{slot} is bound.
Setting a slot's value makes it bound.  Calling @dfn{slot-makeunbound} will
make a slot unbound.
@var{object} can be an instance or a class.
@end defun

@defun eieio-class-name class
Return the class name as a symbol.
@end defun

@defun class-option class option
Return the value in @var{CLASS} of a given @var{OPTION}.
For example:

@example
(class-option eieio-default-superclass :documentation)
@end example

Will fetch the documentation string for @code{eieio-default-superclass}.
@end defun

@defun eieio-object-name obj
Return a string of the form @samp{#<object-class myobjname>} for @var{obj}.
This should look like Lisp symbols from other parts of Emacs such as
buffers and processes, and is shorter and cleaner than printing the
object's record.  It is more useful to use @code{object-print} to get
an object's print form, as this allows the object to add extra display
information into the symbol.
@end defun

@defun eieio-object-class obj
Returns the class symbol from @var{obj}.
@end defun

@defun eieio-object-class-name obj
Returns the symbol of @var{obj}'s class.
@end defun

@defun eieio-class-parents class
Returns the direct parents class of @var{class}.  Returns @code{nil} if
it is a superclass.
@end defun

@defun eieio-class-parents-fast class
Just like @code{eieio-class-parents} except it is a macro and no type checking
is performed.
@end defun

@defun eieio-class-parent class
Deprecated function which returns the first parent of @var{class}.
@end defun

@defun eieio-class-children class
Return the list of classes inheriting from @var{class}.
@end defun

@defun eieio-class-children-fast class
Just like @code{eieio-class-children}, but with no checks.
@end defun

@defun same-class-p obj class
Returns @code{t} if @var{obj}'s class is the same as @var{class}.
@end defun

@defun object-of-class-p obj class
Returns @code{t} if @var{obj} inherits anything from @var{class}.  This
is different from @code{same-class-p} because it checks for inheritance.
@end defun

@defun child-of-class-p child class
Returns @code{t} if @var{child} is a subclass of @var{class}.
@end defun

@defun generic-p method-symbol
Returns @code{t} if @code{method-symbol} is a generic function, as
opposed to a regular Emacs Lisp function.
@end defun

@node Association Lists
@chapter Association Lists

Lisp offers the concept of association lists, with primitives such as
@code{assoc} used to access them.  The following functions can be used
to manage association lists of @eieio{} objects:

@defun object-assoc key slot list
@anchor{object-assoc}
Return an object if @var{key} is @dfn{equal} to SLOT's value of an object in @var{list}.
@var{list} is a list of objects whose slots are searched.
Objects in @var{list} do not need to have a slot named @var{slot}, nor does
@var{slot} need to be bound.  If these errors occur, those objects will
be ignored.
@end defun


@defun object-assoc-list slot list
Return an association list generated by extracting @var{slot} from all
objects in @var{list}.  For each element of @var{list} the @code{car} is
the value of @var{slot}, and the @code{cdr} is the object it was
extracted from.  This is useful for generating completion tables.
@end defun

@defun eieio-build-class-alist &optional base-class
Returns an alist of all currently defined classes.  This alist is
suitable for completion lists used by interactive functions to select a
class.  The optional argument @var{base-class} allows the programmer to
select only a subset of classes which includes @var{base-class} and
all its subclasses.
@end defun

@node Customizing
@chapter Customizing Objects

@eieio{} supports the Custom facility through two new widget types.
If a variable is declared as type @code{object}, then full editing of
slots via the widgets is made possible.  This should be used
carefully, however, because modified objects are cloned, so if there
are other references to these objects, they will no longer be linked
together.

If you want in place editing of objects, use the following methods:

@defun eieio-customize-object object
Create a custom buffer and insert a widget for editing @var{object}.  At
the end, an @code{Apply} and @code{Reset} button are available.  This
will edit the object "in place" so references to it are also changed.
There is no effort to prevent multiple edits of a singular object, so
care must be taken by the user of this function.
@end defun

@defun eieio-custom-widget-insert object flags
This method inserts an edit object into the current buffer in place.
It is implemented as @code{(widget-create 'object-edit :value object)}.
This method is provided as a locale for adding tracking, or
specializing the widget insert procedure for any object.
@end defun

To define a slot with an object in it, use the @code{object} tag.  This
widget type will be automatically converted to @code{object-edit} if you
do in place editing of you object.

If you want to have additional actions taken when a user clicks on the
@code{Apply} button, then overload the method @code{eieio-done-customizing}.
This method does nothing by default, but that may change in the future.
This would be the best way to make your objects persistent when using
in-place editing.

@section Widget extension

When widgets are being created, one new widget extension has been added,
called the @code{:slotofchoices}.  When this occurs in a widget
definition, all elements after it are removed, and the slot is specifies
is queried and converted into a series of constants.

@example
(choice (const :tag "None" nil)
        :slotofchoices morestuff)
@end example

and if the slot @code{morestuff} contains @code{(sym1 sym2 sym3)}, the
above example is converted into:

@example
(choice (const :tag "None" nil)
        (const sym1)
        (const sym2)
        (const sym3))
@end example

This is useful when a given item needs to be selected from a list of
items defined in this second slot.

@node Introspection
@chapter Introspection

Introspection permits a programmer to peek at the contents of a class
without any previous knowledge of that class.  While @eieio{} implements
objects on top of records, and thus everything is technically visible,
some functions have been provided.  None of these functions are a part
of CLOS.

@defun eieio-class-slots obj
Return the list of public slots for @var{obj}.
@end defun

@defun class-slot-initarg class slot
For the given @var{class} return an :initarg associated with
@var{slot}.  Not all slots have initargs, so the return value can be
@code{nil}.
@end defun

@node Base Classes
@chapter Base Classes

All defined classes, if created with no specified parent class,
inherit from a special class called @code{eieio-default-superclass}.
@xref{Default Superclass}.

Often, it is more convenient to inherit from one of the other base
classes provided by @eieio{}, which have useful pre-defined
properties.  (Since @eieio{} supports multiple inheritance, you can
even inherit from more than one of these classes at once.)

@menu
* eieio-instance-inheritor::    Enable value inheritance between instances.
* eieio-instance-tracker::      Enable self tracking instances.
* eieio-singleton::             Only one instance of a given class.
* eieio-persistent::            Enable persistence for a class.
* eieio-named::                 Use the object name as a :name slot.
* eieio-speedbar::              Enable speedbar support in your objects.
@end menu

@node eieio-instance-inheritor
@section @code{eieio-instance-inheritor}

This class is defined in the package @file{eieio-base}.

Instance inheritance is a mechanism whereby the value of a slot in
object instance can reference the parent instance.  If the parent's slot
value is changed, then the child instance is also changed.  If the
child's slot is set, then the parent's slot is not modified.

@deftp {Class} eieio-instance-inheritor parent-instance
A class whose instances are enabled with instance inheritance.
The @var{parent-instance} slot indicates the instance which is
considered the parent of the current instance.  Default is @code{nil}.
@end deftp

@cindex @code{clone}
To use this class, inherit from it with your own class.
To make a new instance that inherits from and existing instance of your
class, use the @code{clone} method with additional parameters
to specify local values.

@cindex @code{slot-unbound}
The @code{eieio-instance-inheritor} class works by causing cloned
objects to have all slots unbound.  This class' @code{slot-unbound}
method will cause references to unbound slots to be redirected to the
parent instance.  If the parent slot is also unbound, then
@code{slot-unbound} will signal an error named @code{slot-unbound}.

@node eieio-instance-tracker
@section @code{eieio-instance-tracker}

This class is defined in the package @file{eieio-base}.

Sometimes it is useful to keep a master list of all instances of a given
class.  The class @code{eieio-instance-tracker} performs this task.

@deftp {Class} eieio-instance-tracker tracking-symbol
Enable instance tracking for this class.
The slot @var{tracking-symbol} should be initialized in inheritors of
this class to a symbol created with @code{defvar}.  This symbol will
serve as the variable used as a master list of all objects of the given
class.
@end deftp

@defmethod eieio-instance-tracker initialize-instance obj slot
This method is defined as an @code{:after} method.
It adds new instances to the master list.
@end defmethod

@defmethod eieio-instance-tracker delete-instance obj
Remove @var{obj} from the master list of instances of this class.
This may let the garbage collector nab this instance.
@end defmethod

@deffn eieio-instance-tracker-find key slot list-symbol
This convenience function lets you find instances.  @var{key} is the
value to search for.  @var{slot} is the slot to compare @var{KEY}
against.  The function @code{equal} is used for comparison.
The parameter @var{list-symbol} is the variable symbol which contains the
list of objects to be searched.
@end deffn

@node eieio-singleton
@section @code{eieio-singleton}

This class is defined in the package @file{eieio-base}.

@deftp {Class} eieio-singleton
Inheriting from the singleton class will guarantee that there will
only ever be one instance of this class.  Multiple calls to
@code{make-instance} will always return the same object.
@end deftp

@node eieio-persistent
@section @code{eieio-persistent}

This class is defined in the package @file{eieio-base}.

If you want an object, or set of objects to be persistent, meaning the
slot values are important to keep saved between sessions, then you will
want your top level object to inherit from @code{eieio-persistent}.

To make sure your persistent object can be moved, make sure all file
names stored to disk are made relative with
@code{eieio-persistent-path-relative}.

@deftp {Class} eieio-persistent file file-header-line
Enables persistence for instances of this class.
Slot @var{file} with initarg @code{:file} is the file name in which this
object will be saved.
Class allocated slot @var{file-header-line} is used with method
@code{object-write} as a header comment.
@end deftp

All objects can write themselves to a file, but persistent objects have
several additional methods that aid in maintaining them.

@defmethod eieio-persistent eieio-persistent-save obj &optional file
Write the object @var{obj} to its file.
If optional argument @var{file} is specified, use that file name
instead.
@end defmethod

@defmethod eieio-persistent eieio-persistent-path-relative obj file
Return a file name derived from @var{file} which is relative to the
stored location of @var{OBJ}.  This method should be used to convert
file names so that they are relative to the save file, making any system
of files movable from one location to another.
@end defmethod

@defmethod eieio-persistent object-write obj &optional comment
Like @code{object-write} for @code{standard-object}, but will derive
a header line comment from the class allocated slot if one is not
provided.
@end defmethod

@defun eieio-persistent-read filename &optional class allow-subclass
Read a persistent object from @var{filename}, and return it.
Signal an error if the object in @var{FILENAME} is not a constructor
for @var{CLASS}.  Optional @var{allow-subclass} says that it is ok for
@code{eieio-persistent-read} to load in subclasses of class instead of
being pedantic.
@end defun

@node eieio-named
@section @code{eieio-named}

This class is defined in the package @file{eieio-base}.

@deftp {Class} eieio-named
Object with a name.
Name storage already occurs in an object.  This object provides get/set
access to it.
@end deftp

@node eieio-speedbar
@section @code{eieio-speedbar}

This class is in package @file{eieio-speedbar}.

If a series of class instances map to a tree structure, it is possible
to cause your classes to be displayable in Speedbar.  @xref{Top,,,speedbar}.
Inheriting from these classes will enable a speedbar major display mode
with a minimum of effort.

@deftp {Class} eieio-speedbar buttontype buttonface
Enables base speedbar display for a class.
@cindex @code{speedbar-make-tag-line}
The slot @var{buttontype} is any of the symbols allowed by the
function @code{speedbar-make-tag-line} for the @var{exp-button-type}
argument @xref{Extending,,,speedbar}.
The slot @var{buttonface} is the face to use for the text of the string
displayed in speedbar.
The slots @var{buttontype} and @var{buttonface} are class allocated
slots, and do not take up space in your instances.
@end deftp

@deftp {Class} eieio-speedbar-directory-button buttontype buttonface
This class inherits from @code{eieio-speedbar} and initializes
@var{buttontype} and @var{buttonface} to appear as directory level lines.
@end deftp

@deftp {Class} eieio-speedbar-file-button buttontype buttonface
This class inherits from @code{eieio-speedbar} and initializes
@var{buttontype} and @var{buttonface} to appear as file level lines.
@end deftp

To use these classes, inherit from one of them in you class.  You can
use multiple inheritance with them safely.  To customize your class for
speedbar display, override the default values for @var{buttontype} and
@var{buttonface} to get the desired effects.

Useful methods to define for your new class include:

@defmethod eieio-speedbar eieio-speedbar-derive-line-path obj depth
Return a string representing a directory associated with an instance
of @var{obj}.  @var{depth} can be used to index how many levels of
indentation have been opened by the user where @var{obj} is shown.
@end defmethod


@defmethod eieio-speedbar eieio-speedbar-description obj
Return a string description of @var{OBJ}.
This is shown in the minibuffer or tooltip when the mouse hovers over
this instance in speedbar.
@end defmethod

@defmethod eieio-speedbar eieio-speedbar-child-description obj
Return a string representing a description of a child node of @var{obj}
when that child is not an object.  It is often useful to just use
item info helper functions such as @code{speedbar-item-info-file-helper}.
@end defmethod

@defmethod eieio-speedbar eieio-speedbar-object-buttonname obj
Return a string which is the text displayed in speedbar for @var{obj}.
@end defmethod

@defmethod eieio-speedbar eieio-speedbar-object-children obj
Return a list of children of @var{obj}.
@end defmethod

@defmethod eieio-speedbar eieio-speedbar-child-make-tag-lines obj depth
This method inserts a list of speedbar tag lines for @var{obj} to
represent its children.  Implement this method for your class
if your children are not objects themselves.  You still need to
implement @code{eieio-speedbar-object-children}.

In this method, use techniques specified in the Speedbar manual.
@xref{Extending,,,speedbar}.
@end defmethod

Some other functions you will need to learn to use are:

@deffn eieio-speedbar-create make-map key-map menu name toplevelfn
Register your object display mode with speedbar.
@var{make-map} is a function which initialized you keymap.
@var{key-map} is a symbol you keymap is installed into.
@var{menu} is an easy menu vector representing menu items specific to your
object display.
@var{name} is a short string to use as a name identifying you mode.
@var{toplevelfn} is a function called which must return a list of
objects representing those in the instance system you wish to browse in
speedbar.

Read the Extending chapter in the speedbar manual for more information
on how speedbar modes work
@xref{Extending,,,speedbar}.
@end deffn

@node Browsing
@chapter Browsing class trees

The command @kbd{M-x eieio-browse} displays a buffer listing all the
currently loaded classes in Emacs.  The classes are listed in an
indented tree structure, starting from @code{eieio-default-superclass}
(@pxref{Default Superclass}).

With a prefix argument, this command prompts for a class name; it then
lists only that class and its subclasses.

Here is a sample tree from our current example:

@example
eieio-default-superclass
  +--data-object
       +--data-object-symbol
@end example

Note: new classes are consed into the inheritance lists, so the tree
comes out upside-down.

@node Class Values
@chapter Class Values

You can use the normal @code{describe-function} command to retrieve
information about a class.  Running it on constructors will show a
full description of the generated class.  If you call it on a generic
function, all implementations of that generic function will be listed,
together with links through which you can directly jump to the source.

@node Default Superclass
@chapter Default Superclass

All defined classes, if created with no specified parent class, will
inherit from a special class stored in
@code{eieio-default-superclass}.  This superclass is quite simple, but
with it, certain default methods or attributes can be added to all
objects.  In CLOS, this would be named @code{STANDARD-CLASS}, and that
symbol is an alias to @code{eieio-default-superclass}.

Currently, the default superclass is defined as follows:

@example
(defclass eieio-default-superclass nil
  nil
  "Default parent class for classes with no specified parent class.
Its slots are automatically adopted by classes with no specified
parents.  This class is not stored in the `parent' slot of a class object."
  :abstract t)
@end example

The default superclass implements several methods providing a default
behavior for all objects created by @eieio{}.

@menu
* Initialization::      How objects are initialized
* Basic Methods::       Clone, print, and write
* Signal Handling::     Methods for managing signals.
@end menu

@node Initialization
@section Initialization

When creating an object of any type, you can use its constructor, or
@code{make-instance}.  This, in turns calls the method
@code{initialize-instance}, which then calls the method
@code{shared-initialize}.

These methods are all implemented on the default superclass so you do
not need to write them yourself, unless you need to override one of
their behaviors.

Users should not need to call @code{initialize-instance} or
@code{shared-initialize}, as these are used by @code{make-instance} to
initialize the object.  They are instead provided so that users can
augment these behaviors.

@defun initialize-instance obj &rest slots
Initialize @var{obj}.  Sets slots of @var{obj} with @var{slots} which
is a list of name/value pairs.  These are actually just passed to
@code{shared-initialize}.
@end defun

@defun shared-initialize obj &rest slots
Sets slots of @var{obj} with @var{slots} which is a list of name/value
pairs.

This is called from the default constructor.
@end defun

@node Basic Methods
@section Basic Methods

Additional useful methods defined on the base subclass are:

@defun clone obj &rest params
@anchor{clone}
Make a copy of @var{obj}, and then apply @var{params}.
@var{params} is a parameter list of the same form as @var{initialize-instance}
which are applied to change the object.  When overloading @dfn{clone}, be
sure to call @dfn{cl-call-next-method} first and modify the returned object.
@end defun

@defun object-print this &rest strings
@anchor{object-print}
Pretty printer for object @var{this}.  Call function @dfn{eieio-object-name} with @var{strings}.
The default method for printing object @var{this} is to use the
function @dfn{eieio-object-name}.

It is sometimes useful to put a summary of the object into the
default #<notation> string when using eieio browsing tools.

Implement this function and specify @var{strings} in a call to
@dfn{cl-call-next-method} to provide additional summary information.
When passing in extra strings from child classes, always remember
to prepend a space.

@example
(defclass data-object ()
   (value)
   "Object containing one data slot.")

(cl-defmethod object-print ((this data-object) &optional strings)
  "Return a string with a summary of the data object as part of the name."
  (apply #'cl-call-next-method this
         (format " value: %s" (render this))
         strings))
@end example

Here is what some output could look like:
@example
(object-print test-object)
   => #<data-object test-object value: 3>
@end example
@end defun

@defun object-write obj &optional comment
Write @var{obj} onto a stream in a readable fashion.  The resulting
output will be Lisp code which can be used with @code{read} and
@code{eval} to recover the object.  Only slots with @code{:initarg}s
are written to the stream.
@end defun

@node Signal Handling
@section Signal Handling

The default superclass defines methods for managing error conditions.
These methods all throw a signal for a particular error condition.

By implementing one of these methods for a class, you can change the
behavior that occurs during one of these error cases, or even ignore
the error by providing some behavior.

@defun slot-missing object slot-name operation &optional new-value
@anchor{slot-missing}
Method invoked when an attempt to access a slot in @var{object} fails.
@var{slot-name} is the name of the failed slot, @var{operation} is the type of access
that was requested, and optional @var{new-value} is the value that was desired
to be set.

This method is called from @code{oref}, @code{oset}, and other functions which
directly reference slots in EIEIO objects.

The default method signals an error of type @code{invalid-slot-name}.
@xref{Signals}.

You may override this behavior, but it is not expected to return in the
current implementation.

This function takes arguments in a different order than in CLOS.
@end defun

@defun slot-unbound object class slot-name fn
@anchor{slot-unbound}
Slot unbound is invoked during an attempt to reference an unbound slot.
@var{object} is the instance of the object being reference.  @var{class} is the
class of @var{object}, and @var{slot-name} is the offending slot.  This function
throws the signal @code{unbound-slot}.  You can overload this function and
return the value to use in place of the unbound value.
Argument @var{fn} is the function signaling this error.
Use @dfn{slot-boundp} to determine if a slot is bound or not.

In @var{clos}, the argument list is (@var{class} @var{object} @var{slot-name}), but
@var{eieio} can only dispatch on the first argument, so the first two are swapped.
@end defun

@defun cl-no-applicable-method generic &rest args
@anchor{cl-no-applicable-method}
Called if there are no methods applicable for @var{args} in the generic
function @var{generic}.
@var{args} are the arguments that were passed to @var{generic}.

Implement this for a class to block this signal.  The return
value becomes the return value of the original method call.
@end defun

@defun cl-no-primary-method generic &rest args
@anchor{cl-no-primary-method}
Called if there are methods applicable for @var{args} in the generic
function @var{generic} but they are all qualified.
@var{args} are the arguments that were passed to @var{generic}.

Implement this for a class to block this signal.  The return
value becomes the return value of the original method call.
@end defun

@defun cl-no-next-method generic method &rest args
@anchor{cl-no-next-method}
Called from @dfn{cl-call-next-method} when no additional methods are available.
@var{generic} is the generic function being called on
@dfn{cl-call-next-method}, @var{method} is the method where
@dfn{cl-call-next-method} was called, and
@var{args} are the arguments it is called by.
This method signals @dfn{cl-no-next-method} by default.  Override this
method to not throw an error, and its return value becomes the
return value of @dfn{cl-call-next-method}.
@end defun

@node Signals
@chapter Signals

There are new condition names (signals) that can be caught when using
@eieio{}.

@deffn Signal invalid-slot-name obj-or-class slot
This signal is called when an attempt to reference a slot in an
@var{obj-or-class} is made, and the @var{slot} is not defined for
it.
@end deffn

@deffn Signal cl-no-applicable-method generic arguments
This signal is called when @var{generic} is called, with @var{arguments}
and nothing is resolved.  This occurs when @var{generic} has been
defined, but the arguments make it impossible for @eieio{} to determine
which method body to run.

To prevent this signal from occurring in your class, implement the
method @code{cl-no-applicable-method} for your class.  This method is
called when to throw this signal, so implementing this for your class
allows you block the signal, and perform some work.
@end deffn

@deffn Signal cl-no-primary-method generic arguments
Like @code{cl-no-applicable-method} but applies when there are some applicable
methods, but none of them are primary.  You can similarly block it by
implementing a @code{cl-no-primary-method} method.
@end deffn

@deffn Signal cl-no-next-method class arguments
This signal is called if the function @code{cl-call-next-method} is called
and there is no next method to be called.

Overload the method @code{cl-no-next-method} to protect against this signal.
@end deffn

@deffn Signal invalid-slot-type slot spec value
This signal is called when an attempt to set @var{slot} is made, and
@var{value} doesn't match the specified type @var{spec}.

In @eieio{}, this is also used if a slot specifier has an invalid value
during a @code{defclass}.
@end deffn

@deffn Signal unbound-slot object class slot
This signal is called when an attempt to reference @var{slot} in
@var{object} is made, and that instance is currently unbound.
@end deffn

@node Naming Conventions
@chapter Naming Conventions

@xref{Tips,,Tips and Conventions,elisp,GNU Emacs Lisp Reference
Manual}, for a description of Emacs Lisp programming conventions.
These conventions help ensure that Emacs packages work nicely one
another, so an @eieio{}-based program should follow them.  Here are
some conventions that apply specifically to @eieio{}-based programs:

@itemize

@item Come up with a package prefix that is relatively short.  Prefix
all classes, and methods with your prefix.  This is a standard
convention for functions and variables in Emacs.

@item Do not prefix method names with the class name.  All methods in
@eieio{} are ``virtual'', and are dynamically dispatched.  Anyone can
override your methods at any time.  Your methods should be prefixed
with your package name.

@item Do not prefix slots in your class.  The slots are always locally
scoped to your class, and need no prefixing.

@item If your library inherits from other libraries of classes, you
must ``require'' that library with the @code{require} command.

@end itemize

@node CLOS compatibility
@chapter CLOS compatibility

Currently, the following functions should behave almost as expected from
CLOS.

@table @code

@item defclass
All slot keywords are available but not all work correctly.
Slot keyword differences are:

@table @asis

@item :reader, and :writer tags
Create methods that signal errors instead of creating an unqualified
method.  You can still create new ones to do its business.

@item :accessor
This should create an unqualified method to access a slot, but
instead pre-builds a method that gets the slot's value.

@item :type
Specifier uses the @code{typep} function from the @file{cl}
package.  @xref{Type Predicates,,,cl,Common Lisp Extensions}.
It therefore has the same issues as that package.  Extensions include
the ability to provide object names.
@end table

defclass also supports class options, but does not currently use values
of @code{:metaclass}, and @code{:default-initargs}.

@item make-instance
Make instance works as expected, however it just uses the @eieio{} instance
creator automatically generated when a new class is created.
@xref{Making New Objects}.

@item cl-defgeneric
Creates the desired symbol, and accepts most of the expected arguments of
CLOS's @code{defgeneric}.

@item cl-defmethod
Accepts most of the expected arguments of CLOS's @code{defmethod}.  To type
cast against a class, the class must exist before @code{cl-defmethod}
is called.

@item cl-call-next-method
Works just like CLOS's @code{call-next-method}.
@end table

CLOS supports the @code{describe} command, but @eieio{} provides
support for using the standard @code{describe-function} command on a
constructor or generic function.

When creating a new class (@pxref{Building Classes}) there are several
new keywords supported by @eieio{}.

In @eieio{} tags are in lower case, not mixed case.

@node Wish List
@chapter Wish List

@eieio{} is an incomplete implementation of CLOS@.  Finding ways to
improve the compatibility would help make CLOS style programs run
better in Emacs.

Some important compatibility features that would be good to add are:

@enumerate
@item
Support for metaclasses.
@item
Improve integration with the @file{cl} package.
@end enumerate

There are also improvements to be made to allow @eieio{} to operate
better in the Emacs environment.

@enumerate
@item
Allow subclassing of Emacs built-in types, such as faces, markers, and
buffers.
@item
Allow method overloading of method-like functions in Emacs.
@end enumerate

@node GNU Free Documentation License
@appendix GNU Free Documentation License
@include doclicense.texi

@node Function Index
@unnumbered Function Index

@printindex fn

@contents
@bye