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diff --git a/doc/ref/goops.texi b/doc/ref/goops.texi new file mode 100644 index 000000000..c0a828f71 --- /dev/null +++ b/doc/ref/goops.texi @@ -0,0 +1,2790 @@ +@c -*-texinfo-*- +@c This is part of the GNU Guile Reference Manual. +@c Copyright (C) 2008, 2009 +@c Free Software Foundation, Inc. +@c See the file guile.texi for copying conditions. + +@macro goops +GOOPS +@end macro + +@macro guile +Guile +@end macro + +@node GOOPS +@chapter GOOPS + +@goops{} is the object oriented extension to @guile{}. Its +implementation is derived from @w{STk-3.99.3} by Erick Gallesio and +version 1.3 of Gregor Kiczales @cite{Tiny-Clos}. It is very close in +spirit to CLOS, the Common Lisp Object System (@cite{CLtL2}) but is +adapted for the Scheme language. While GOOPS is not compatible with any +of these systems, GOOPS contains a compatibility module which allows for +execution of STKlos programs. + +Briefly stated, the @goops{} extension gives the user a full object +oriented system with multiple inheritance and generic functions with +multi-method dispatch. Furthermore, the implementation relies on a true +meta object protocol, in the spirit of the one defined for CLOS +(@cite{Gregor Kiczales: A Metaobject Protocol}). + +@menu +* Quick Start:: +* Tutorial:: +* Reference Manual:: +* MOP Specification:: +@end menu + +@node Quick Start +@section Quick Start + +To give an immediate flavour of what GOOPS can do, here is a very +brief introduction to its main operations. + +To start using GOOPS, load the @code{(oop goops)} module: + +@lisp +(use-modules (oop goops)) +@end lisp + +We're now ready to try some basic GOOPS functionality. + +@menu +* Methods:: +* User-defined types:: +* Asking for the type of an object:: +@end menu + +@node Methods +@subsection Methods + +A GOOPS method is like a Scheme procedure except that it is +specialized for a particular set of argument types. + +@lisp +(define-method (+ (x <string>) (y <string>)) + (string-append x y)) + +(+ "abc" "de") @result{} "abcde" +@end lisp + +If @code{+} is used with arguments that do not match the method's +types, Guile falls back to using the normal Scheme @code{+} procedure. + +@lisp +(+ 1 2) @result{} 3 +@end lisp + + +@node User-defined types +@subsection User-defined types + +@lisp +(define-class <2D-vector> () + (x #:init-value 0 #:accessor x-component #:init-keyword #:x) + (y #:init-value 0 #:accessor y-component #:init-keyword #:y)) + +@group +(use-modules (ice-9 format)) + +(define-method (write (obj <2D-vector>) port) + (format port "<~S, ~S>" (x-component obj) (y-component obj))) + +(define v (make <2D-vector> #:x 3 #:y 4)) + +v @result{} <3, 4> +@end group + +@group +(define-method (+ (x <2D-vector>) (y <2D-vector>)) + (make <2D-vector> + #:x (+ (x-component x) (x-component y)) + #:y (+ (y-component x) (y-component y)))) + +(+ v v) @result{} <6, 8> +@end group +@end lisp + +@node Asking for the type of an object +@subsection Types + +@example +(class-of v) @result{} #<<class> <2D-vector> 40241ac0> +<2D-vector> @result{} #<<class> <2D-vector> 40241ac0> +(class-of 1) @result{} #<<class> <integer> 401b2a98> +<integer> @result{} #<<class> <integer> 401b2a98> + +(is-a? v <2D-vector>) @result{} #t +@end example + +@node Tutorial +@section Tutorial +@include goops-tutorial.texi + +@node Reference Manual +@section Reference Manual + +This chapter is the GOOPS reference manual. It aims to describe all the +syntax, procedures, options and associated concepts that a typical +application author would need to understand in order to use GOOPS +effectively in their application. It also describes what is meant by +the GOOPS ``metaobject protocol'' (aka ``MOP''), and indicates how +authors can use the metaobject protocol to customize the behaviour of +GOOPS itself. + +For a detailed specification of the GOOPS metaobject protocol, see +@ref{MOP Specification}. + +@menu +* Introductory Remarks:: +* Defining New Classes:: +* Creating Instances:: +* Accessing Slots:: +* Creating Generic Functions:: +* Adding Methods to Generic Functions:: +* Invoking Generic Functions:: +* Redefining a Class:: +* Changing the Class of an Instance:: +* Introspection:: +* Miscellaneous Functions:: +@end menu + +@node Introductory Remarks +@subsection Introductory Remarks + +GOOPS is an object-oriented programming system based on a ``metaobject +protocol'' derived from the ones used in CLOS (the Common Lisp Object +System), tiny-clos (a small Scheme implementation of a subset of CLOS +functionality) and STKlos. + +GOOPS can be used by application authors at a basic level without any +need to understand what the metaobject protocol (aka ``MOP'') is and how +it works. On the other hand, the MOP underlies even the customizations +that application authors are likely to make use of very quickly --- such +as defining an @code{initialize} method to customize the initialization +of instances of an application-defined class --- and an understanding of +the MOP makes it much easier to explain such customizations in a precise +way. And in the long run, understanding the MOP is the key both to +understanding GOOPS at a deeper level and to taking full advantage of +GOOPS' power, by customizing the behaviour of GOOPS itself. + +Each of the following sections of the reference manual is arranged +such that the most basic usage is introduced first, and then subsequent +subsubsections discuss the related internal functions and metaobject +protocols, finishing with a description of how to customize that area of +functionality. + +These introductory remarks continue with a few words about metaobjects +and the MOP. Readers who do not want to be bothered yet with the MOP +and customization could safely skip this subsubsection on a first reading, +and should correspondingly skip subsequent subsubsections that are +concerned with internals and customization. + +In general, this reference manual assumes familiarity with standard +object oriented concepts and terminology. However, some of the terms +used in GOOPS are less well known, so the Terminology subsubsection +provides definitions for these terms. + +@menu +* Metaobjects and the Metaobject Protocol:: +* Terminology:: +@end menu + +@node Metaobjects and the Metaobject Protocol +@subsubsection Metaobjects and the Metaobject Protocol + +The conceptual building blocks of GOOPS are classes, slot definitions, +instances, generic functions and methods. A class is a grouping of +inheritance relations and slot definitions. An instance is an object +with slots that are allocated following the rules implied by its class's +superclasses and slot definitions. A generic function is a collection +of methods and rules for determining which of those methods to apply +when the generic function is invoked. A method is a procedure and a set +of specializers that specify the type of arguments to which the +procedure is applicable. + +Of these entities, GOOPS represents classes, generic functions and +methods as ``metaobjects''. In other words, the values in a GOOPS +program that describe classes, generic functions and methods, are +themselves instances (or ``objects'') of special GOOPS classes that +encapsulate the behaviour, respectively, of classes, generic functions, +and methods. + +(The other two entities are slot definitions and instances. Slot +definitions are not strictly instances, but every slot definition is +associated with a GOOPS class that specifies the behaviour of the slot +as regards accessibility and protection from garbage collection. +Instances are of course objects in the usual sense, and there is no +benefit from thinking of them as metaobjects.) + +The ``metaobject protocol'' (aka ``MOP'') is the specification of the +generic functions which determine the behaviour of these metaobjects and +the circumstances in which these generic functions are invoked. + +For a concrete example of what this means, consider how GOOPS calculates +the set of slots for a class that is being defined using +@code{define-class}. The desired set of slots is the union of the new +class's direct slots and the slots of all its superclasses. But +@code{define-class} itself does not perform this calculation. Instead, +there is a method of the @code{initialize} generic function that is +specialized for instances of type @code{<class>}, and it is this method +that performs the slot calculation. + +@code{initialize} is a generic function which GOOPS calls whenever a new +instance is created, immediately after allocating memory for a new +instance, in order to initialize the new instance's slots. The sequence +of steps is as follows. + +@itemize @bullet +@item +@code{define-class} uses @code{make} to make a new instance of the +@code{<class>}, passing as initialization arguments the superclasses, +slot definitions and class options that were specified in the +@code{define-class} form. + +@item +@code{make} allocates memory for the new instance, and then invokes the +@code{initialize} generic function to initialize the new instance's +slots. + +@item +The @code{initialize} generic function applies the method that is +specialized for instances of type @code{<class>}, and this method +performs the slot calculation. +@end itemize + +In other words, rather than being hardcoded in @code{define-class}, the +behaviour of class definition is encapsulated by generic function +methods that are specialized for the class @code{<class>}. + +It is possible to create a new class that inherits from @code{<class>}, +which is called a ``metaclass'', and to write a new @code{initialize} +method that is specialized for instances of the new metaclass. Then, if +the @code{define-class} form includes a @code{#:metaclass} class option +whose value is the new metaclass, the class that is defined by the +@code{define-class} form will be an instance of the new metaclass rather +than of the default @code{<class>}, and will be defined in accordance +with the new @code{initialize} method. Thus the default slot +calculation, as well as any other aspect of the new class's relationship +with its superclasses, can be modified or overridden. + +In a similar way, the behaviour of generic functions can be modified or +overridden by creating a new class that inherits from the standard +generic function class @code{<generic>}, writing appropriate methods +that are specialized to the new class, and creating new generic +functions that are instances of the new class. + +The same is true for method metaobjects. And the same basic mechanism +allows the application class author to write an @code{initialize} method +that is specialized to their application class, to initialize instances +of that class. + +Such is the power of the MOP. Note that @code{initialize} is just one +of a large number of generic functions that can be customized to modify +the behaviour of application objects and classes and of GOOPS itself. +Each subsequent section of the reference manual covers a particular area +of GOOPS functionality, and describes the generic functions that are +relevant for customization of that area. + +We conclude this subsubsection by emphasizing a point that may seem +obvious, but contrasts with the corresponding situation in some other +MOP implementations, such as CLOS. The point is simply that an +identifier which represents a GOOPS class or generic function is a +variable with a first-class value, the value being an instance of class +@code{<class>} or @code{<generic>}. (In CLOS, on the other hand, a +class identifier is a symbol that indexes the corresponding class +metaobject in a separate namespace for classes.) This is, of course, +simply an extension of the tendency in Scheme to avoid the unnecessary +use of, on the one hand, syntactic forms that require unevaluated +arguments and, on the other, separate identifier namespaces (e.g. for +class names), but it is worth noting that GOOPS conforms fully to this +Schemely principle. + +@node Terminology +@subsubsection Terminology + +It is assumed that the reader is already familiar with standard object +orientation concepts such as classes, objects/instances, +inheritance/subclassing, generic functions and methods, encapsulation +and polymorphism. + +This section explains some of the less well known concepts and +terminology that GOOPS uses, which are assumed by the following sections +of the reference manual. + +@subsubheading Metaclass + +A @dfn{metaclass} is the class of an object which represents a GOOPS +class. Put more succinctly, a metaclass is a class's class. + +Most GOOPS classes have the metaclass @code{<class>} and, by default, +any new class that is created using @code{define-class} has the +metaclass @code{<class>}. + +But what does this really mean? To find out, let's look in more detail +at what happens when a new class is created using @code{define-class}: + +@example +(define-class <my-class> (<object>) . slots) +@end example + +GOOPS actually expands the @code{define-class} form to something like +this + +@example +(define <my-class> (class (<object>) . slots)) +@end example + +and thence to + +@example +(define <my-class> + (make <class> #:supers (list <object>) #:slots slots)) +@end example + +In other words, the value of @code{<my-class>} is in fact an instance of +the class @code{<class>} with slot values specifying the superclasses +and slot definitions for the class @code{<my-class>}. (@code{#:supers} +and @code{#:slots} are initialization keywords for the @code{dsupers} +and @code{dslots} slots of the @code{<class>} class.) + +In order to take advantage of the full power of the GOOPS metaobject +protocol (@pxref{MOP Specification}), it is sometimes desirable to +create a new class with a metaclass other than the default +@code{<class>}. This is done by writing: + +@example +(define-class <my-class2> (<object>) + slot @dots{} + #:metaclass <my-metaclass>) +@end example + +GOOPS expands this to something like: + +@example +(define <my-class2> + (make <my-metaclass> #:supers (list <object>) #:slots slots)) +@end example + +In this case, the value of @code{<my-class2>} is an instance of the more +specialized class @code{<my-metaclass>}. Note that +@code{<my-metaclass>} itself must previously have been defined as a +subclass of @code{<class>}. For a full discussion of when and how it is +useful to define new metaclasses, see @ref{MOP Specification}. + +Now let's make an instance of @code{<my-class2>}: + +@example +(define my-object (make <my-class2> ...)) +@end example + +All of the following statements are correct expressions of the +relationships between @code{my-object}, @code{<my-class2>}, +@code{<my-metaclass>} and @code{<class>}. + +@itemize @bullet +@item +@code{my-object} is an instance of the class @code{<my-class2>}. + +@item +@code{<my-class2>} is an instance of the class @code{<my-metaclass>}. + +@item +@code{<my-metaclass>} is an instance of the class @code{<class>}. + +@item +The class of @code{my-object} is @code{<my-class2>}. + +@item +The metaclass of @code{my-object} is @code{<my-metaclass>}. + +@item +The class of @code{<my-class2>} is @code{<my-metaclass>}. + +@item +The metaclass of @code{<my-class2>} is @code{<class>}. + +@item +The class of @code{<my-metaclass>} is @code{<class>}. + +@item +The metaclass of @code{<my-metaclass>} is @code{<class>}. + +@item +@code{<my-class2>} is not a metaclass, since it is does not inherit from +@code{<class>}. + +@item +@code{<my-metaclass>} is a metaclass, since it inherits from +@code{<class>}. +@end itemize + +@subsubheading Class Precedence List + +The @dfn{class precedence list} of a class is the list of all direct and +indirect superclasses of that class, including the class itself. + +In the absence of multiple inheritance, the class precedence list is +ordered straightforwardly, beginning with the class itself and ending +with @code{<top>}. + +For example, given this inheritance hierarchy: + +@example +(define-class <invertebrate> (<object>) @dots{}) +(define-class <echinoderm> (<invertebrate>) @dots{}) +(define-class <starfish> (<echinoderm>) @dots{}) +@end example + +the class precedence list of <starfish> would be + +@example +(<starfish> <echinoderm> <invertebrate> <object> <top>) +@end example + +With multiple inheritance, the algorithm is a little more complicated. +A full description is provided by the GOOPS Tutorial: see @ref{Class +precedence list}. + +``Class precedence list'' is often abbreviated, in documentation and +Scheme variable names, to @dfn{cpl}. + +@subsubheading Accessor + +An @dfn{accessor} is a generic function with both reference and setter +methods. + +@example +(define-accessor perimeter) +@end example + +Reference methods for an accessor are defined in the same way as generic +function methods. + +@example +(define-method (perimeter (s <square>)) + (* 4 (side-length s))) +@end example + +Setter methods for an accessor are defined by specifying ``(setter +<accessor-name>)'' as the first parameter of the @code{define-method} +call. + +@example +(define-method ((setter perimeter) (s <square>) (n <number>)) + (set! (side-length s) (/ n 4))) +@end example + +Once an appropriate setter method has been defined in this way, it can +be invoked using the generalized @code{set!} syntax, as in: + +@example +(set! (perimeter s1) 18.3) +@end example + +@node Defining New Classes +@subsection Defining New Classes + +[ *fixme* Somewhere in this manual there needs to be an introductory +discussion about GOOPS classes, generic functions and methods, covering + +@itemize @bullet +@item +how classes encapsulate related items of data in @dfn{slots} + +@item +why it is that, unlike in C++ and Java, a class does not encapsulate the +methods that act upon the class (at least not in the C++/Java sense) + +@item +how generic functions provide a more general solution that provides for +dispatch on all argument types, and avoids idiosyncracies like C++'s +friend classes + +@item +how encapsulation in the sense of data- and code-hiding, or of +distinguishing interface from implementation, is treated in Guile as an +orthogonal concept to object orientation, and is the responsibility of +the module system. +@end itemize + +Some of this is covered in the Tutorial chapter, in @ref{Generic +functions and methods} - perhaps the best solution would be to expand +the discussion there. ] + +@menu +* Basic Class Definition:: +* Class Options:: +* Slot Options:: +* Class Definition Internals:: +* Customizing Class Definition:: +* STKlos Compatibility:: +@end menu + +@node Basic Class Definition +@subsubsection Basic Class Definition + +New classes are defined using the @code{define-class} syntax, with +arguments that specify the classes that the new class should inherit +from, the direct slots of the new class, and any required class options. + +@deffn syntax define-class name (super @dots{}) slot-definition @dots{} . options +Define a class called @var{name} that inherits from @var{super}s, with +direct slots defined by @var{slot-definition}s and class options +@var{options}. The newly created class is bound to the variable name +@var{name} in the current environment. + +Each @var{slot-definition} is either a symbol that names the slot or a +list, + +@example +(@var{slot-name-symbol} . @var{slot-options}) +@end example + +where @var{slot-name-symbol} is a symbol and @var{slot-options} is a +list with an even number of elements. The even-numbered elements of +@var{slot-options} (counting from zero) are slot option keywords; the +odd-numbered elements are the corresponding values for those keywords. + +@var{options} is a similarly structured list containing class option +keywords and corresponding values. +@end deffn + +The standard GOOPS class and slot options are described in the following +subsubsections: see @ref{Class Options} and @ref{Slot Options}. + +Example 1. Define a class that combines two pre-existing classes by +inheritance but adds no new slots. + +@example +(define-class <combined> (<tree> <bicycle>)) +@end example + +Example 2. Define a @code{regular-polygon} class with slots for side +length and number of sides that have default values and can be accessed +via the generic functions @code{side-length} and @code{num-sides}. + +@example +(define-class <regular-polygon> () + (sl #:init-value 1 #:accessor side-length) + (ns #:init-value 5 #:accessor num-sides)) +@end example + +Example 3. Define a class whose behavior (and that of its instances) is +customized via an application-defined metaclass. + +@example +(define-class <tcpip-fsm> () + (s #:init-value #f #:accessor state) + ... + #:metaclass <finite-state-class>) +@end example + +@node Class Options +@subsubsection Class Options + +@deffn {class option} #:metaclass metaclass +The @code{#:metaclass} class option specifies the metaclass of the class +being defined. @var{metaclass} must be a class that inherits from +@code{<class>}. For an introduction to the use of metaclasses, see +@ref{Metaobjects and the Metaobject Protocol} and @ref{Terminology}. + +If the @code{#:metaclass} option is absent, GOOPS reuses or constructs a +metaclass for the new class by calling @code{ensure-metaclass} +(@pxref{Class Definition Internals,, ensure-metaclass}). +@end deffn + +@deffn {class option} #:name name +The @code{#:name} class option specifies the new class's name. This +name is used to identify the class whenever related objects - the class +itself, its instances and its subclasses - are printed. + +If the @code{#:name} option is absent, GOOPS uses the first argument to +@code{define-class} as the class name. +@end deffn + +@deffn {class option} #:environment environment +*fixme* Not sure about this one, but I think that the +@code{#:environment} option specifies the environment in which the +class's getters and setters are computed and evaluated. + +If the @code{#:environment} option is not specified, the class's +environment defaults to the top-level environment in which the +@code{define-class} form appears. +@end deffn + +@node Slot Options +@subsubsection Slot Options + +@deffn {slot option} #:allocation allocation +The @code{#:allocation} option tells GOOPS how to allocate storage for +the slot. Possible values for @var{allocation} are + +@itemize @bullet +@item @code{#:instance} + +Indicates that GOOPS should create separate storage for this slot in +each new instance of the containing class (and its subclasses). + +@item @code{#:class} + +Indicates that GOOPS should create storage for this slot that is shared +by all instances of the containing class (and its subclasses). In other +words, a slot in class @var{C} with allocation @code{#:class} is shared +by all @var{instance}s for which @code{(is-a? @var{instance} @var{c})}. + +@item @code{#:each-subclass} + +Indicates that GOOPS should create storage for this slot that is shared +by all @emph{direct} instances of the containing class, and that +whenever a subclass of the containing class is defined, GOOPS should +create a new storage for the slot that is shared by all @emph{direct} +instances of the subclass. In other words, a slot with allocation +@code{#:each-subclass} is shared by all instances with the same +@code{class-of}. + +@item @code{#:virtual} + +Indicates that GOOPS should not allocate storage for this slot. The +slot definition must also include the @code{#:slot-ref} and +@code{#:slot-set!} options to specify how to reference and set the value +for this slot. +@end itemize + +The default value is @code{#:instance}. + +Slot allocation options are processed when defining a new class by the +generic function @code{compute-get-n-set}, which is specialized by the +class's metaclass. Hence new types of slot allocation can be +implemented by defining a new metaclass and a method for +@code{compute-get-n-set} that is specialized for the new metaclass. For +an example of how to do this, see @ref{Customizing Class Definition}. +@end deffn + +@deffn {slot option} #:slot-ref getter +@deffnx {slot option} #:slot-set! setter +The @code{#:slot-ref} and @code{#:slot-set!} options must be specified +if the slot allocation is @code{#:virtual}, and are ignored otherwise. + +@var{getter} should be a closure taking a single @var{instance} parameter +that returns the current slot value. @var{setter} should be a closure +taking two parameters - @var{instance} and @var{new-val} - that sets the +slot value to @var{new-val}. +@end deffn + +@deffn {slot option} #:getter getter +@deffnx {slot option} #:setter setter +@deffnx {slot option} #:accessor accessor +These options, if present, tell GOOPS to create generic function and +method definitions that can be used to get and set the slot value more +conveniently than by using @code{slot-ref} and @code{slot-set!}. + +@var{getter} specifies a generic function to which GOOPS will add a +method for getting the slot value. @var{setter} specifies a generic +function to which GOOPS will add a method for setting the slot value. +@var{accessor} specifies an accessor to which GOOPS will add methods for +both getting and setting the slot value. + +So if a class includes a slot definition like this: + +@example +(c #:getter get-count #:setter set-count #:accessor count) +@end example + +GOOPS defines generic function methods such that the slot value can be +referenced using either the getter or the accessor - + +@example +(let ((current-count (get-count obj))) @dots{}) +(let ((current-count (count obj))) @dots{}) +@end example + +- and set using either the setter or the accessor - + +@example +(set-count obj (+ 1 current-count)) +(set! (count obj) (+ 1 current-count)) +@end example + +Note that + +@itemize @bullet +@item +with an accessor, the slot value is set using the generalized +@code{set!} syntax + +@item +in practice, it is unusual for a slot to use all three of these options: +read-only, write-only and read-write slots would typically use only +@code{#:getter}, @code{#:setter} and @code{#:accessor} options +respectively. +@end itemize + +If the specified names are already bound in the top-level environment to +values that cannot be upgraded to generic functions, those values are +overwritten during evaluation of the @code{define-class} that contains +the slot definition. For details, see @ref{Generic Function Internals,, +ensure-generic}. +@end deffn + +@deffn {slot option} #:init-value init-value +@deffnx {slot option} #:init-form init-form +@deffnx {slot option} #:init-thunk init-thunk +@deffnx {slot option} #:init-keyword init-keyword +These options provide various ways to specify how to initialize the +slot's value at instance creation time. @var{init-value} is a fixed +value (shared across all new instances of the class). +@var{init-thunk} is a procedure of no arguments that is called +when a new instance is created and should return the desired initial +slot value. @var{init-form} is an unevaluated expression that gets +evaluated when a new instance is created and should return the desired +initial slot value. @var{init-keyword} is a keyword that can be used +to pass an initial slot value to @code{make} when creating a new +instance. + +Note that, since an @code{init-value} value is shared across all +instances of a class, you should only use it when the initial value is +an immutable value, like a constant. If you want to initialize a slot +with a fresh, independently mutable value, you should use +@code{init-thunk} or @code{init-form} instead. Consider the following +example. + +@example +(define-class <chbouib> () + (hashtab #:init-value (make-hash-table))) +@end example + +@noindent +Here only one hash table is created and all instances of +@code{<chbouib>} have their @code{hashtab} slot refer to it. In order +to have each instance of @code{<chbouib>} refer to a new hash table, you +should instead write: + +@example +(define-class <chbouib> () + (hashtab #:init-thunk make-hash-table)) +@end example + +@noindent +or: + +@example +(define-class <chbouib> () + (hashtab #:init-form (make-hash-table))) +@end example + +If more than one of these options is specified for the same slot, the +order of precedence, highest first is + +@itemize @bullet +@item +@code{#:init-keyword}, if @var{init-keyword} is present in the options +passed to @code{make} + +@item +@code{#:init-thunk}, @code{#:init-form} or @code{#:init-value}. +@end itemize + +If the slot definition contains more than one initialization option of +the same precedence, the later ones are ignored. If a slot is not +initialized at all, its value is unbound. + +In general, slots that are shared between more than one instance are +only initialized at new instance creation time if the slot value is +unbound at that time. However, if the new instance creation specifies +a valid init keyword and value for a shared slot, the slot is +re-initialized regardless of its previous value. + +Note, however, that the power of GOOPS' metaobject protocol means that +everything written here may be customized or overridden for particular +classes! The slot initializations described here are performed by the least +specialized method of the generic function @code{initialize}, whose +signature is + +@example +(define-method (initialize (object <object>) initargs) ...) +@end example + +The initialization of instances of any given class can be customized by +defining a @code{initialize} method that is specialized for that class, +and the author of the specialized method may decide to call +@code{next-method} - which will result in a call to the next less +specialized @code{initialize} method - at any point within the +specialized code, or maybe not at all. In general, therefore, the +initialization mechanisms described here may be modified or overridden by +more specialized code, or may not be supported at all for particular +classes. +@end deffn + +@node Class Definition Internals +@subsubsection Class Definition Internals + +Implementation notes: @code{define-class} expands to an expression which + +@itemize @bullet +@item +checks that it is being evaluated only at top level + +@item +defines any accessors that are implied by the @var{slot-definition}s + +@item +uses @code{class} to create the new class (@pxref{Class Definition +Internals,, class}) + +@item +checks for a previous class definition for @var{name} and, if found, +handles the redefinition by invoking @code{class-redefinition} +(@pxref{Redefining a Class}). +@end itemize + +@deffn syntax class name (super @dots{}) slot-definition @dots{} . options +Return a newly created class that inherits from @var{super}s, with +direct slots defined by @var{slot-definition}s and class options +@var{options}. For the format of @var{slot-definition}s and +@var{options}, see @ref{Basic Class Definition,, define-class}. +@end deffn + +Implementation notes: @code{class} expands to an expression which + +@itemize @bullet +@item +processes the class and slot definition options to check that they are +well-formed, to convert the @code{#:init-form} option to an +@code{#:init-thunk} option, to supply a default environment parameter +(the current top-level environment) and to evaluate all the bits that +need to be evaluated + +@item +calls @code{make-class} to create the class with the processed and +evaluated parameters. +@end itemize + +@deffn procedure make-class supers slots . options +Return a newly created class that inherits from @var{supers}, with +direct slots defined by @var{slots} and class options @var{options}. +For the format of @var{slots} and @var{options}, see @ref{Basic Class +Definition,, define-class}, except note that for @code{make-class}, +@var{slots} and @var{options} are separate list parameters: @var{slots} +here is a list of slot definitions. +@end deffn + +Implementation notes: @code{make-class} + +@itemize @bullet +@item +adds @code{<object>} to the @var{supers} list if @var{supers} is empty +or if none of the classes in @var{supers} have @code{<object>} in their +class precedence list + +@item +defaults the @code{#:environment}, @code{#:name} and @code{#:metaclass} +options, if they are not specified by @var{options}, to the current +top-level environment, the unbound value, and @code{(ensure-metaclass +@var{supers})} respectively (@pxref{Class Definition Internals,, +ensure-metaclass}) + +@item +checks for duplicate classes in @var{supers} and duplicate slot names in +@var{slots}, and signals an error if there are any duplicates + +@item +calls @code{make}, passing the metaclass as the first parameter and all +other parameters as option keywords with values. +@end itemize + +@deffn procedure ensure-metaclass supers env +Return a metaclass suitable for a class that inherits from the list of +classes in @var{supers}. The returned metaclass is the union by +inheritance of the metaclasses of the classes in @var{supers}. + +In the simplest case, where all the @var{supers} are straightforward +classes with metaclass @code{<class>}, the returned metaclass is just +@code{<class>}. + +For a more complex example, suppose that @var{supers} contained one +class with metaclass @code{<operator-class>} and one with metaclass +@code{<foreign-object-class>}. Then the returned metaclass would be a +class that inherits from both @code{<operator-class>} and +@code{<foreign-object-class>}. + +If @var{supers} is the empty list, @code{ensure-metaclass} returns the +default GOOPS metaclass @code{<class>}. + +GOOPS keeps a list of the metaclasses created by +@code{ensure-metaclass}, so that each required type of metaclass only +has to be created once. + +The @code{env} parameter is ignored. +@end deffn + +@deffn procedure ensure-metaclass-with-supers meta-supers +@code{ensure-metaclass-with-supers} is an internal procedure used by +@code{ensure-metaclass} (@pxref{Class Definition Internals,, +ensure-metaclass}). It returns a metaclass that is the union by +inheritance of the metaclasses in @var{meta-supers}. +@end deffn + +The internals of @code{make}, which is ultimately used to create the new +class object, are described in @ref{Customizing Instance Creation}, +which covers the creation and initialization of instances in general. + +@node Customizing Class Definition +@subsubsection Customizing Class Definition + +During the initialization of a new class, GOOPS calls a number of generic +functions with the newly allocated class instance as the first +argument. Specifically, GOOPS calls the generic function + +@itemize @bullet +@item +(initialize @var{class} @dots{}) +@end itemize + +where @var{class} is the newly allocated class instance, and the default +@code{initialize} method for arguments of type @code{<class>} calls the +generic functions + +@itemize @bullet +@item +(compute-cpl @var{class}) + +@item +(compute-slots @var{class}) + +@item +(compute-get-n-set @var{class} @var{slot-def}), for each of the slot +definitions returned by @code{compute-slots} + +@item +(compute-getter-method @var{class} @var{slot-def}), for each of the +slot definitions returned by @code{compute-slots} that includes a +@code{#:getter} or @code{#:accessor} slot option + +@item +(compute-setter-method @var{class} @var{slot-def}), for each of the +slot definitions returned by @code{compute-slots} that includes a +@code{#:setter} or @code{#:accessor} slot option. +@end itemize + +If the metaclass of the new class is something more specialized than the +default @code{<class>}, then the type of @var{class} in the calls above +is more specialized than @code{<class>}, and hence it becomes possible +to define generic function methods, specialized for the new class's +metaclass, that can modify or override the default behaviour of +@code{initialize}, @code{compute-cpl} or @code{compute-get-n-set}. + +@code{compute-cpl} computes the class precedence list (``CPL'') for the +new class (@pxref{Class precedence list}), and returns it as a list of +class objects. The CPL is important because it defines a superclass +ordering that is used, when a generic function is invoked upon an +instance of the class, to decide which of the available generic function +methods is the most specific. Hence @code{compute-cpl} could be +customized in order to modify the CPL ordering algorithm for all classes +with a special metaclass. + +The default CPL algorithm is encapsulated by the @code{compute-std-cpl} +procedure, which is in turn called by the default @code{compute-cpl} +method. + +@deffn procedure compute-std-cpl class +Compute and return the class precedence list for @var{class} according +to the algorithm described in @ref{Class precedence list}. +@end deffn + +@code{compute-slots} computes and returns a list of all slot definitions +for the new class. By default, this list includes the direct slot +definitions from the @code{define-class} form, plus the slot definitions +that are inherited from the new class's superclasses. The default +@code{compute-slots} method uses the CPL computed by @code{compute-cpl} +to calculate this union of slot definitions, with the rule that slots +inherited from superclasses are shadowed by direct slots with the same +name. One possible reason for customizing @code{compute-slots} would be +to implement an alternative resolution strategy for slot name conflicts. + +@code{compute-get-n-set} computes the low-level closures that will be +used to get and set the value of a particular slot, and returns them in +a list with two elements. + +The closures returned depend on how storage for that slot is allocated. +The standard @code{compute-get-n-set} method, specialized for classes of +type @code{<class>}, handles the standard GOOPS values for the +@code{#:allocation} slot option (@pxref{Slot Options,, allocation}). By +defining a new @code{compute-get-n-set} method for a more specialized +metaclass, it is possible to support new types of slot allocation. + +Suppose you wanted to create a large number of instances of some class +with a slot that should be shared between some but not all instances of +that class - say every 10 instances should share the same slot storage. +The following example shows how to implement and use a new type of slot +allocation to do this. + +@example +(define-class <batched-allocation-metaclass> (<class>)) + +(let ((batch-allocation-count 0) + (batch-get-n-set #f)) + (define-method (compute-get-n-set + (class <batched-allocation-metaclass>) s) + (case (slot-definition-allocation s) + ((#:batched) + ;; If we've already used the same slot storage for 10 instances, + ;; reset variables. + (if (= batch-allocation-count 10) + (begin + (set! batch-allocation-count 0) + (set! batch-get-n-set #f))) + ;; If we don't have a current pair of get and set closures, + ;; create one. make-closure-variable returns a pair of closures + ;; around a single Scheme variable - see goops.scm for details. + (or batch-get-n-set + (set! batch-get-n-set (make-closure-variable))) + ;; Increment the batch allocation count. + (set! batch-allocation-count (+ batch-allocation-count 1)) + batch-get-n-set) + + ;; Call next-method to handle standard allocation types. + (else (next-method))))) + +(define-class <class-using-batched-slot> () + ... + (c #:allocation #:batched) + ... + #:metaclass <batched-allocation-metaclass>) +@end example + +The usage of @code{compute-getter-method} and @code{compute-setter-method} +is described in @ref{MOP Specification}. + +@code{compute-cpl} and @code{compute-get-n-set} are called by the +standard @code{initialize} method for classes whose metaclass is +@code{<class>}. But @code{initialize} itself can also be modified, by +defining an @code{initialize} method specialized to the new class's +metaclass. Such a method could complete override the standard +behaviour, by not calling @code{(next-method)} at all, but more +typically it would perform additional class initialization steps before +and/or after calling @code{(next-method)} for the standard behaviour. + +@node STKlos Compatibility +@subsubsection STKlos Compatibility + +If the STKlos compatibility module is loaded, @code{define-class} is +overwritten by a STKlos-specific definition; the standard GOOPS +definition of @code{define-class} remains available in +@code{standard-define-class}. + +@deffn syntax standard-define-class name (super @dots{}) slot-definition @dots{} . options +@code{standard-define-class} is equivalent to the standard GOOPS +@code{define-class}. +@end deffn + +@node Creating Instances +@subsection Creating Instances + +@menu +* Basic Instance Creation:: +* Customizing Instance Creation:: +@end menu + +@node Basic Instance Creation +@subsubsection Basic Instance Creation + +To create a new instance of any GOOPS class, use the generic function +@code{make} or @code{make-instance}, passing the required class and any +appropriate instance initialization arguments as keyword and value +pairs. Note that @code{make} and @code{make-instances} are aliases for +each other - their behaviour is identical. + +@deffn generic make +@deffnx method make (class <class>) . initargs +Create and return a new instance of class @var{class}, initialized using +@var{initargs}. + +In theory, @var{initargs} can have any structure that is understood by +whatever methods get applied when the @code{initialize} generic function +is applied to the newly allocated instance. + +In practice, specialized @code{initialize} methods would normally call +@code{(next-method)}, and so eventually the standard GOOPS +@code{initialize} methods are applied. These methods expect +@var{initargs} to be a list with an even number of elements, where +even-numbered elements (counting from zero) are keywords and +odd-numbered elements are the corresponding values. + +GOOPS processes initialization argument keywords automatically for slots +whose definition includes the @code{#:init-keyword} option (@pxref{Slot +Options,, init-keyword}). Other keyword value pairs can only be +processed by an @code{initialize} method that is specialized for the new +instance's class. Any unprocessed keyword value pairs are ignored. +@end deffn + +@deffn generic make-instance +@deffnx method make-instance (class <class>) . initargs +@code{make-instance} is an alias for @code{make}. +@end deffn + +@node Customizing Instance Creation +@subsubsection Customizing Instance Creation + +@code{make} itself is a generic function. Hence the @code{make} +invocation itself can be customized in the case where the new instance's +metaclass is more specialized than the default @code{<class>}, by +defining a @code{make} method that is specialized to that metaclass. + +Normally, however, the method for classes with metaclass @code{<class>} +will be applied. This method calls two generic functions: + +@itemize @bullet +@item +(allocate-instance @var{class} . @var{initargs}) + +@item +(initialize @var{instance} . @var{initargs}) +@end itemize + +@code{allocate-instance} allocates storage for and returns the new +instance, uninitialized. You might customize @code{allocate-instance}, +for example, if you wanted to provide a GOOPS wrapper around some other +object programming system. + +To do this, you would create a specialized metaclass, which would act as +the metaclass for all classes and instances from the other system. Then +define an @code{allocate-instance} method, specialized to that +metaclass, which calls a Guile primitive C function, which in turn +allocates the new instance using the interface of the other object +system. + +In this case, for a complete system, you would also need to customize a +number of other generic functions like @code{make} and +@code{initialize}, so that GOOPS knows how to make classes from the +other system, access instance slots, and so on. + +@code{initialize} initializes the instance that is returned by +@code{allocate-instance}. The standard GOOPS methods perform +initializations appropriate to the instance class. + +@itemize @bullet +@item +At the least specialized level, the method for instances of type +@code{<object>} performs internal GOOPS instance initialization, and +initializes the instance's slots according to the slot definitions and +any slot initialization keywords that appear in @var{initargs}. + +@item +The method for instances of type @code{<class>} calls +@code{(next-method)}, then performs the class initializations described +in @ref{Customizing Class Definition}. + +@item +and so on for generic functions, method, operator classes @dots{} +@end itemize + +Similarly, you can customize the initialization of instances of any +application-defined class by defining an @code{initialize} method +specialized to that class. + +Imagine a class whose instances' slots need to be initialized at +instance creation time by querying a database. Although it might be +possible to achieve this a combination of @code{#:init-thunk} keywords +and closures in the slot definitions, it is neater to write an +@code{initialize} method for the class that queries the database once +and initializes all the dependent slot values according to the results. + +@node Accessing Slots +@subsection Accessing Slots + +The definition of a slot contains at the very least a slot name, and may +also contain various slot options, including getter, setter and/or +accessor functions for the slot. + +It is always possible to access slots by name, using the various +``slot-ref'' and ``slot-set!'' procedures described in the following +subsubsections. For example, + +@example +(define-class <my-class> () ;; Define a class with slots + (count #:init-value 0) ;; named "count" and "cache". + (cache #:init-value '()) + @dots{}) + +(define inst (make <my-class>)) ;; Make an instance of this class. + +(slot-set! inst 'count 5) ;; Set the value of the "count" + ;; slot to 5. + +(slot-set! inst 'cache ;; Modify the value of the + (cons (cons "^it" "It") ;; "cache" slot. + (slot-ref inst 'cache))) +@end example + +If a slot definition includes a getter, setter or accessor function, +these can be used instead of @code{slot-ref} and @code{slot-set!} to +access the slot. + +@example +(define-class <adv-class> () ;; Define a new class whose slots + (count #:setter set-count) ;; use a getter, a setter and + (cache #:accessor cache) ;; an accessor. + (csize #:getter cache-size) + @dots{}) + +(define inst (make <adv-class>)) ;; Make an instance of this class. + +(set-count inst 5) ;; Set the value of the "count" + ;; slot to 5. + +(set! (cache inst) ;; Modify the value of the + (cons (cons "^it" "It") ;; "cache" slot. + (cache inst))) + +(let ((size (cache-size inst))) ;; Get the value of the "csize" + @dots{}) ;; slot. +@end example + +Whichever of these methods is used to access slots, GOOPS always calls +the low-level @dfn{getter} and @dfn{setter} closures for the slot to get +and set its value. These closures make sure that the slot behaves +according to the @code{#:allocation} type that was specified in the slot +definition (@pxref{Slot Options,, allocation}). (For more about these +closures, see @ref{Customizing Class Definition,, compute-get-n-set}.) + +@menu +* Instance Slots:: +* Class Slots:: +* Handling Slot Access Errors:: +@end menu + +@node Instance Slots +@subsubsection Instance Slots + +Any slot, regardless of its allocation, can be queried, referenced and +set using the following four primitive procedures. + +@deffn {primitive procedure} slot-exists? obj slot-name +Return @code{#t} if @var{obj} has a slot with name @var{slot-name}, +otherwise @code{#f}. +@end deffn + +@deffn {primitive procedure} slot-bound? obj slot-name +Return @code{#t} if the slot named @var{slot-name} in @var{obj} has a +value, otherwise @code{#f}. + +@code{slot-bound?} calls the generic function @code{slot-missing} if +@var{obj} does not have a slot called @var{slot-name} (@pxref{Handling +Slot Access Errors, slot-missing}). +@end deffn + +@deffn {primitive procedure} slot-ref obj slot-name +Return the value of the slot named @var{slot-name} in @var{obj}. + +@code{slot-ref} calls the generic function @code{slot-missing} if +@var{obj} does not have a slot called @var{slot-name} (@pxref{Handling +Slot Access Errors, slot-missing}). + +@code{slot-ref} calls the generic function @code{slot-unbound} if the +named slot in @var{obj} does not have a value (@pxref{Handling Slot +Access Errors, slot-unbound}). +@end deffn + +@deffn {primitive procedure} slot-set! obj slot-name value +Set the value of the slot named @var{slot-name} in @var{obj} to @var{value}. + +@code{slot-set!} calls the generic function @code{slot-missing} if +@var{obj} does not have a slot called @var{slot-name} (@pxref{Handling +Slot Access Errors, slot-missing}). +@end deffn + +GOOPS stores information about slots in class metaobjects. Internally, +all of these procedures work by looking up the slot definition for the +slot named @var{slot-name} in the class metaobject for @code{(class-of +@var{obj})}, and then using the slot definition's ``getter'' and +``setter'' closures to get and set the slot value. + +The next four procedures differ from the previous ones in that they take +the class metaobject as an explicit argument, rather than assuming +@code{(class-of @var{obj})}. Therefore they allow you to apply the +``getter'' and ``setter'' closures of a slot definition in one class to +an instance of a different class. + +[ *fixme* I have no idea why this is useful! Perhaps when a slot in +@code{(class-of @var{obj})} shadows a slot with the same name in one of +its superclasses? There should be an enlightening example here. ] + +@deffn {primitive procedure} slot-exists-using-class? class obj slot-name +Return @code{#t} if the class metaobject @var{class} has a slot +definition for a slot with name @var{slot-name}, otherwise @code{#f}. +@end deffn + +@deffn {primitive procedure} slot-bound-using-class? class obj slot-name +Return @code{#t} if applying @code{slot-ref-using-class} to the same +arguments would call the generic function @code{slot-unbound}, otherwise +@code{#f}. + +@code{slot-bound-using-class?} calls the generic function +@code{slot-missing} if @var{class} does not have a slot definition for a +slot called @var{slot-name} (@pxref{Handling Slot Access Errors, +slot-missing}). +@end deffn + +@deffn {primitive procedure} slot-ref-using-class class obj slot-name +Apply the ``getter'' closure for the slot named @var{slot-name} in +@var{class} to @var{obj}, and return its result. + +@code{slot-ref-using-class} calls the generic function +@code{slot-missing} if @var{class} does not have a slot definition for a +slot called @var{slot-name} (@pxref{Handling Slot Access Errors, +slot-missing}). + +@code{slot-ref-using-class} calls the generic function +@code{slot-unbound} if the application of the ``getter'' closure to +@var{obj} returns an unbound value (@pxref{Handling Slot Access Errors, +slot-unbound}). +@end deffn + +@deffn {primitive procedure} slot-set-using-class! class obj slot-name value +Apply the ``setter'' closure for the slot named @var{slot-name} in +@var{class} to @var{obj} and @var{value}. + +@code{slot-set-using-class!} calls the generic function +@code{slot-missing} if @var{class} does not have a slot definition for a +slot called @var{slot-name} (@pxref{Handling Slot Access Errors, +slot-missing}). +@end deffn + +@node Class Slots +@subsubsection Class Slots + +Slots whose allocation is per-class rather than per-instance can be +referenced and set without needing to specify any particular instance. + +@deffn procedure class-slot-ref class slot-name +Return the value of the slot named @var{slot-name} in class @var{class}. +The named slot must have @code{#:class} or @code{#:each-subclass} +allocation (@pxref{Slot Options,, allocation}). + +If there is no such slot with @code{#:class} or @code{#:each-subclass} +allocation, @code{class-slot-ref} calls the @code{slot-missing} generic +function with arguments @var{class} and @var{slot-name}. Otherwise, if +the slot value is unbound, @code{class-slot-ref} calls the +@code{slot-missing} generic function, with the same arguments. +@end deffn + +@deffn procedure class-slot-set! class slot-name value +Set the value of the slot named @var{slot-name} in class @var{class} to +@var{value}. The named slot must have @code{#:class} or +@code{#:each-subclass} allocation (@pxref{Slot Options,, allocation}). + +If there is no such slot with @code{#:class} or @code{#:each-subclass} +allocation, @code{class-slot-ref} calls the @code{slot-missing} generic +function with arguments @var{class} and @var{slot-name}. +@end deffn + +@node Handling Slot Access Errors +@subsubsection Handling Slot Access Errors + +GOOPS calls one of the following generic functions when a ``slot-ref'' +or ``slot-set!'' call specifies a non-existent slot name, or tries to +reference a slot whose value is unbound. + +@deffn generic slot-missing +@deffnx method slot-missing (class <class>) slot-name +@deffnx method slot-missing (class <class>) (object <object>) slot-name +@deffnx method slot-missing (class <class>) (object <object>) slot-name value +When an application attempts to reference or set a class or instance +slot by name, and the slot name is invalid for the specified @var{class} +or @var{object}, GOOPS calls the @code{slot-missing} generic function. + +The default methods all call @code{goops-error} with an appropriate +message. +@end deffn + +@deffn generic slot-unbound +@deffnx method slot-unbound (object <object>) +@deffnx method slot-unbound (class <class>) slot-name +@deffnx method slot-unbound (class <class>) (object <object>) slot-name +When an application attempts to reference a class or instance slot, and +the slot's value is unbound, GOOPS calls the @code{slot-unbound} generic +function. + +The default methods all call @code{goops-error} with an appropriate +message. +@end deffn + +@node Creating Generic Functions +@subsection Creating Generic Functions + +A generic function is a collection of methods, with rules for +determining which of the methods should be applied for any given +invocation of the generic function. + +GOOPS represents generic functions as metaobjects of the class +@code{<generic>} (or one of its subclasses). + +@menu +* Basic Generic Function Creation:: +* Generic Function Internals:: +* Extending Guiles Primitives:: +@end menu + +@node Basic Generic Function Creation +@subsubsection Basic Generic Function Creation + +The following forms may be used to bind a variable to a generic +function. Depending on that variable's pre-existing value, the generic +function may be created empty - with no methods - or it may contain +methods that are inferred from the pre-existing value. + +It is not, in general, necessary to use @code{define-generic} or +@code{define-accessor} before defining methods for the generic function +using @code{define-method}, since @code{define-method} will +automatically interpolate a @code{define-generic} call, or upgrade an +existing generic to an accessor, if that is implied by the +@code{define-method} call. Note in particular that, +if the specified variable already has a @emph{generic function} value, +@code{define-generic} and @code{define-accessor} will @emph{discard} it! +Obviously it is application-dependent whether this is desirable or not. + +If, for example, you wanted to extend @code{+} for a class representing +a new numerical type, you probably want to inherit any existing methods +for @code{+} and so should not use @code{define-generic}. If, on the +other hand, you do not want to risk inheriting methods whose behaviour +might surprise you, you can use @code{define-generic} or +@code{define-accessor} to wipe the slate clean. + +@deffn syntax define-generic symbol +Create a generic function with name @var{symbol} and bind it to the +variable @var{symbol}. + +If the variable @var{symbol} was previously bound to a Scheme procedure +(or procedure-with-setter), the old procedure (and setter) is +incorporated into the new generic function as its default procedure (and +setter). Any other previous value that was bound to @var{symbol}, +including an existing generic function, is overwritten by the new +generic function. +@end deffn + +@deffn syntax define-accessor symbol +Create an accessor with name @var{symbol} and bind it to the variable +@var{symbol}. + +If the variable @var{symbol} was previously bound to a Scheme procedure +(or procedure-with-setter), the old procedure (and setter) is +incorporated into the new accessor as its default procedure (and +setter). Any other previous value that was bound to @var{symbol}, +including an existing generic function or accessor, is overwritten by +the new definition. +@end deffn + +It is sometimes tempting to use GOOPS accessors with short names. For +example, it is tempting to use the name @code{x} for the x-coordinate +in vector packages. + +Assume that we work with a graphical package which needs to use two +independent vector packages for 2D and 3D vectors respectively. If +both packages export @code{x} we will encounter a name collision. + +This can be resolved automagically with the duplicates handler +@code{merge-generics} which gives the module system license to merge +all generic functions sharing a common name: + +@lisp +(define-module (math 2D-vectors) + #:use-module (oop goops) + #:export (x y ...)) + +(define-module (math 3D-vectors) + #:use-module (oop goops) + #:export (x y z ...)) + +(define-module (my-module) + #:use-module (math 2D-vectors) + #:use-module (math 3D-vectors) + #:duplicates merge-generics) +@end lisp + +The generic function @code{x} in @code{(my-module)} will now share +methods with @code{x} in both imported modules. + +There will, in fact, now be three distinct generic functions named +@code{x}: @code{x} in @code{(2D-vectors)}, @code{x} in +@code{(3D-vectors)}, and @code{x} in @code{(my-module)}. The last +function will be an @code{<extended-generic>}, extending the previous +two functions. + +Let's call the imported generic functions the "ancestor functions". +The generic function @code{x} in @code{(my-module)} is, in turn, a +"descendant function" of the imported functions, extending its +ancestors. + +For any generic function G, the applicable methods are selected from +the union of the methods of the descendant functions, the methods of G +itself and the methods of the ancestor functions. + +This, ancestor functions share methods with their descendants and vice +versa. This implies that @code{x} in @code{(math 2D-vectors)} will +share the methods of @code{x} in @code{(my-module)} and vice versa, +while @code{x} in @code{(math 2D-vectors)} doesn't share the methods +of @code{x} in @code{(math 3D-vectors)}, thus preserving modularity. + +Sharing is dynamic, so that adding new methods to a descendant implies +adding it to the ancestor. + +If duplicates checking is desired in the above example, the following +form of the @code{#:duplicates} option can be used instead: + +@lisp + #:duplicates (merge-generics check) +@end lisp + +@node Generic Function Internals +@subsubsection Generic Function Internals + +@code{define-generic} calls @code{ensure-generic} to upgrade a +pre-existing procedure value, or @code{make} with metaclass +@code{<generic>} to create a new generic function. + +@code{define-accessor} calls @code{ensure-accessor} to upgrade a +pre-existing procedure value, or @code{make-accessor} to create a new +accessor. + +@deffn procedure ensure-generic old-definition [name] +Return a generic function with name @var{name}, if possible by using or +upgrading @var{old-definition}. If unspecified, @var{name} defaults to +@code{#f}. + +If @var{old-definition} is already a generic function, it is returned +unchanged. + +If @var{old-definition} is a Scheme procedure or procedure-with-setter, +@code{ensure-generic} returns a new generic function that uses +@var{old-definition} for its default procedure and setter. + +Otherwise @code{ensure-generic} returns a new generic function with no +defaults and no methods. +@end deffn + +@deffn procedure make-generic [name] +Return a new generic function with name @code{(car @var{name})}. If +unspecified, @var{name} defaults to @code{#f}. +@end deffn + +@code{ensure-generic} calls @code{make} with metaclasses +@code{<generic>} and @code{<generic-with-setter>}, depending on the +previous value of the variable that it is trying to upgrade. + +@code{make-generic} is a simple wrapper for @code{make} with metaclass +@code{<generic>}. + +@deffn procedure ensure-accessor proc [name] +Return an accessor with name @var{name}, if possible by using or +upgrading @var{proc}. If unspecified, @var{name} defaults to @code{#f}. + +If @var{proc} is already an accessor, it is returned unchanged. + +If @var{proc} is a Scheme procedure, procedure-with-setter or generic +function, @code{ensure-accessor} returns an accessor that reuses the +reusable elements of @var{proc}. + +Otherwise @code{ensure-accessor} returns a new accessor with no defaults +and no methods. +@end deffn + +@deffn procedure make-accessor [name] +Return a new accessor with name @code{(car @var{name})}. If +unspecified, @var{name} defaults to @code{#f}. +@end deffn + +@code{ensure-accessor} calls @code{make} with +metaclass @code{<generic-with-setter>}, as well as calls to +@code{ensure-generic}, @code{make-accessor} and (tail recursively) +@code{ensure-accessor}. + +@code{make-accessor} calls @code{make} twice, first +with metaclass @code{<generic>} to create a generic function for the +setter, then with metaclass @code{<generic-with-setter>} to create the +accessor, passing the setter generic function as the value of the +@code{#:setter} keyword. + +@node Extending Guiles Primitives +@subsubsection Extending Guile's Primitives + +When GOOPS is loaded, many of Guile's primitive procedures can be +extended by giving them a generic function definition that operates +in conjunction with their normal C-coded implementation. For +primitives that are extended in this way, the result from the user- +or application-level point of view is that the extended primitive +behaves exactly like a generic function with the C-coded implementation +as its default method. + +The @code{generic-capability?} predicate should be used to determine +whether a particular primitive is extensible in this way. + +@deffn {primitive procedure} generic-capability? primitive +Return @code{#t} if @var{primitive} can be extended by giving it a +generic function definition, otherwise @code{#f}. +@end deffn + +Even when a primitive procedure is extensible like this, its generic +function definition is not created until it is needed by a call to +@code{define-method}, or until the application explicitly requests it +by calling @code{enable-primitive-generic!}. + +@deffn {primitive procedure} enable-primitive-generic! primitive +Force the creation of a generic function definition for +@var{primitive}. +@end deffn + +Once the generic function definition for a primitive has been created, +it can be retrieved using @code{primitive-generic-generic}. + +@deffn {primitive procedure} primitive-generic-generic primitive +Return the generic function definition of @var{primitive}. + +@code{primitive-generic-generic} raises an error if @var{primitive} +is not a primitive with generic capability, or if its generic capability +has not yet been enabled, whether implicitly (by @code{define-method}) +or explicitly (by @code{enable-primitive-generic!}). +@end deffn + +Note that the distinction between, on the one hand, primitives with +additional generic function definitions and, on the other hand, generic +functions with a default method, may disappear when GOOPS is fully +integrated into the core of Guile. Consequently, the +procedures described in this section may disappear as well. + +@node Adding Methods to Generic Functions +@subsection Adding Methods to Generic Functions + +@menu +* Basic Method Definition:: +* Method Definition Internals:: +@end menu + +@node Basic Method Definition +@subsubsection Basic Method Definition + +To add a method to a generic function, use the @code{define-method} form. + +@deffn syntax define-method (generic parameter @dots{}) . body +Define a method for the generic function or accessor @var{generic} with +parameters @var{parameter}s and body @var{body}. + +@var{generic} is a generic function. If @var{generic} is a variable +which is not yet bound to a generic function object, the expansion of +@code{define-method} will include a call to @code{define-generic}. If +@var{generic} is @code{(setter @var{generic-with-setter})}, where +@var{generic-with-setter} is a variable which is not yet bound to a +generic-with-setter object, the expansion will include a call to +@code{define-accessor}. + +Each @var{parameter} must be either a symbol or a two-element list +@code{(@var{symbol} @var{class})}. The symbols refer to variables in +the @var{body} that will be bound to the parameters supplied by the +caller when calling this method. The @var{class}es, if present, +specify the possible combinations of parameters to which this method +can be applied. + +@var{body} is the body of the method definition. +@end deffn + +@code{define-method} expressions look a little like normal Scheme +procedure definitions of the form + +@example +(define (name formals @dots{}) . body) +@end example + +The most important difference is that each formal parameter, apart from the +possible ``rest'' argument, can be qualified by a class name: +@code{@var{formal}} becomes @code{(@var{formal} @var{class})}. The +meaning of this qualification is that the method being defined +will only be applicable in a particular generic function invocation if +the corresponding argument is an instance of @code{@var{class}} (or one of +its subclasses). If more than one of the formal parameters is qualified +in this way, then the method will only be applicable if each of the +corresponding arguments is an instance of its respective qualifying class. + +Note that unqualified formal parameters act as though they are qualified +by the class @code{<top>}, which GOOPS uses to mean the superclass of +all valid Scheme types, including both primitive types and GOOPS classes. + +For example, if a generic function method is defined with +@var{parameter}s @code{((s1 <square>) (n <number>))}, that method is +only applicable to invocations of its generic function that have two +parameters where the first parameter is an instance of the +@code{<square>} class and the second parameter is a number. + +If a generic function is invoked with a combination of parameters for which +there is no applicable method, GOOPS raises an error. For more about +invocation error handling, and generic function invocation in general, +see @ref{Invoking Generic Functions}. + +@node Method Definition Internals +@subsubsection Method Definition Internals + +@code{define-method} + +@itemize @bullet +@item +checks the form of the first parameter, and applies the following steps +to the accessor's setter if it has the @code{(setter @dots{})} form + +@item +interpolates a call to @code{define-generic} or @code{define-accessor} +if a generic function is not already defined with the supplied name + +@item +calls @code{method} with the @var{parameter}s and @var{body}, to make a +new method instance + +@item +calls @code{add-method!} to add this method to the relevant generic +function. +@end itemize + +@deffn syntax method (parameter @dots{}) . body +Make a method whose specializers are defined by the classes in +@var{parameter}s and whose procedure definition is constructed from the +@var{parameter} symbols and @var{body} forms. + +The @var{parameter} and @var{body} parameters should be as for +@code{define-method} (@pxref{Basic Method Definition,, define-method}). +@end deffn + +@code{method} + +@itemize @bullet +@item +extracts formals and specializing classes from the @var{parameter}s, +defaulting the class for unspecialized parameters to @code{<top>} + +@item +creates a closure using the formals and the @var{body} forms + +@item +calls @code{make} with metaclass @code{<method>} and the specializers +and closure using the @code{#:specializers} and @code{#:procedure} +keywords. +@end itemize + +@deffn procedure make-method specializers procedure +Make a method using @var{specializers} and @var{procedure}. + +@var{specializers} should be a list of classes that specifies the +parameter combinations to which this method will be applicable. + +@var{procedure} should be the closure that will applied to the generic +function parameters when this method is invoked. +@end deffn + +@code{make-method} is a simple wrapper around @code{make} with metaclass +@code{<method>}. + +@deffn generic add-method! target method +Generic function for adding method @var{method} to @var{target}. +@end deffn + +@deffn method add-method! (generic <generic>) (method <method>) +Add method @var{method} to the generic function @var{generic}. +@end deffn + +@deffn method add-method! (proc <procedure>) (method <method>) +If @var{proc} is a procedure with generic capability (@pxref{Extending +Guiles Primitives,, generic-capability?}), upgrade it to a +primitive generic and add @var{method} to its generic function +definition. +@end deffn + +@deffn method add-method! (pg <primitive-generic>) (method <method>) +Add method @var{method} to the generic function definition of @var{pg}. + +Implementation: @code{(add-method! (primitive-generic-generic pg) method)}. +@end deffn + +@deffn method add-method! (whatever <top>) (method <method>) +Raise an error indicating that @var{whatever} is not a valid generic +function. +@end deffn + +@node Invoking Generic Functions +@subsection Invoking Generic Functions + +When a variable with a generic function definition appears as the first +element of a list that is being evaluated, the Guile evaluator tries +to apply the generic function to the arguments obtained by evaluating +the remaining elements of the list. [ *fixme* How do I put this in a +more Schemely and less Lispy way? ] + +Usually a generic function contains several method definitions, with +varying degrees of formal parameter specialization (@pxref{Basic +Method Definition,, define-method}). So it is necessary to sort these +methods by specificity with respect to the supplied arguments, and then +apply the most specific method definition. Less specific methods +may be applied subsequently if a method that is being applied calls +@code{next-method}. + +@menu +* Determining Which Methods to Apply:: +* Handling Invocation Errors:: +@end menu + +@node Determining Which Methods to Apply +@subsubsection Determining Which Methods to Apply + +[ *fixme* Sorry - this is the area of GOOPS that I understand least of +all, so I'm afraid I have to pass on this section. Would some other +kind person consider filling it in? ] + +@deffn generic apply-generic +@deffnx method apply-generic (gf <generic>) args +@end deffn + +@deffn generic compute-applicable-methods +@deffnx method compute-applicable-methods (gf <generic>) args +@end deffn + +@deffn generic sort-applicable-methods +@deffnx method sort-applicable-methods (gf <generic>) methods args +@end deffn + +@deffn generic method-more-specific? +@deffnx method method-more-specific? (m1 <method>) (m2 <method>) args +@end deffn + +@deffn generic apply-method +@deffnx method apply-method (gf <generic>) methods build-next args +@end deffn + +@deffn generic apply-methods +@deffnx method apply-methods (gf <generic>) (l <list>) args +@end deffn + +@node Handling Invocation Errors +@subsubsection Handling Invocation Errors + +@deffn generic no-method +@deffnx method no-method (gf <generic>) args +When an application invokes a generic function, and no methods at all +have been defined for that generic function, GOOPS calls the +@code{no-method} generic function. The default method calls +@code{goops-error} with an appropriate message. +@end deffn + +@deffn generic no-applicable-method +@deffnx method no-applicable-method (gf <generic>) args +When an application applies a generic function to a set of arguments, +and no methods have been defined for those argument types, GOOPS calls +the @code{no-applicable-method} generic function. The default method +calls @code{goops-error} with an appropriate message. +@end deffn + +@deffn generic no-next-method +@deffnx method no-next-method (gf <generic>) args +When a generic function method calls @code{(next-method)} to invoke the +next less specialized method for that generic function, and no less +specialized methods have been defined for the current generic function +arguments, GOOPS calls the @code{no-next-method} generic function. The +default method calls @code{goops-error} with an appropriate message. +@end deffn + +@node Redefining a Class +@subsection Redefining a Class + +Suppose that a class @code{<my-class>} is defined using @code{define-class} +(@pxref{Basic Class Definition,, define-class}), with slots that have +accessor functions, and that an application has created several instances +of @code{<my-class>} using @code{make} (@pxref{Basic Instance Creation,, +make}). What then happens if @code{<my-class>} is redefined by calling +@code{define-class} again? + +@menu +* Default Class Redefinition Behaviour:: +* Customizing Class Redefinition:: +@end menu + +@node Default Class Redefinition Behaviour +@subsubsection Default Class Redefinition Behaviour + +GOOPS' default answer to this question is as follows. + +@itemize @bullet +@item +All existing direct instances of @code{<my-class>} are converted to be +instances of the new class. This is achieved by preserving the values +of slots that exist in both the old and new definitions, and initializing the +values of new slots in the usual way (@pxref{Basic Instance Creation,, +make}). + +@item +All existing subclasses of @code{<my-class>} are redefined, as though +the @code{define-class} expressions that defined them were re-evaluated +following the redefinition of @code{<my-class>}, and the class +redefinition process described here is applied recursively to the +redefined subclasses. + +@item +Once all of its instances and subclasses have been updated, the class +metaobject previously bound to the variable @code{<my-class>} is no +longer needed and so can be allowed to be garbage collected. +@end itemize + +To keep things tidy, GOOPS also needs to do a little housekeeping on +methods that are associated with the redefined class. + +@itemize @bullet +@item +Slot accessor methods for slots in the old definition should be removed +from their generic functions. They will be replaced by accessor methods +for the slots of the new class definition. + +@item +Any generic function method that uses the old @code{<my-class>} metaobject +as one of its formal parameter specializers must be updated to refer to +the new @code{<my-class>} metaobject. (Whenever a new generic function +method is defined, @code{define-method} adds the method to a list stored +in the class metaobject for each class used as a formal parameter +specializer, so it is easy to identify all the methods that must be +updated when a class is redefined.) +@end itemize + +If this class redefinition strategy strikes you as rather counter-intuitive, +bear in mind that it is derived from similar behaviour in other object +systems such as CLOS, and that experience in those systems has shown it to be +very useful in practice. + +Also bear in mind that, like most of GOOPS' default behaviour, it can +be customized@dots{} + +@node Customizing Class Redefinition +@subsubsection Customizing Class Redefinition + +When @code{define-class} notices that a class is being redefined, +it constructs the new class metaobject as usual, and then invokes the +@code{class-redefinition} generic function with the old and new classes +as arguments. Therefore, if the old or new classes have metaclasses +other than the default @code{<class>}, class redefinition behaviour can +be customized by defining a @code{class-redefinition} method that is +specialized for the relevant metaclasses. + +@deffn generic class-redefinition +Handle the class redefinition from @var{old-class} to @var{new-class}, +and return the new class metaobject that should be bound to the +variable specified by @code{define-class}'s first argument. +@end deffn + +@deffn method class-redefinition (old-class <class>) (new-class <class>) +Implements GOOPS' default class redefinition behaviour, as described in +@ref{Default Class Redefinition Behaviour}. Returns the metaobject +for the new class definition. +@end deffn + +An alternative class redefinition strategy could be to leave all +existing instances as instances of the old class, but accepting that the +old class is now ``nameless'', since its name has been taken over by the +new definition. In this strategy, any existing subclasses could also +be left as they are, on the understanding that they inherit from a nameless +superclass. + +This strategy is easily implemented in GOOPS, by defining a new metaclass, +that will be used as the metaclass for all classes to which the strategy +should apply, and then defining a @code{class-redefinition} method that +is specialized for this metaclass: + +@example +(define-class <can-be-nameless> (<class>)) + +(define-method (class-redefinition (old <can-be-nameless>) + (new <class>)) + new) +@end example + +When customization can be as easy as this, aren't you glad that GOOPS +implements the far more difficult strategy as its default! + +Finally, note that, if @code{class-redefinition} itself is not customized, +the default @code{class-redefinition} method invokes three further +generic functions that could be individually customized: + +@itemize @bullet +@item +(remove-class-accessors! @var{old-class}) + +@item +(update-direct-method! @var{method} @var{old-class} @var{new-class}) + +@item +(update-direct-subclass! @var{subclass} @var{old-class} @var{new-class}) +@end itemize + +and the default methods for these generic functions invoke further +generic functions, and so on@dots{} The detailed protocol for all of these +is described in @ref{MOP Specification}. + +@node Changing the Class of an Instance +@subsection Changing the Class of an Instance + +You can change the class of an existing instance by invoking the +generic function @code{change-class} with two arguments: the instance +and the new class. + +@deffn generic change-class +@end deffn + +The default method for @code{change-class} decides how to implement the +change of class by looking at the slot definitions for the instance's +existing class and for the new class. If the new class has slots with +the same name as slots in the existing class, the values for those slots +are preserved. Slots that are present only in the existing class are +discarded. Slots that are present only in the new class are initialized +using the corresponding slot definition's init function (@pxref{Classes,, +slot-init-function}). + +@deffn {method} change-class (obj <object>) (new <class>) +Modify instance @var{obj} to make it an instance of class @var{new}. + +The value of each of @var{obj}'s slots is preserved only if a similarly named +slot exists in @var{new}; any other slot values are discarded. + +The slots in @var{new} that do not correspond to any of @var{obj}'s +pre-existing slots are initialized according to @var{new}'s slot definitions' +init functions. +@end deffn + +Customized change of class behaviour can be implemented by defining +@code{change-class} methods that are specialized either by the class +of the instances to be modified or by the metaclass of the new class. + +When a class is redefined (@pxref{Redefining a Class}), and the default +class redefinition behaviour is not overridden, GOOPS (eventually) +invokes the @code{change-class} generic function for each existing +instance of the redefined class. + +@node Introspection +@subsection Introspection + +@dfn{Introspection}, also known as @dfn{reflection}, is the name given +to the ability to obtain information dynamically about GOOPS metaobjects. +It is perhaps best illustrated by considering an object oriented language +that does not provide any introspection, namely C++. + +Nothing in C++ allows a running program to obtain answers to the following +types of question: + +@itemize @bullet +@item +What are the data members of this object or class? + +@item +What classes does this class inherit from? + +@item +Is this method call virtual or non-virtual? + +@item +If I invoke @code{Employee::adjustHoliday()}, what class contains the +@code{adjustHoliday()} method that will be applied? +@end itemize + +In C++, answers to such questions can only be determined by looking at +the source code, if you have access to it. GOOPS, on the other hand, +includes procedures that allow answers to these questions --- or their +GOOPS equivalents --- to be obtained dynamically, at run time. + +@menu +* Classes:: +* Slots:: +* Instances:: +* Generic Functions:: +* Generic Function Methods:: +@end menu + +@node Classes +@subsubsection Classes + +@deffn {primitive procedure} class-name class +Return the name of class @var{class}. +This is the value of the @var{class} metaobject's @code{name} slot. +@end deffn + +@deffn {primitive procedure} class-direct-supers class +Return a list containing the direct superclasses of @var{class}. +This is the value of the @var{class} metaobject's +@code{direct-supers} slot. +@end deffn + +@deffn {primitive procedure} class-direct-slots class +Return a list containing the slot definitions of the direct slots of +@var{class}. +This is the value of the @var{class} metaobject's @code{direct-slots} +slot. +@end deffn + +@deffn {primitive procedure} class-direct-subclasses class +Return a list containing the direct subclasses of @var{class}. +This is the value of the @var{class} metaobject's +@code{direct-subclasses} slot. +@end deffn + +@deffn {primitive procedure} class-direct-methods class +Return a list of all the generic function methods that use @var{class} +as a formal parameter specializer. +This is the value of the @var{class} metaobject's @code{direct-methods} +slot. +@end deffn + +@deffn {primitive procedure} class-precedence-list class +Return the class precedence list for class @var{class} (@pxref{Class +precedence list}). +This is the value of the @var{class} metaobject's @code{cpl} slot. +@end deffn + +@deffn {primitive procedure} class-slots class +Return a list containing the slot definitions for all @var{class}'s slots, +including any slots that are inherited from superclasses. +This is the value of the @var{class} metaobject's @code{slots} slot. +@end deffn + +@deffn {primitive procedure} class-environment class +Return the value of @var{class}'s @code{environment} slot. +[ *fixme* I don't know what this value is used for. ] +@end deffn + +@deffn procedure class-subclasses class +Return a list of all subclasses of @var{class}. +@end deffn + +@deffn procedure class-methods class +Return a list of all methods that use @var{class} or a subclass of +@var{class} as one of its formal parameter specializers. +@end deffn + +@node Slots +@subsubsection Slots + +@deffn procedure class-slot-definition class slot-name +Return the slot definition for the slot named @var{slot-name} in class +@var{class}. @var{slot-name} should be a symbol. +@end deffn + +@deffn procedure slot-definition-name slot-def +Extract and return the slot name from @var{slot-def}. +@end deffn + +@deffn procedure slot-definition-options slot-def +Extract and return the slot options from @var{slot-def}. +@end deffn + +@deffn procedure slot-definition-allocation slot-def +Extract and return the slot allocation option from @var{slot-def}. This +is the value of the @code{#:allocation} keyword (@pxref{Slot Options,, +allocation}), or @code{#:instance} if the @code{#:allocation} keyword is +absent. +@end deffn + +@deffn procedure slot-definition-getter slot-def +Extract and return the slot getter option from @var{slot-def}. This is +the value of the @code{#:getter} keyword (@pxref{Slot Options,, +getter}), or @code{#f} if the @code{#:getter} keyword is absent. +@end deffn + +@deffn procedure slot-definition-setter slot-def +Extract and return the slot setter option from @var{slot-def}. This is +the value of the @code{#:setter} keyword (@pxref{Slot Options,, +setter}), or @code{#f} if the @code{#:setter} keyword is absent. +@end deffn + +@deffn procedure slot-definition-accessor slot-def +Extract and return the slot accessor option from @var{slot-def}. This +is the value of the @code{#:accessor} keyword (@pxref{Slot Options,, +accessor}), or @code{#f} if the @code{#:accessor} keyword is absent. +@end deffn + +@deffn procedure slot-definition-init-value slot-def +Extract and return the slot init-value option from @var{slot-def}. This +is the value of the @code{#:init-value} keyword (@pxref{Slot Options,, +init-value}), or the unbound value if the @code{#:init-value} keyword is +absent. +@end deffn + +@deffn procedure slot-definition-init-form slot-def +Extract and return the slot init-form option from @var{slot-def}. This +is the value of the @code{#:init-form} keyword (@pxref{Slot Options,, +init-form}), or the unbound value if the @code{#:init-form} keyword is +absent. +@end deffn + +@deffn procedure slot-definition-init-thunk slot-def +Extract and return the slot init-thunk option from @var{slot-def}. This +is the value of the @code{#:init-thunk} keyword (@pxref{Slot Options,, +init-thunk}), or @code{#f} if the @code{#:init-thunk} keyword is absent. +@end deffn + +@deffn procedure slot-definition-init-keyword slot-def +Extract and return the slot init-keyword option from @var{slot-def}. +This is the value of the @code{#:init-keyword} keyword (@pxref{Slot +Options,, init-keyword}), or @code{#f} if the @code{#:init-keyword} +keyword is absent. +@end deffn + +@deffn procedure slot-init-function class slot-name +Return the initialization function for the slot named @var{slot-name} in +class @var{class}. @var{slot-name} should be a symbol. + +The returned initialization function incorporates the effects of the +standard @code{#:init-thunk}, @code{#:init-form} and @code{#:init-value} +slot options. These initializations can be overridden by the +@code{#:init-keyword} slot option or by a specialized @code{initialize} +method, so, in general, the function returned by +@code{slot-init-function} may be irrelevant. For a fuller discussion, +see @ref{Slot Options,, init-value}. +@end deffn + +@node Instances +@subsubsection Instances + +@deffn {primitive procedure} class-of value +Return the GOOPS class of any Scheme @var{value}. +@end deffn + +@deffn {primitive procedure} instance? object +Return @code{#t} if @var{object} is any GOOPS instance, otherwise +@code{#f}. +@end deffn + +@deffn procedure is-a? object class +Return @code{#t} if @var{object} is an instance of @var{class} or one of +its subclasses. +@end deffn + +Implementation notes: @code{is-a?} uses @code{class-of} and +@code{class-precedence-list} to obtain the class precedence list for +@var{object}. + +@node Generic Functions +@subsubsection Generic Functions + +@deffn {primitive procedure} generic-function-name gf +Return the name of generic function @var{gf}. +@end deffn + +@deffn {primitive procedure} generic-function-methods gf +Return a list of the methods of generic function @var{gf}. +This is the value of the @var{gf} metaobject's @code{methods} slot. +@end deffn + +@node Generic Function Methods +@subsubsection Generic Function Methods + +@deffn {primitive procedure} method-generic-function method +Return the generic function that @var{method} belongs to. +This is the value of the @var{method} metaobject's +@code{generic-function} slot. +@end deffn + +@deffn {primitive procedure} method-specializers method +Return a list of @var{method}'s formal parameter specializers . +This is the value of the @var{method} metaobject's +@code{specializers} slot. +@end deffn + +@deffn {primitive procedure} method-procedure method +Return the procedure that implements @var{method}. +This is the value of the @var{method} metaobject's +@code{procedure} slot. +@end deffn + +@deffn generic method-source +@deffnx method method-source (m <method>) +Return an expression that prints to show the definition of method +@var{m}. + +@example +(define-generic cube) + +(define-method (cube (n <number>)) + (* n n n)) + +(map method-source (generic-function-methods cube)) +@result{} +((method ((n <number>)) (* n n n))) +@end example +@end deffn + +@node Miscellaneous Functions +@subsection Miscellaneous Functions + +@menu +* Administrative Functions:: +* GOOPS Error Handling:: +* Object Comparisons:: +* Cloning Objects:: +* Write and Display:: +@end menu + +@node Administrative Functions +@subsubsection Administration Functions + +This section describes administrative, non-technical GOOPS functions. + +@deffn primitive goops-version +Return the current GOOPS version as a string, for example ``0.2''. +@end deffn + +@node GOOPS Error Handling +@subsubsection Error Handling + +The procedure @code{goops-error} is called to raise an appropriate error +by the default methods of the following generic functions: + +@itemize @bullet +@item +@code{slot-missing} (@pxref{Handling Slot Access Errors,, slot-missing}) + +@item +@code{slot-unbound} (@pxref{Handling Slot Access Errors,, slot-unbound}) + +@item +@code{no-method} (@pxref{Handling Invocation Errors,, no-method}) + +@item +@code{no-applicable-method} (@pxref{Handling Invocation Errors,, +no-applicable-method}) + +@item +@code{no-next-method} (@pxref{Handling Invocation Errors,, +no-next-method}) +@end itemize + +If you customize these functions for particular classes or metaclasses, +you may still want to use @code{goops-error} to signal any error +conditions that you detect. + +@deffn procedure goops-error format-string . args +Raise an error with key @code{goops-error} and error message constructed +from @var{format-string} and @var{args}. Error message formatting is +as done by @code{scm-error}. +@end deffn + +@node Object Comparisons +@subsubsection Object Comparisons + +@deffn generic eqv? +@deffnx method eqv? ((x <top>) (y <top>)) +@deffnx generic equal? +@deffnx method equal? ((x <top>) (y <top>)) +@deffnx generic = +@deffnx method = ((x <number>) (y <number>)) +Generic functions and default (unspecialized) methods for comparing two +GOOPS objects. + +The default method for @code{eqv?} returns @code{#t} for all values +that are equal in the sense defined by R5RS and the Guile reference +manual, otherwise @code{#f}. The default method for @code{equal?} +returns @code{#t} or @code{#f} in the sense defined by R5RS and the +Guile reference manual. If no such comparison is defined, +@code{equal?} returns the result of a call to @code{eqv?}. The +default method for = returns @code{#t} if @var{x} and @var{y} are +numerically equal, otherwise @code{#f}. + +Application class authors may wish to define specialized methods for +@code{eqv?}, @code{equal?} and @code{=} that compare instances of the +same class for equality in whatever sense is useful to the +application. Such methods will only be called if the arguments have +the same class and the result of the comparison isn't defined by R5RS +and the Guile reference manual. +@end deffn + +@node Cloning Objects +@subsubsection Cloning Objects + +@deffn generic shallow-clone +@deffnx method shallow-clone (self <object>) +Return a ``shallow'' clone of @var{self}. The default method makes a +shallow clone by allocating a new instance and copying slot values from +self to the new instance. Each slot value is copied either as an +immediate value or by reference. +@end deffn + +@deffn generic deep-clone +@deffnx method deep-clone (self <object>) +Return a ``deep'' clone of @var{self}. The default method makes a deep +clone by allocating a new instance and copying or cloning slot values +from self to the new instance. If a slot value is an instance +(satisfies @code{instance?}), it is cloned by calling @code{deep-clone} +on that value. Other slot values are copied either as immediate values +or by reference. +@end deffn + +@node Write and Display +@subsubsection Write and Display + +@deffn {primitive generic} write object port +@deffnx {primitive generic} display object port +When GOOPS is loaded, @code{write} and @code{display} become generic +functions with special methods for printing + +@itemize @bullet +@item +objects - instances of the class @code{<object>} + +@item +foreign objects - instances of the class @code{<foreign-object>} + +@item +classes - instances of the class @code{<class>} + +@item +generic functions - instances of the class @code{<generic>} + +@item +methods - instances of the class @code{<method>}. +@end itemize + +@code{write} and @code{display} print non-GOOPS values in the same way +as the Guile primitive @code{write} and @code{display} functions. +@end deffn + +@node MOP Specification +@section MOP Specification + +For an introduction to metaobjects and the metaobject protocol, +see @ref{Metaobjects and the Metaobject Protocol}. + +The aim of the MOP specification in this chapter is to specify all the +customizable generic function invocations that can be made by the standard +GOOPS syntax, procedures and methods, and to explain the protocol for +customizing such invocations. + +A generic function invocation is customizable if the types of the arguments +to which it is applied are not all determined by the lexical context in +which the invocation appears. For example, + +@itemize @bullet +@item +the @code{(initialize @var{instance} @var{initargs})} invocation in the +default @code{make-instance} method is customizable, because the type of the +@code{@var{instance}} argument is determined by the class that was passed to +@code{make-instance}. + +@item +the @code{(make <generic> #:name ',name)} invocation in @code{define-generic} +is not customizable, because all of its arguments have lexically determined +types. +@end itemize + +When using this rule to decide whether a given generic function invocation +is customizable, we ignore arguments that are expected to be handled in +method definitions as a single ``rest'' list argument. + +For each customizable generic function invocation, the @dfn{invocation +protocol} is explained by specifying + +@itemize @bullet +@item +what, conceptually, the applied method is intended to do + +@item +what assumptions, if any, the caller makes about the applied method's side +effects + +@item +what the caller expects to get as the applied method's return value. +@end itemize + +@menu +* Class Definition:: +* Instance Creation:: +* Class Redefinition:: +* Method Definition:: +* Generic Function Invocation:: +@end menu + +@node Class Definition +@subsection Class Definition + +@code{define-class} (syntax) + +@itemize @bullet +@item +@code{class} (syntax) + +@itemize @bullet +@item +@code{make-class} (procedure) + +@itemize @bullet +@item +@code{make @var{metaclass} @dots{}} (generic) + +@var{metaclass} is the metaclass of the class being defined, either +taken from the @code{#:metaclass} class option or computed by +@code{ensure-metaclass}. The applied method must create and return the +fully initialized class metaobject for the new class definition. +@end itemize + +@end itemize + +@item +@code{class-redefinition @var{old-class} @var{new-class}} (generic) + +@code{define-class} calls @code{class-redefinition} if the variable +specified by its first argument already held a GOOPS class definition. +@var{old-class} and @var{new-class} are the old and new class metaobjects. +The applied method should perform whatever is necessary to handle the +redefinition, and should return the class metaobject that is to be bound +to @code{define-class}'s variable. The default class redefinition +protocol is described in @ref{Class Redefinition}. +@end itemize + +The @code{(make @var{metaclass} @dots{})} invocation above will create +an class metaobject with metaclass @var{metaclass}. By default, this +metaobject will be initialized by the @code{initialize} method that is +specialized for instances of type @code{<class>}. + +@code{initialize <class> @var{initargs}} (method) + +@itemize @bullet +@item +@code{compute-cpl @var{class}} (generic) + +The applied method should compute and return the class precedence list +for @var{class} as a list of class metaobjects. When @code{compute-cpl} +is called, the following @var{class} metaobject slots have all been +initialized: @code{name}, @code{direct-supers}, @code{direct-slots}, +@code{direct-subclasses} (empty), @code{direct-methods}. The value +returned by @code{compute-cpl} will be stored in the @code{cpl} slot. + +@item +@code{compute-slots @var{class}} (generic) + +The applied method should compute and return the slots (union of direct +and inherited) for @var{class} as a list of slot definitions. When +@code{compute-slots} is called, all the @var{class} metaobject slots +mentioned for @code{compute-cpl} have been initialized, plus the +following: @code{cpl}, @code{redefined} (@code{#f}), @code{environment}. +The value returned by @code{compute-slots} will be stored in the +@code{slots} slot. + +@item +@code{compute-get-n-set @var{class} @var{slot-def}} (generic) + +@code{initialize} calls @code{compute-get-n-set} for each slot computed +by @code{compute-slots}. The applied method should compute and return a +pair of closures that, respectively, get and set the value of the specified +slot. The get closure should have arity 1 and expect a single argument +that is the instance whose slot value is to be retrieved. The set closure +should have arity 2 and expect two arguments, where the first argument is +the instance whose slot value is to be set and the second argument is the +new value for that slot. The closures should be returned in a two element +list: @code{(list @var{get} @var{set})}. + +The closures returned by @code{compute-get-n-set} are stored as part of +the value of the @var{class} metaobject's @code{getters-n-setters} slot. +Specifically, the value of this slot is a list with the same number of +elements as there are slots in the class, and each element looks either like + +@example +@code{(@var{slot-name-symbol} @var{init-function} . @var{index})} +@end example + +or like + +@example +@code{(@var{slot-name-symbol} @var{init-function} @var{get} @var{set})} +@end example + +Where the get and set closures are replaced by @var{index}, the slot is +an instance slot and @var{index} is the slot's index in the underlying +structure: GOOPS knows how to get and set the value of such slots and so +does not need specially constructed get and set closures. Otherwise, +@var{get} and @var{set} are the closures returned by @code{compute-get-n-set}. + +The structure of the @code{getters-n-setters} slot value is important when +understanding the next customizable generic functions that @code{initialize} +calls@dots{} + +@item +@code{compute-getter-method @var{class} @var{gns}} (generic) + +@code{initialize} calls @code{compute-getter-method} for each of the class's +slots (as determined by @code{compute-slots}) that includes a +@code{#:getter} or @code{#:accessor} slot option. @var{gns} is the +element of the @var{class} metaobject's @code{getters-n-setters} slot that +specifies how the slot in question is referenced and set, as described +above under @code{compute-get-n-set}. The applied method should create +and return a method that is specialized for instances of type @var{class} +and uses the get closure to retrieve the slot's value. [ *fixme Need +to insert something here about checking that the value is not unbound. ] +@code{initialize} uses @code{add-method!} to add the returned method to +the generic function named by the slot definition's @code{#:getter} or +@code{#:accessor} option. + +@item +@code{compute-setter-method @var{class} @var{gns}} (generic) + +@code{compute-setter-method} is invoked with the same arguments as +@code{compute-getter-method}, for each of the class's slots that includes +a @code{#:setter} or @code{#:accessor} slot option. The applied method +should create and return a method that is specialized for instances of +type @var{class} and uses the set closure to set the slot's value. +@code{initialize} then uses @code{add-method!} to add the returned method +to the generic function named by the slot definition's @code{#:setter} +or @code{#:accessor} option. +@end itemize + +@node Instance Creation +@subsection Instance Creation + +@code{make <class> . @var{initargs}} (method) + +@itemize @bullet +@item +@code{allocate-instance @var{class} @var{initargs}} (generic) + +The applied @code{allocate-instance} method should allocate storage for +a new instance of class @var{class} and return the uninitialized instance. + +@item +@code{initialize @var{instance} @var{initargs}} (generic) + +@var{instance} is the uninitialized instance returned by +@code{allocate-instance}. The applied method should initialize the new +instance in whatever sense is appropriate for its class. The method's +return value is ignored. +@end itemize + +@node Class Redefinition +@subsection Class Redefinition + +The default @code{class-redefinition} method, specialized for classes +with the default metaclass @code{<class>}, has the following internal +protocol. + +@code{class-redefinition (@var{old <class>}) (@var{new <class>})} +(method) + +@itemize @bullet +@item +@code{remove-class-accessors! @var{old}} (generic) + +@item +@code{update-direct-method! @var{method} @var{old} @var{new}} (generic) + +@item +@code{update-direct-subclass! @var{subclass} @var{old} @var{new}} (generic) +@end itemize + +This protocol cleans up things that the definition of the old class +once changed and modifies things to work with the new class. + +The default @code{remove-class-accessors!} method removes the +accessor methods of the old class from all classes which they +specialize. + +The default @code{update-direct-method!} method substitutes the new +class for the old in all methods specialized to the old class. + +The default @code{update-direct-subclass!} method invokes +@code{class-redefinition} recursively to handle the redefinition of +subclasses. + +When a class is redefined, any existing instance of the redefined class +will be modified for the new class definition before the next time that +any of the instance's slot is referenced or set. GOOPS modifies each +instance by calling the generic function @code{change-class}. + +The default @code{change-class} method copies slot values from the old +to the modified instance, and initializes new slots, as described in +@ref{Changing the Class of an Instance}. After doing so, it makes a +generic function invocation that can be used to customize the instance +update algorithm. + +@code{change-class (@var{old-instance <object>}) (@var{new <class>})} (method) + +@itemize @bullet +@item +@code{update-instance-for-different-class @var{old-instance} @var{new-instance}} (generic) + +@code{change-class} invokes @code{update-instance-for-different-class} +as the last thing that it does before returning. The applied method can +make any further adjustments to @var{new-instance} that are required to +complete or modify the change of class. The return value from the +applied method is ignored. + +The default @code{update-instance-for-different-class} method does +nothing. +@end itemize + +@node Method Definition +@subsection Method Definition + +@code{define-method} (syntax) + +@itemize @bullet +@item +@code{add-method! @var{target} @var{method}} (generic) + +@code{define-method} invokes the @code{add-method!} generic function to +handle adding the new method to a variety of possible targets. GOOPS +includes methods to handle @var{target} as + +@itemize @bullet +@item +a generic function (the most common case) + +@item +a procedure + +@item +a primitive generic (@pxref{Extending Guiles Primitives}) +@end itemize + +By defining further methods for @code{add-method!}, you can +theoretically handle adding methods to further types of target. +@end itemize + +@node Generic Function Invocation +@subsection Generic Function Invocation + +[ *fixme* Description required here. ] + +@code{apply-generic} + +@itemize @bullet +@item +@code{no-method} + +@item +@code{compute-applicable-methods} + +@item +@code{sort-applicable-methods} + +@item +@code{apply-methods} + +@item +@code{no-applicable-method} +@end itemize + +@code{sort-applicable-methods} + +@itemize @bullet +@item +@code{method-more-specific?} +@end itemize + +@code{apply-methods} + +@itemize @bullet +@item +@code{apply-method} +@end itemize + +@code{next-method} + +@itemize @bullet +@item +@code{no-next-method} +@end itemize |