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Continue separating simple and more complex GOOPS doc

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* doc/ref/goops.texi (Defining New Classes): Move `Class Definition
  Internals' and `Customizing Class Definition' notes to the
  metaobject protocol section.
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Neil Jerram 2010-10-02 15:36:46 +01:00
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doc/ref/goops.texi
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@ -255,8 +255,6 @@ subsections.
@menu @menu
* Slot Options:: * Slot Options::
* Class Options:: * Class Options::
* Class Definition Internals::
* Customizing Class Definition::
* STKlos Compatibility:: * STKlos Compatibility::
@end menu @end menu
@ -495,255 +493,6 @@ If the @code{#:name} option is absent, GOOPS uses the first argument to
@code{define-class} as the class name. @code{define-class} as the class name.
@end deffn @end deffn
@node Class Definition Internals
@subsection 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{Defining New Classes,, 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{Defining New
Classes,, 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
@subsection 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 @node STKlos Compatibility
@subsection STKlos Compatibility @subsection STKlos Compatibility
@ -2146,6 +1895,8 @@ GOOPS' power, by customizing the behaviour of GOOPS itself.
* Terminology:: * Terminology::
* MOP Specification:: * MOP Specification::
* Class Definition:: * Class Definition::
* Class Definition Internals::
* Customizing Class Definition::
* Instance Creation:: * Instance Creation::
* Class Redefinition:: * Class Redefinition::
* Method Definition:: * Method Definition::
@ -2607,6 +2358,255 @@ to the generic function named by the slot definition's @code{#:setter}
or @code{#:accessor} option. or @code{#:accessor} option.
@end itemize @end itemize
@node Class Definition Internals
@subsection Class Definition Internals
@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{Defining New Classes,, define-class}.
@end deffn
@noindent @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{Defining New
Classes,, 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
@noindent @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
@subsection 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 Instance Creation @node Instance Creation
@subsection Instance Creation @subsection Instance Creation