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This manual documents SOS 1.9.
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Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License.”
SOS is a Scheme object system derived from Tiny CLOS 1, which in turn was loosely derived from CLOS, the Common Lisp Object System. Its basic design and philosophy is closely related to Tiny CLOS, but there are differences in naming and interface.
This document is a reference manual, and as such does not attempt to teach the reader about object-oriented programming. It is assumed that you already have a passing familiarity with CLOS and with Scheme.
In the procedure descriptions that follow, certain argument names imply restrictions on the corresponding argument. Here is a table of those names. The parenthesised name in each entry is the name of the predicate procedure that the argument must satisfy.
The argument must be a class (class?).
The argument must be an instance (instance?).
The argument must be a symbol (symbol?); sometimes this is also
allowed to be #f (false?).
The argument must be a generic procedure
(generic-procedure?).
The argument must be a method (method?).
The argument must be a method specializer (specializer?).
The argument must be a procedure (procedure?).
The argument must be a slot descriptor (slot-descriptor?).
Next: Instances, Previous: Introduction, Up: SOS [Contents][Index]
A class is an object that determines the structure and behavior of
a set of other objects, which are called its instances. However,
in this document, the word instance usually means an instance of
the class <instance>.
A class can inherit structure and behavior from other classes. A class whose definition refers to other classes for the purpose of inheriting from them is said to be a subclass of each of those classes. The classes that are designated for purposes of inheritance are said to be superclasses of the inheriting class.
A class can have a name. The procedure class-name takes a
class object and returns its name. The name of an anonymous class is
#f.
A class C_1 is a direct superclass of a class C_2 if C_2 explicitly designates C_1 as a superclass in its definition. In this case, C_2 is a direct subclass of C_1. A class C_n is a superclass of a class C_1 if there exists a series of classes C_2, …, C_n-1 such that C_i+1 is a direct superclass of C_i for all i between 1 and n. In this case, C_1 is a subclass of C_n. A class is considered neither a superclass nor a subclass of itself. That is, if C_1 is a superclass of C_2, then C_1 is different from C_2. The set of classes consisting of some given class C along with all of its superclasses is called “C and its superclasses.”
Each class has a class precedence list, which is a total ordering on the set of the given class and its superclasses. The total ordering is expressed as a list ordered from the most specific to the least specific. The class precedence list is used in several ways. In general, more specific classes can shadow, or override, features that would otherwise be inherited from less specific classes. The method selection and combination process uses the class precedence list to order methods from most specific to least specific.
When a class is defined, the order in which its direct superclasses are mentioned in the defining form is important. Each class has a local precedence order, which is a list consisting of the class followed by its direct superclasses in the order mentioned in the defining form.
A class precedence list is always consistent with the local precedence order of each class in the list. The classes in each local precedence order appear within the class precedence list in the same order. If the local precedence orders are inconsistent with each other, no class precedence list can be constructed, and an error is signalled.
Classes are organized into a directed acyclic graph. There are
two distinguished classes, named <object> and <instance>.
The class named <object> has no superclasses. It is a superclass
of every class except itself. The class named <instance> is a
direct subclass of <object> and is the base class for
instance objects. Instances are special because SOS has
efficient mechanisms for dispatching on them and for accessing their
slots.
Next: Predefined Classes, Previous: Classes, Up: Classes [Contents][Index]
The procedures in this section may be used to construct and inspect classes.
Creates and returns a new class object.
Name is used for debugging: it is a symbol that appears in the
printed representation of the class and has no role in the semantics of
the class. Alternatively, name may be #f to indicate that
the class is anonymous.
Direct-superclasses must be a list of class objects. The new
class inherits both methods and slots from the classes in this list.
Specifying the empty list for direct-superclasses is equivalent to
specifying (list <instance>).
Direct-slots describes additional slots that instances of this class will have. It is a list, each element of which must have one of the following forms:
name (name . plist)
where name is a symbol, and plist is a property list. The first of these two forms is equivalent to the second with an empty plist.
Each of the elements of direct-slots defines one slot named name. Plist is used to describe additional properties of that slot. The following properties are recognized:
initial-valueThis property specifies the default initial value for the slot, i.e.
the value stored in the slot when an instance is created and no value is
explicitly specified by the instance constructor. If neither the
initial-value nor the initializer property is specified,
the slot has no default initial value.
initializerThis property specifies a procedure of no arguments that is called by an
instance constructor whenever an instance containing this slot is
created. The value returned by the initializer procedure is the
initial value of the slot.
accessorThis property specifies a generic procedure; make-class will add
an accessor method for this slot to the procedure. See Slots.
modifierThis property specifies a generic procedure; make-class will add
a modifier method for this slot to the procedure. See Slots.
initpredThis property specifies a generic procedure; make-class will add
an “initialized?” predicate method for this slot to the procedure.
See Slots.
Slot properties are combined in slightly complicated ways.
initial-value and
initializer for a slot in a given call to make-class; at
most one of these properties may be given.
initial-value and
initializer shadow one another. In other words, regardless of
the inherited slot properties, the resulting slot has at most one of
these two properties.
Examples of make-class:
(define <cell> (make-class '<cell> '() '()))
(define-generic cell-name (cell)) (define-generic cell-width (cell)) (define-generic cell-height (cell)) (define-generic cell-components (cell)) (define-generic set-cell-components! (cell components))
(define <contact>
(make-class '<contact>
(list <cell>)
`((name accessor ,cell-name))))
(define <compound-cell>
(make-class '<compound-cell>
(list <cell>)
`((width accessor ,cell-width)
(height accessor ,cell-height)
(components accessor ,cell-components
modifier ,set-cell-components!
initial-value ()))))
Define name to be a class. In its basic form, define-class
might have been defined by
(define-syntax define-class
(syntax-rules ()
((define-class name (class ...) slot ...)
(define name
(make-class (quote name)
(list class ...)
(quote (slot ...)))))))
Note that slot properties are handled specially by define-class.
If a direct-slot specifies a slot properties property list, the
keys of the property list (i.e. the even-numbered elements) are not
evaluated, while the datums of the property list are evaluated.
The expansion above does not show the proper treatment of slot
properties.
In addition to the slot properties recognized by make-class,
define-class recognizes a special slot property, called
define. The define property specifies that some or all of
the slot accessors should be defined here; that is, generic procedures
should be constructed and bound to variables, and then the accessor
methods added to them.
The argument to the define property is a list containing any
combination of the symbols accessor, modifier, and
initpred. As an abbreviation, the argument may be one of these
symbols by itself, which is equivalent to the list containing that
symbol. Also, the argument may be the symbol standard, which is
equivalent to (accessor modifier).
The argument to define specifies the accessors that will be
defined by this form. The accessors are defined using default names,
unless the names are overridden by the corresponding slot property. The
default names for a class <foo> and a slot bar are
foo-bar, set-foo-bar!, and foo-bar-initialized?,
respectively for the accessor, modifier, and initpred. For example,
(define-class foo () (bar define accessor))
defines an accessor called foo-bar, but
(define-class foo () (bar define accessor accessor foo/bar))
instead defines an accessor called foo/bar. Finally,
(define-class foo () (bar accessor foo/bar))
doesn’t define any accessor, but assumes that foo/bar is a
previously-defined generic procedure and adds an accessor method to it.
define-class permits the specification of class options,
which are options that pertain to the class as a whole. Class options
are specified by overloading name: instead of a symbol, specify a
pair whose CAR is a symbol and whose CDR is an alist. The
following class options are recognized:
(predicate [name]) ¶Specifies that a predicate procedure should be defined for this class.
Name must be either a symbol or #f: a symbol specifies the
name that will be bound to the predicate procedure, and #f
specifies that no predicate procedure should be defined. If name
is omitted, or if no predicate option is specified, a predicate
procedure is defined by appending ? to the name of the class. If
the class name is surrounded by angle brackets, they are stripped off
first. For example, the default predicate name for the class
<foo> is foo?.
(constructor [name] slot-names [n-init-args]) ¶Specifies that a constructor procedure should be defined for this class.
Name must be a symbol, which is the name that will be bound to the
constructor procedure; if omitted, a default name is formed by
prepending make- to the name of the class. If the class name is
surrounded by angle brackets, they are stripped off first. For example,
the default constructor name for the class <foo> is
make-foo.
Slot-names and n-init-args correspond to the arguments of
the respective names accepted by instance-constructor, and can
take any of the allowed forms for those arguments.
(separator string) ¶Specifies how names for slot accessors are constructed. If this option
isn’t given, the name of a slot accessor is formed by concatenating the
name of the class with the name of the slot, with a hyphen between them.
When this option is given, string is used instead of the hyphen.
For example, normally a slot accessor for the slot bar in the
class foo is called foo-bar. A class option
(separator ".") will cause the slot accessor to be called
foo.bar, the modifier to be called set-foo.bar!, and the
initialization predicate to be called foo.bar?.
Examples of define-class (compare these to the similar examples
for make-class):
(define-class <cell> ())
(define-generic cell-name (cell)) (define-generic cell-width (cell)) (define-generic cell-height (cell)) (define-generic cell-components (cell)) (define-generic set-cell-components! (cell components))
(define-class (<contact> (constructor (name) no-init)) (<cell>) (name accessor cell-name))
(define-class (<compound-cell> (constructor ())) (<cell>)
(width accessor cell-width)
(height accessor cell-height)
(components accessor cell-components
modifier set-cell-components!
initial-value '()))
This convenience procedure makes a subclass that defines no new slots, and that inherits from the given superclasses. It is equivalent to the following
(make-class (class-name superclass1)
(list superclass1 superclass2 …)
'())
Returns #t if object is a class, otherwise returns
#f.
Returns #t if class is a subclass of specializer,
otherwise returns #f. If specializer is a class, the
result follows from the above definition of subclass, except that a
class is a subclass of itself. If specializer is a record type,
it is equivalent to having used the record-type-class of the
record type. Finally, if specializer is a union specializer,
subclass? is true if class is a subclass of one or more of
the component classes of specializer.
Returns the class of object. Object may be any Scheme
object; if object is known to be an instance,
instance-class is faster than object-class.
Returns the name of class. This is the name argument passed
to make-class when class was created.
Returns a list of the direct superclasses of class. If a
non-empty direct-superclasses argument was passed to
make-class when class was created, this list is
equal? to that argument. The returned value must not be
modified.
Returns a list of symbols that are the names of the direct slots of
class. This list contains only those slots that were defined in
the call to make-class that created class; it does not
contain slots that were inherited. The returned value must not be
modified.
Returns a list of the superclasses of class. The order of this
list is significant: it is the method resolution order. This list will
always have class as its first element, and <object> as its
last element. The returned value must not be modified.
Next: Record Classes, Previous: Class Datatype, Up: Classes [Contents][Index]
SOS provides a rich set of predefined classes that can be used to specialize methods to any of Scheme’s built-in datatypes.
This is the class of all Scheme objects. It has no direct superclasses, and all other classes are subclasses of this class.
This is the class of instances. It is a direct subclass of
<object>. The members of this class are the objects that satisfy
the predicate instance?.
These are the classes of their respective Scheme objects. They are all
direct subclasses of <object>. The members of each class are the
objects that satisfy the corresponding predicate; for example, the
members of <procedure> are the objects that satisfy
procedure?.
This is the class of generic procedure instances. It is a direct
subclass of <procedure>.
This is the class of method objects. It is a direct subclass of
<instance>.
These classes specify additional method objects with special properties.
Each class is a subclass of <method>.
The following are the classes of Scheme numbers. Note that
object-class will never return one of these classes; instead it
returns an implementation-specific class that is associated with a
particular numeric representation. The implementation-specific class is
a subclass of one or more of these implementation-independent classes,
so you should use these classes for specialization.
These are the classes of the Scheme numeric tower. <number> is a
direct subclass of <math-object>, <complex> is a direct
subclass of <number>, <real> is a direct subclass of
<complex>, etc.
These are the classes of exact numbers. <exact> is a direct
subclass of <number>, <exact-complex> is a direct
subclass of <exact> and <complex>, and in general, each is
a direct subclass of preceding class and of the class without the
exact- prefix.
These are the classes of inexact numbers. <inexact> is a direct
subclass of <number>, <inexact-complex> is a direct
subclass of <inexact> and <complex>, and in general, each
is a direct subclass of preceding class and of the class without the
inexact- prefix.
Next: Specializers, Previous: Predefined Classes, Up: Classes [Contents][Index]
SOS allows generic procedures to discriminate on record types.
This means that a record structure defined by means of
make-record-type or define-structure can be passed as an
argument to a generic procedure, and the generic procedure can use the
record’s type to determine which method to be invoked.2
In order to support this, SOS accepts record type descriptors in all contexts that accept classes. Additionally, every record type descriptor has an associated SOS class; either the class or the record type can be used with equivalent results.
Record-type must be a record type descriptor (in other words, it
must satisfy the predicate record-type?). Returns the class
associated with record-type.
Record must be a record (in other words, it must satisfy the
predicate record?). Returns the class associated with
record. This is equivalent to
(record-type-class (record-type-descriptor record))
Previous: Record Classes, Up: Classes [Contents][Index]
A specializer is a generalization of a class. A specializer is any one of the following:
A specializer may be used in many contexts where a class is required,
specifically, as a method specializer (hence the name), as the second
argument to subclass?, and elsewhere.
Returns #t if object is a specializer, otherwise returns
#f.
Returns a list of the classes in specializer. If specializer is a class, the result is a list of that class. If specializer is a record type, the result is a list of the record type’s class. If specializer is a union specializer, the result is a list of the component classes of the specializer.
Returns #t if specializer1 and specializer2 are
equivalent, otherwise returns #f. Two specializers are
equivalent if the lists returned by specializer-classes contain
the same elements.
Returns a union specializer consisting of the union of the classes of the arguments. This is equivalent to converting all of the specializer arguments to sets of classes, then taking the union of those sets.
Returns #t if object is a union specializer, otherwise
returns #f.
Returns #t if object is a list of specializers, otherwise
returns #f.
Specializers1 and specializers2 must be lists of
specializers. Returns #t if specializers1 and
specializers2 are equivalent, otherwise returns #f.
Two specializers lists are equivalent if each of their corresponding
elements is equivalent.
An instance is a compound data structure much like a record, except that it is defined by a class rather than a record type descriptor. Instances are more powerful than records, because their representation is designed to support inheritance, while the representation of records is not.
Creates and returns a procedure that, when called, will create and return a newly allocated instance of class.
Class must be a subclass of <instance>. Slot-names
must be a list of symbols, each of which must be the name of a slot in
class. N-init-args will be described below.
In its basic operation, instance-constructor works much like
record-constructor: the slot-names argument specifies how
many arguments the returned constructor accepts, and each of those
arguments is stored in the corresponding slot of the returned instance.
Any slots that are not specified in slot-names are given their
initial values, as specified by the initial-value or
initializer slot properties; otherwise they are left
uninitialized.
After the new instance is created and its slots filled in, but before it
is returned, it is passed to the generic procedure
initialize-instance. Normally, initialize-instance does
nothing, but because it is always called, the programmer can add methods
to it to specify an initialization that is to be performed on every
instance of the class.
By default, initialize-instance is called with one argument, the
newly created instance. However, the optional argument
n-init-args can be used to specify additional arguments that will
be passed to initialize-instance.
The way this works is that the returned constructor procedure accepts
additional arguments after the specified number of slot values, and
passes these extra arguments to initialize-instance. When
n-init-args is not supplied or is #t, any number of extra
arguments are accepted and passed along. When n-init-args is an
exact non-negative integer, exactly that number of extra arguments must
be supplied when the constructor is called. Finally, if
n-init-args is the symbol no-initialize-instance, then the
constructor accepts no extra arguments and does not call
initialize-instance at all; this is desirable when
initialize-instance is not needed, because it makes the
constructor significantly faster.
For notational convenience, n-init-args may take two other forms.
First, it may be a list of symbols, which is equivalent to the integer
that is the length of the list. Second, it may be the symbol
no-init, which is an abbreviation for
no-initialize-instance.
Note that the default method on initialize-instance accepts no
extra arguments and does nothing.
Examples of instance-constructor:
(define-class <simple-reference> (<reference>) (from accessor reference-from) (to accessor reference-to) (cx accessor reference-cx) (cy accessor reference-cy))
(define make-simple-reference
(instance-constructor <simple-reference>
'(from to cx cy)
'no-init))
(define-class <simple-wirenet> (<wirenet>)
(cell accessor wirenet-cell)
(wires accessor wirenet-wires
modifier set-wirenet-wires!
initial-value '()))
(define make-simple-wirenet (instance-constructor <simple-wirenet> '(cell)))
Returns #t if object is an instance, otherwise returns
#f.
Returns the class of instance. This is faster than
object-class, but it works only for instances, and not for other
objects.
Returns #t if object is a general instance of
specializer, otherwise returns #f. This is equivalent to
(subclass? (object-class object) specializer)
Returns a predicate procedure for specializer. The returned
procedure accepts one argument and returns #t if the argument is
an instance of specializer and #f otherwise.
Next: Generic Procedures, Previous: Instances, Up: SOS [Contents][Index]
An instance has zero or more named slots; the name of a slot is a symbol. The slots of an instance are determined by its class.
Each slot can hold one value. When a slot does not have a value, the
slot is said to be uninitialized. The default initial value for a
slot is defined by the initial-value and initializer slot
properties.
A slot is said to be accessible in an instance of a class if the
slot is defined by the class of the instance or is inherited from a
superclass of that class. At most one slot of a given name can be
accessible in an instance. Slots are accessed by means of slot-access
methods (usually generated by make-class).
Next: Slot Access Methods, Previous: Slots, Up: Slots [Contents][Index]
Slots are represented by slot descriptors, which are data structures providing information about the slots, such as their name. Slot descriptors are stored inside of classes, and may be retrieved from there and subsequently inspected.
Returns a list of the slot descriptors for class. This contains all slots for class, both direct slots and inherited slots. The returned value must not be modified.
Returns the slot descriptor for the slot named name in
class. If there is no such slot: if error? is #f,
returns #f, otherwise signals an error of type
condition-type:no-such-slot.
Returns #t if object is a slot descriptor, otherwise
returns #f.
Returns the name of slot.
Returns the class of slot. This is the class with which
slot is associated. This is not necessarily the class that
defines slot; it could also be a subclass of that class. If the
slot was returned from class-slots or class-slot, then
this class is the argument passed to that procedure.
Returns an alist of the properties of slot. This list must not be modified.
If slot has a property named name, it is returned; otherwise default is returned.
Returns #t if slot has an initial value, and #f
otherwise. The initial value is specified by the initial-value
slot property when a class is made.
Returns the initial value for slot, if it has one; otherwise it
returns an unspecified value. The initial value is specified by the
initial-value slot property when a class is made.
Returns the initializer for slot; the initializer is specified by
the initializer slot property when a class is made. This is a
procedure of no arguments that is called to produce an initial value for
slot. The result may also be #f meaning that the slot has
no initializer.
Next: Slot Access Constructors, Previous: Slot Descriptors, Up: Slots [Contents][Index]
The procedure make-class provides slot properties that generate
methods to read and write slots. If an accessor is requested, a
method is automatically generated for reading the value of the slot. If
a modifier is requested, a method is automatically generated for
storing a value into the slot. When an accessor or modifier is
specified for a slot, the generic procedure to which the generated
method belongs is directly specified. The procedure specified for the
accessor takes one argument, the instance. The procedure specified for
the modifier takes two arguments, the instance and the new value, in
that order.
All of the procedures described here signal an error of type
condition-type:no-such-slot if the given class or object does not
have a slot of the given name.
Slot-access methods can be generated by the procedures
slot-accessor-method, slot-modifier-method, and
slot-initpred-method. These methods may be added to a generic
procedure by passing them as arguments to add-method. The
methods generated by these procedures are equivalent to those generated
by the slot properties in make-class.
Returns an accessor method for the slot name in class. The
returned method has one required argument, an instance of class,
and the specializer for that argument is class. When invoked, the
method returns the contents of the slot specified by name in the
instance; if the slot is uninitialized, an error of type
condition-type:uninitialized-slot is signalled.
(define-generic get-bar (object)) (add-method get-bar (slot-accessor-method <foo> 'bar))
Returns a modifier method for the slot name in class. The returned method has two required arguments, an instance of class and an object. The specializer for the first argument is class and the second argument is not specialized. When invoked, the method stores the second argument in the slot specified by name in the instance.
(define-generic set-bar! (object bar)) (add-method set-bar! (slot-modifier-method <foo> 'bar))
Returns an “initialized?” predicate method for the slot name
in class. The returned method has one required argument, an
instance of class, and the specializer for that argument is
class. When invoked, the method returns #t if the slot
specified by name is initialized in the instance; otherwise it
returns #f.
(define-generic has-bar? (object)) (add-method has-bar? (slot-initpred-method <foo> 'bar))
Next: Slot Access Procedures, Previous: Slot Access Methods, Up: Slots [Contents][Index]
For convenience, and for consistency with the record-accessor procedures
record-accessor and record-modifier, each of the above
method-generating procedures has a corresponding accessor-generator.
Each of these procedures creates a generic procedure, adds an
appropriate method to it by calling the corresponding method-generating
procedure, and returns the generic procedure. Thus, for example, the
following are equivalent:
(slot-accessor <foo> 'bar) (let ((g (make-generic-procedure 1))) (add-method g (slot-accessor-method <foo> 'bar)) g)
Returns a generic procedure of one argument that is an accessor for the
slot name in class. The argument to the returned procedure
must be an instance of class. When the procedure is called, it
returns the contents of the slot name in that instance; if the
slot is uninitialized, an error of type
condition-type:uninitialized-slot is signalled.
Returns a generic procedure of two arguments that is a modifier for the slot name in class. The first argument to the returned procedure must be an instance of class, and the second argument may be any object. When the procedure is called, it modifies the slot name in the instance to contain the second argument.
Returns a generic procedure of one argument that is an
“initialized?” predicate for the slot name in class.
The argument to the returned procedure must be an instance of
class. When the procedure is called, it returns #t if the
slot name in that instance is initialized, otherwise it returns
#f.
Previous: Slot Access Constructors, Up: Slots [Contents][Index]
Finally, there is another set of three procedures, which access the contents of a slot directly, given an instance and a slot name. These procedures are very slow by comparison with the above.
However, note the following. You can use these procedures in the body
of a define-method special form in an efficient way. If the
define-method specifies the correct number of arguments, the body
of the form contains a call to one of these procedures and nothing else,
and the specified slot name is quoted, the form is rewritten during
macro-expansion time as a call to the corresponding method-generating
procedure. For example, the following are equivalent:
(define-method p ((v <foo>))
(slot-value v 'bar))
(add-method p
(slot-accessor-method <foo> 'bar))
Returns the contents of the slot name in instance; if the
slot is uninitialized, an error of type
condition-type:uninitialized-slot is signalled.
Modifies the slot name in instance to contain object.
Returns #t if the slot name in instance is
initialized, otherwise returns #f.
Like an ordinary Scheme procedure, a generic procedure takes arguments, performs a series of operations, and perhaps returns useful values. An ordinary procedure has a single body of code that is always executed when the procedure is called. A generic procedure has a set of multiple bodies of code, called methods, from which a subset is selected for execution. The selected bodies of code and the manner of their combination are determined by the classes of one or more of the arguments to the generic procedure.
Ordinary procedures and generic procedures are called with identical procedure-call syntax.
Generic procedures are true procedures that can be passed as arguments,
returned as values, and otherwise used in all the ways an ordinary
procedure may be used. In particular, generic procedures satisfy the
predicate procedure?.
Next: Method Storage, Previous: Generic Procedures, Up: Generic Procedures [Contents][Index]
The following definitions are used to construct and inspect generic procedures.
Creates and returns a new generic procedure. The generic procedure requires arity arguments.
Arity may take one of the following forms. An exact positive
integer specifies that the procedure will accept exactly that number of
arguments. A pair of two exact positive integers specifies inclusive
lower and upper bounds, respectively, on the number of arguments
accepted; the CDR may be #f indicating no upper bound.
Name is used for debugging: it is a symbol that has no role in the
semantics of the generic procedure. Name may be #f to
indicate that the generic procedure is anonymous. If name is not
specified, it defaults to #f.
Examples:
(define foo-bar (make-generic-procedure 2)) (define foo-baz (make-generic-procedure '(1 . 2) 'foo-baz)) (define foo-mum (make-generic-procedure '(1 . #f)))
Defines name to be a generic procedure. Lambda-list is an
ordinary parameter list, which is exactly like the parameter list in a
lambda special form. This expands into
(define name
(make-generic-procedure arity
(quote name)))
where arity is determined from lambda-list.
Examples (compare to examples of make-generic-procedure):
(define-generic foo-bar (x y)) (define-generic foo-baz (x #!optional y)) (define-generic foo-mum (x . y))
Returns #t if object is a generic procedure, otherwise
returns #f. Note that every generic procedure satisfies the
predicate procedure?.
Returns the arity of generic-procedure, as specified in the call
to make-generic-procedure. The returned arity must not be
modified.
Returns the name of generic-procedure, as specified in the call to
make-generic-procedure.
Next: Effective Method Procedure, Previous: Generic Procedure Datatype, Up: Generic Procedures [Contents][Index]
Methods are stored in generic procedures. When a generic procedure is
called, it selects a subset of its stored methods (using
method-applicable?), and arranges to invoke one or more of the
methods as necessary. The following definitions provide the means for
adding methods to and removing them from a generic procedure.
Adds method to generic-procedure. If generic-procedure already has a method with the same specializers as method, then the old method is discarded and method is used in its place.
Removes method from generic-procedure. Does nothing if generic-procedure does not contain method.
Adds methods, which must be a list of methods, to
generic-procedure. Equivalent to calling add-method on
each method in methods.
Returns a list of the methods contained in generic-procedure. The returned list must not be modified.
Previous: Method Storage, Up: Generic Procedures [Contents][Index]
When a generic procedure is called, it arranges to invoke a subset of
its methods. This is done by combining the selected methods into
an effective method procedure, or EMP, then tail-recursively
invoking the EMP. compute-effective-method-procedure is the
procedure that is called to select the applicable methods and combine
them into an EMP.
Collects the applicable methods of generic-procedure by calling
method-applicable? on each method and on classes. Combines
the resulting methods together into an effective method procedure, and
returns that EMP.
This procedure is like compute-effective-method-procedure, except
that it returns the result as a method whose specializers are
classes.
compute-method is equivalent to
(make-method classes
(compute-effective-method-procedure generic-procedure
classes))
Next: Printing, Previous: Generic Procedures, Up: SOS [Contents][Index]
A method contains a method procedure and a sequence of parameter specializers that specify when the given method is applicable.
A method is not a procedure and cannot be invoked as a procedure. Methods are invoked by the effective method procedure when a generic procedure is called.
Next: Method Syntax, Previous: Methods, Up: Methods [Contents][Index]
The following procedures are used to construct and inspect methods.
Creates and returns a new method. Note that specializers may have
fewer elements than the number of required parameters in
procedure; the trailing parameters are considered to be
specialized by <object>.
After the returned method is stored in a generic procedure, Procedure is called by the effective method procedure of the generic procedure when the generic procedure is called with arguments satisfying specializers. In simple cases, when no method combination occurs, procedure is the effective method procedure.
Returns #t iff object is a method, otherwise returns
#f.
Returns the specializers of method. This list must not be modified.
Returns the procedure of method.
This predicate is used to determine the applicability of method. When a method is contained in a generic procedure, and the procedure is applied to some arguments, the method is applicable if each argument is an instance of the corresponding method specializer, or equivalently, if each argument’s class is a subclass of the corresponding method specializer.
method-applicable? determines whether method would be
applicable if the given arguments had the classes specified by
classes. It returns #t if each element of classes is
a subclass of the corresponding specializer of method, and
#f otherwise.
Next: Chained Methods, Previous: Method Datatype, Up: Methods [Contents][Index]
The following syntactic form greatly simplifies the definition of methods, and of adding them to generic procedures.
Defines a method of generic-procedure. Lambda-list is like
the parameter list of a lambda special form, except that the
required parameters may have associated specializers. A parameter with
an associated specializer is written as a list of two elements: the
first element is the parameter’s name, and the second element is an
expression that evaluates to a class.
Lambda-list must contain at least one required parameter, and at least one required parameter must be specialized.
A define-method special form expands into the following:
(add-method generic-procedure
(make-method (list specializer …)
(lambda (call-next-method . stripped-lambda-list)
body …)))
where stripped-lambda-list is lambda-list with the
specialized parameters replaced by their names, and the
specializers are the corresponding expressions from the
specialized parameters. If necessary, the specializers are
interspersed with references to <object> in order to make them
occur in the correct position in the sequence.
For example,
(define-method add ((x <integer>) (y <rational>)) ...)
expands into
(add-method add
(make-method (list <integer> <rational>)
(lambda (call-next-method x y) ...)))
Note that the list of specializers passed to make-method will
correspond to the required parameters of the method; the specializer
corresponding to a non-specialized required parameter is
<object>.
Further note that, within the body of a define-method special
form, the free variable call-next-method is bound to a
“call-next-method” procedure (see make-chained-method for
details). If the define-method body refers to this variable, the
defined method is a chained method, otherwise it is an ordinary method.
Next: Computed Methods, Previous: Method Syntax, Up: Methods [Contents][Index]
Sometimes it is useful to have a method that adds functionality to existing methods. Chained methods provide a mechanism to accomplish this. A chained method, when invoked, can call the method that would have been called had this method not been defined: it is passed a procedure that will call the inherited method. The chained method can run arbitrary code both before and after calling the inherited method.
Create and return a chained method. Procedure must be a procedure of one argument that returns a procedure. When the chained method is combined, its procedure will be called with one argument, a “call-next-method” procedure; it must then return another procedure that will be called when the method is invoked. The “call-next-method” procedure may called by the method procedure at any time, which will invoke the next less-specific method. The “call-next-method” procedure must be called with the same number of arguments as the method procedure; normally these are the same arguments, but that is not required.
Returns #t if object is a chained method, otherwise returns
#f. Note that every chained method satisfies method?.
Previous: Chained Methods, Up: Methods [Contents][Index]
A computed method is a powerful mechanism that provides the ability to generate methods “on the fly”. A computed method is like an ordinary method, except that its procedure is called during method combination, and is passed the classes of the arguments in place of the arguments themselves. Based on these classes, the computed method returns an ordinary method, which is combined in the usual way.
Note that computed methods and computed EMPs both satisfy the
predicate method?. They are not really methods in that they
cannot be combined with other methods to form an effective method
procedure; however, they are treated as methods by procedures such as
add-method and method-specializers.
Create and return a computed method. Procedure will be called during method combination with the classes of the generic-procedure arguments as its arguments. It must return one of the following:
make-method or
make-chained-method). The returned method’s specializers must be
restrictions of specializers, i.e. each specializer in the
returned method must be a subclass of the corresponding specializer in
specializers. In the usual case, the returned method’s
specializers are the same as specializers.
make-method on specializers and the returned procedure.
#f, which means that the computed method declines to generate a
method.
Returns #t if object is a computed method, otherwise
returns #f.
A computed EMP takes the computed-method mechanism one step
further. A computed EMP is like a computed method, except that it
returns an effective method procedure rather than a method.
compute-effective-method-procedure tries each of the applicable
computed EMPs, and if exactly one of them returns an EMP, that
is the resulting effective method procedure.
Create and return a computed EMP. Procedure will be called
during method combination with the classes of the generic-procedure
arguments as its arguments. It must return either an EMP or
#f.
Key is an arbitrary object that is used to identify the computed
EMP. The key is used by add-method and
delete-method to decide whether two computed EMPs are
the same; they are the same if their keys are equal?. This
is necessary because a generic procedure may have more than one computed
EMP with the same specializers.
Returns #t if object is a computed EMP, otherwise
returns #f.
Returns the key for computed-emp.
Next: GNU Free Documentation License, Previous: Methods, Up: SOS [Contents][Index]
The following procedures can be used to define a custom printed
representation for an instance. It is highly recommended that instances
be printed by write-instance-helper, as this ensures a uniform
appearance for all objects.
This is called by the runtime system to generate the printed representation of instance. The methods of this procedure should write the representation to port.
This writes a standardized “frame” for a printed representation method. It generates the following output on port:
#[name hash-number…]
where hash-number is the result of calling hash on
instance, and … is the output generated by thunk.
Next: Binding Index, Previous: Printing, Up: SOS [Contents][Index]
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