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
#fto 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-value- This 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-valuenor theinitializerproperty is specified, the slot has no default initial value.initializer- This 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
initializerprocedure is the initial value of the slot.accessor- This property specifies a generic procedure;
make-classwill add an accessor method for this slot to the procedure. See Slots.modifier- This property specifies a generic procedure;
make-classwill add a modifier method for this slot to the procedure. See Slots.initpred- This property specifies a generic procedure;
make-classwill add an “initialized?” predicate method for this slot to the procedure. See Slots.Slot properties are combined in slightly complicated ways.
- It is not allowed to specify both
initial-valueandinitializerfor a slot in a given call tomake-class; at most one of these properties may be given.- If a slot is specified for a given class, and a slot of the same name is inherited from a superclass, then the slot properties for the two slots are combined. Slot properties from the subclass shadow those of the superclass. However, if a superclass has a slot property, and the subclass does not, the property is inherited. The resulting class never has more than one slot of a given name.
- When combining superclass and subclass slots,
initial-valueandinitializershadow 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-classmight 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-classrecognizes a special slot property, calleddefine. Thedefineproperty 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
defineproperty is a list containing any combination of the symbolsaccessor,modifier, andinitpred. 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 symbolstandard, which is equivalent to(accessor modifier).The argument to
definespecifies 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 slotbararefoo-bar,set-foo-bar!, andfoo-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/baris a previously-defined generic procedure and adds an accessor method to it.
define-classpermits 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#fspecifies that no predicate procedure should be defined. If name is omitted, or if nopredicateoption 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>isfoo?.(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>ismake-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.(separatorstring)- 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
barin the classfoois calledfoo-bar. A class option(separator ".")will cause the slot accessor to be calledfoo.bar, the modifier to be calledset-foo.bar!, and the initialization predicate to be calledfoo.bar?.Examples of
define-class(compare these to the similar examples formake-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
#tif 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 therecord-type-classof 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-classis faster thanobject-class.
Returns the name of class. This is the name argument passed to
make-classwhen class was created.
Returns a list of the direct superclasses of class. If a non-empty direct-superclasses argument was passed to
make-classwhen class was created, this list isequal?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-classthat created class; it does not contain slots that were inherited. The returned value must not be modified.