4.1 Class Definition

define-class (syntax)

• class (syntax)
  • make-class (procedure)
    • make metaclass … (generic)

      metaclass is the metaclass of the class being defined, either taken from the #:metaclass class option or computed by ensure-metaclass. The applied method must create and return the fully initialized class metaobject for the new class definition.

• class-redefinition old-class new-class (generic)

  define-class calls class-redefinition if the variable specified by its first argument already held a GOOPS class definition. old-class and 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 define-class's variable. The default class redefinition protocol is described in Class Redefinition.

The (make metaclass …) invocation above will create an class metaobject with metaclass metaclass. By default, this metaobject will be initialized by the initialize method that is specialized for instances of type <class>.

initialize <class> initargs (method)

• compute-cpl class (generic)

  The applied method should compute and return the class precedence list for class as a list of class metaobjects. When compute-cpl is called, the following class metaobject slots have all been initialized: name, direct-supers, direct-slots, direct-subclasses (empty), direct-methods. The value returned by compute-cpl will be stored in the cpl slot.

• compute-slots class (generic)

  The applied method should compute and return the slots (union of direct and inherited) for class as a list of slot definitions. When compute-slots is called, all the class metaobject slots mentioned for compute-cpl have been initialized, plus the following: cpl, redefined (#f), environment. The value returned by compute-slots will be stored in the slots slot.

• compute-get-n-set class slot-def (generic)

  initialize calls compute-get-n-set for each slot computed by 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: (list get set).

  The closures returned by compute-get-n-set are stored as part of the value of the class metaobject's 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

  (slot-name-symbol init-function . index)

  or like

  (slot-name-symbol init-function get set)

  Where the get and set closures are replaced by index, the slot is an instance slot and 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, get and set are the closures returned by compute-get-n-set.

  The structure of the getters-n-setters slot value is important when understanding the next customizable generic functions that initialize calls…

• compute-getter-method class gns (generic)

  initialize calls compute-getter-method for each of the class's slots (as determined by compute-slots) that includes a #:getter or #:accessor slot option. gns is the element of the class metaobject's getters-n-setters slot that specifies how the slot in question is referenced and set, as described above under compute-get-n-set. The applied method should create and return a method that is specialized for instances of type 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. ] initialize uses add-method! to add the returned method to the generic function named by the slot definition's #:getter or #:accessor option.

• compute-setter-method class gns (generic)

  compute-setter-method is invoked with the same arguments as compute-getter-method, for each of the class's slots that includes a #:setter or #:accessor slot option. The applied method should create and return a method that is specialized for instances of type class and uses the set closure to set the slot's value. initialize then uses add-method! to add the returned method to the generic function named by the slot definition's #:setter or #:accessor option.