A new class is defined with the
(define-class class (superclass …) slot-description … class-option …)
class is the class being defined. The list of superclasses specifies which existing classes, if any, to inherit slots and properties from. Slots hold per-instance30 data, for instances of that class — like “fields” or “member variables” in other object oriented systems. Each slot-description gives the name of a slot and optionally some “properties” of this slot; for example its initial value, the name of a function which will access its value, and so on. Class options, slot descriptions and inheritance are discussed more below.
Define a class called name that inherits from supers, with direct slots defined by slot-definitions and class-options. The newly created class is bound to the variable name name in the current environment.
Each slot-definition is either a symbol that names the slot or a list,
(slot-name-symbol . slot-options)
where slot-name-symbol is a symbol and slot-options is a list with an even number of elements. The even-numbered elements of slot-options (counting from zero) are slot option keywords; the odd-numbered elements are the corresponding values for those keywords.
Each class-option is an option keyword and corresponding value.
As an example, let us define a type for representing a complex number in terms of two real numbers.31 This can be done with the following class definition:
(define-class <my-complex> (<number>) r i)
This binds the variable
<my-complex> to a new class whose
instances will contain two slots. These slots are called
i and will hold the real and imaginary parts of a complex
number. Note that this class inherits from
<number>, which is a
Slot options are described in the next section. The possible class options are as follows.
#:metaclass class option specifies the metaclass of the class
being defined. metaclass must be a class that inherits from
<class>. For the use of metaclasses, see Metaobjects and the Metaobject Protocol and Metaclasses.
#:metaclass option is absent, GOOPS reuses or constructs a
metaclass for the new class by calling
#: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.
#:name option is absent, GOOPS uses the first argument to
define-class as the class name.
Usually — but
see also the
#:allocation slot option.
Of course Guile already
provides complex numbers, and
<complex> is in fact a predefined
class in GOOPS; but the definition here is still useful as an
<number> is the direct superclass of
the predefined class
<complex> is the
<real> is the superclass of