This is goops.info, produced by makeinfo version 4.3 from goops.texi. INFO-DIR-SECTION The Algorithmic Language Scheme START-INFO-DIR-ENTRY * GOOPS: (goops). The GOOPS reference manual. END-INFO-DIR-ENTRY This file documents GOOPS, an object oriented extension for Guile. Copyright (C) 1999, 2000, 2001, 2003 Free Software Foundation Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies.  File: goops.info, Node: Tutorial, Next: Concept Index, Prev: MOP Specification, Up: Top Tutorial ******** This is chapter was originally written by Erick Gallesio as an appendix for the STk reference manual, and subsequently adapted to GOOPS. * Menu: * Copyright:: * Intro:: * Class definition and instantiation:: * Inheritance:: * Generic functions::  File: goops.info, Node: Copyright, Next: Intro, Prev: Tutorial, Up: Tutorial Copyright ========= Original attribution: STk Reference manual (Appendix: An Introduction to STklos) Copyright © 1993-1999 Erick Gallesio - I3S-CNRS/ESSI Permission to use, copy, modify, distribute,and license this software and its documentation for any purpose is hereby granted, provided that existing copyright notices are retained in all copies and that this notice is included verbatim in any distributions. No written agreement, license, or royalty fee is required for any of the authorized uses. This software is provided "AS IS" without express or implied warranty. Adapted for use in Guile with the authors permission  File: goops.info, Node: Intro, Next: Class definition and instantiation, Prev: Copyright, Up: Tutorial Introduction ============ GOOPS is the object oriented extension to Guile. Its implementation is derived from STk-3.99.3 by Erick Gallesio and version 1.3 of the Gregor Kiczales `Tiny-Clos'. It is very close to CLOS, the Common Lisp Object System (`CLtL2') but is adapted for the Scheme language. Briefly stated, the GOOPS extension gives the user a full object oriented system with multiple inheritance and generic functions with multi-method dispatch. Furthermore, the implementation relies on a true meta object protocol, in the spirit of the one defined for CLOS (`Gregor Kiczales: A Metaobject Protocol'). The purpose of this tutorial is to introduce briefly the GOOPS package and in no case will it replace the GOOPS reference manual (which needs to be urgently written now ...). Note that the operations described in this tutorial resides in modules that may need to be imported before being available. The main module is imported by evaluating: (use-modules (oop goops))  File: goops.info, Node: Class definition and instantiation, Next: Inheritance, Prev: Intro, Up: Tutorial Class definition and instantiation ================================== * Menu: * Class definition::  File: goops.info, Node: Class definition, Prev: Class definition and instantiation, Up: Class definition and instantiation Class definition ---------------- A new class is defined with the `define-class'(1) macro. The syntax of `define-class' is close to CLOS `defclass': (define-class CLASS (SUPERCLASS ...) SLOT-DESCRIPTION ... CLASS-OPTION ...) Class options will not be discussed in this tutorial. The list of SUPERCLASSes specifies which classes to inherit properties from CLASS (see *Note Inheritance:: for more details). A SLOT-DESCRIPTION gives the name of a slot and, eventually, some "properties" of this slot (such as its initial value, the function which permit to access its value, ...). Slot descriptions will be discussed in *Note Slot description::. As an example, let us define a type for representation of complex numbers in terms of real numbers. This can be done with the following class definition: (define-class () r i) This binds the variable `'(2) to a new class whose instances contain two slots. These slots are called `r' an `i' and we suppose here that they contain respectively the real part and the imaginary part of a complex number. Note that this class inherits from `' which is a pre-defined class. (`' is the direct super class of the pre-defined class `' which, in turn, is the super class of `' which is the super of `'.)(3). ---------- Footnotes ---------- (1) Don't forget to import the `(oop goops)' module (2) `' is in fact a builtin class in GOOPS. Because of this, GOOPS will create a new class. The old class will still serve as the type for Guile's native complex numbers. (3) With the new definition of `', a `' is not a `' since `' inherits from ` ' rather than `'. In practice, inheritance could be modified _a posteriori_, if needed. However, this necessitates some knowledge of the meta object protocol and it will not be shown in this document  File: goops.info, Node: Inheritance, Next: Generic functions, Prev: Class definition and instantiation, Up: Tutorial Inheritance =========== * Menu: * Class hierarchy and inheritance of slots:: * Instance creation and slot access:: * Slot description:: * Class precedence list::  File: goops.info, Node: Class hierarchy and inheritance of slots, Next: Instance creation and slot access, Prev: Inheritance, Up: Inheritance Class hierarchy and inheritance of slots ---------------------------------------- Inheritance is specified upon class definition. As said in the introduction, GOOPS supports multiple inheritance. Here are some class definitions: (define-class A () a) (define-class B () b) (define-class C () c) (define-class D (A B) d a) (define-class E (A C) e c) (define-class F (D E) f) `A', `B', `C' have a null list of super classes. In this case, the system will replace it by the list which only contains `', the root of all the classes defined by `define-class'. `D', `E', `F' use multiple inheritance: each class inherits from two previously defined classes. Those class definitions define a hierarchy which is shown in Figure 1. In this figure, the class `' is also shown; this class is the super class of all Scheme objects. In particular, `' is the super class of all standard Scheme types. / \\\_____________________ / \\___________ \ / \ \ \ / | \ | / | \ | A B C |\__/__ | | \ / \ / | D E \ / | F | _Fig 1: A class hierarchy_ The set of slots of a given class is calculated by taking the union of the slots of all its super class. For instance, each instance of the class D, defined before will have three slots (`a', `b' and `d'). The slots of a class can be obtained by the `class-slots' primitive. For instance, (class-slots A) => ((a)) (class-slots E) => ((a) (e) (c)) (class-slots F) => ((e) (c) (b) (d) (a) (f)) _Note: _ The order of slots is not significant.  File: goops.info, Node: Instance creation and slot access, Next: Slot description, Prev: Class hierarchy and inheritance of slots, Up: Inheritance Instance creation and slot access --------------------------------- Creation of an instance of a previously defined class can be done with the `make' procedure. This procedure takes one mandatory parameter which is the class of the instance which must be created and a list of optional arguments. Optional arguments are generally used to initialize some slots of the newly created instance. For instance, the following form (define c (make )) will create a new `' object and will bind it to the `c' Scheme variable. Accessing the slots of the new complex number can be done with the `slot-ref' and the `slot-set!' primitives. `Slot-set!' primitive permits to set the value of an object slot and `slot-ref' permits to get its value. (slot-set! c 'r 10) (slot-set! c 'i 3) (slot-ref c 'r) => 10 (slot-ref c 'i) => 3 Using the `describe' function is a simple way to see all the slots of an object at one time: this function prints all the slots of an object on the standard output. First load the module `(oop goops describe)': `(use-modules (oop goops describe))' The expression (describe c) will now print the following information on the standard output: #< 401d8638> is an instance of class Slots are: r = 10 i = 3  File: goops.info, Node: Slot description, Next: Class precedence list, Prev: Instance creation and slot access, Up: Inheritance Slot description ---------------- When specifying a slot, a set of options can be given to the system. Each option is specified with a keyword. The list of authorized keywords is given below: * `#:init-value' permits to supply a default value for the slot. This default value is obtained by evaluating the form given after the `#:init-form' in the global environment, at class definition time. * `#:init-thunk' permits to supply a thunk that will provide a default value for the slot. The value is obtained by evaluating the thunk a instance creation time. * `#:init-keyword' permits to specify the keyword for initializing a slot. The init-keyword may be provided during instance creation (i.e. in the `make' optional parameter list). Specifying such a keyword during instance initialization will supersede the default slot initialization possibly given with `#:init-form'. * `#:getter' permits to supply the name for the slot getter. The name binding is done in the environment of the `define-class' macro. * `#:setter' permits to supply the name for the slot setter. The name binding is done in the environment of the `define-class' macro. * `#:accessor' permits to supply the name for the slot accessor. The name binding is done in the global environment. An accessor permits to get and set the value of a slot. Setting the value of a slot is done with the extended version of `set!'. * `#:allocation' permits to specify how storage for the slot is allocated. Three kinds of allocation are provided. They are described below: - `#:instance' indicates that each instance gets its own storage for the slot. This is the default. - `#:class' indicates that there is one storage location used by all the direct and indirect instances of the class. This permits to define a kind of global variable which can be accessed only by (in)direct instances of the class which defines this slot. - `#:each-subclass' indicates that there is one storage location used by all the direct instances of the class. In other words, if two classes are not siblings in the class hierarchy, they will not see the same value. - `#:virtual' indicates that no storage will be allocated for this slot. It is up to the user to define a getter and a setter function for this slot. Those functions must be defined with the `#:slot-ref' and `#:slot-set!' options. See the example below. To illustrate slot description, we shall redefine the `' class seen before. A definition could be: (define-class () (r #:init-value 0 #:getter get-r #:setter set-r! #:init-keyword #:r) (i #:init-value 0 #:getter get-i #:setter set-i! #:init-keyword #:i)) With this definition, the `r' and `i' slot are set to 0 by default. Value of a slot can also be specified by calling `make' with the `#:r' and `#:i' keywords. Furthermore, the generic functions `get-r' and `set-r!' (resp. `get-i' and `set-i!') are automatically defined by the system to read and write the `r' (resp. `i') slot. (define c1 (make #:r 1 #:i 2)) (get-r c1) => 1 (set-r! c1 12) (get-r c1) => 12 (define c2 (make #:r 2)) (get-r c2) => 2 (get-i c2) => 0 Accessors provide an uniform access for reading and writing an object slot. Writing a slot is done with an extended form of `set!' which is close to the Common Lisp `setf' macro. So, another definition of the previous `' class, using the `#:accessor' option, could be: (define-class () (r #:init-value 0 #:accessor real-part #:init-keyword #:r) (i #:init-value 0 #:accessor imag-part #:init-keyword #:i)) Using this class definition, reading the real part of the `c' complex can be done with: (real-part c) and setting it to the value contained in the `new-value' variable can be done using the extended form of `set!'. (set! (real-part c) new-value) Suppose now that we have to manipulate complex numbers with rectangular coordinates as well as with polar coordinates. One solution could be to have a definition of complex numbers which uses one particular representation and some conversion functions to pass from one representation to the other. A better solution uses virtual slots. A complete definition of the `' class using virtual slots is given in Figure 2. (define-class () ;; True slots use rectangular coordinates (r #:init-value 0 #:accessor real-part #:init-keyword #:r) (i #:init-value 0 #:accessor imag-part #:init-keyword #:i) ;; Virtual slots access do the conversion (m #:accessor magnitude #:init-keyword #:magn #:allocation #:virtual #:slot-ref (lambda (o) (let ((r (slot-ref o 'r)) (i (slot-ref o 'i))) (sqrt (+ (* r r) (* i i))))) #:slot-set! (lambda (o m) (let ((a (slot-ref o 'a))) (slot-set! o 'r (* m (cos a))) (slot-set! o 'i (* m (sin a)))))) (a #:accessor angle #:init-keyword #:angle #:allocation #:virtual #:slot-ref (lambda (o) (atan (slot-ref o 'i) (slot-ref o 'r))) #:slot-set! (lambda(o a) (let ((m (slot-ref o 'm))) (slot-set! o 'r (* m (cos a))) (slot-set! o 'i (* m (sin a))))))) _Fig 2: A `' number class definition using virtual slots_ This class definition implements two real slots (`r' and `i'). Values of the `m' and `a' virtual slots are calculated from real slot values. Reading a virtual slot leads to the application of the function defined in the `#:slot-ref' option. Writing such a slot leads to the application of the function defined in the `#:slot-set!' option. For instance, the following expression (slot-set! c 'a 3) permits to set the angle of the `c' complex number. This expression conducts, in fact, to the evaluation of the following expression ((lambda o m) (let ((m (slot-ref o 'm))) (slot-set! o 'r (* m (cos a))) (slot-set! o 'i (* m (sin a)))) c 3) A more complete example is given below: (define c (make #:r 12 #:i 20)) (real-part c) => 12 (angle c) => 1.03037682652431 (slot-set! c 'i 10) (set! (real-part c) 1) (describe c) => #< 401e9b58> is an instance of class Slots are: r = 1 i = 10 m = 10.0498756211209 a = 1.47112767430373 Since initialization keywords have been defined for the four slots, we can now define the `make-rectangular' and `make-polar' standard Scheme primitives. (define make-rectangular (lambda (x y) (make #:r x #:i y))) (define make-polar (lambda (x y) (make #:magn x #:angle y)))  File: goops.info, Node: Class precedence list, Prev: Slot description, Up: Inheritance Class precedence list --------------------- A class may have more than one superclass. (1) With single inheritance (one superclass), it is easy to order the super classes from most to least specific. This is the rule: Rule 1: Each class is more specific than its superclasses. With multiple inheritance, ordering is harder. Suppose we have (define-class X () (x #:init-value 1)) (define-class Y () (x #:init-value 2)) (define-class Z (X Y) (...)) In this case, the `Z' class is more specific than the `X' or `Y' class for instances of `Z'. However, the `#:init-value' specified in `X' and `Y' leads to a problem: which one overrides the other? The rule in GOOPS, as in CLOS, is that the superclasses listed earlier are more specific than those listed later. So: Rule 2: For a given class, superclasses listed earlier are more specific than those listed later. These rules are used to compute a linear order for a class and all its superclasses, from most specific to least specific. This order is called the "class precedence list" of the class. Given these two rules, we can claim that the initial form for the `x' slot of previous example is 1 since the class `X' is placed before `Y' in class precedence list of `Z'. These two rules are not always enough to determine a unique order, however, but they give an idea of how things work. Taking the `F' class shown in Figure 1, the class precedence list is (f d e a c b ) However, it is usually considered a bad idea for programmers to rely on exactly what the order is. If the order for some superclasses is important, it can be expressed directly in the class definition. The precedence list of a class can be obtained by the function `class-precedence-list'. This function returns a ordered list whose first element is the most specific class. For instance, (class-precedence-list B) => (#< B 401b97c8> #< 401e4a10> #< 4026a9d8>) However, this result is not too much readable; using the function `class-name' yields a clearer result: (map class-name (class-precedence-list B)) => (B ) ---------- Footnotes ---------- (1) This section is an adaptation of Jeff Dalton's (J.Dalton@ed.ac.uk) `Brief introduction to CLOS'  File: goops.info, Node: Generic functions, Prev: Inheritance, Up: Tutorial Generic functions ================= * Menu: * Generic functions and methods:: * Next-method:: * Example::  File: goops.info, Node: Generic functions and methods, Next: Next-method, Prev: Generic functions, Up: Generic functions Generic functions and methods ----------------------------- Neither GOOPS nor CLOS use the message mechanism for methods as most Object Oriented language do. Instead, they use the notion of "generic functions". A generic function can be seen as a methods "tanker". When the evaluator requested the application of a generic function, all the methods of this generic function will be grabbed and the most specific among them will be applied. We say that a method M is _more specific_ than a method M' if the class of its parameters are more specific than the M' ones. To be more precise, when a generic function must be "called" the system will: 1. search among all the generic function those which are applicable 2. sort the list of applicable methods in the "most specific" order 3. call the most specific method of this list (i.e. the first method of the sorted methods list). The definition of a generic function is done with the `define-generic' macro. Definition of a new method is done with the `define-method' macro. Note that `define-method' automatically defines the generic function if it has not been defined before. Consequently, most of the time, the `define-generic' needs not be used. Consider the following definitions: (define-generic G) (define-method (G (a ) b) 'integer) (define-method (G (a ) b) 'real) (define-method (G a b) 'top) The `define-generic' call defines G as a generic function. Note that the signature of the generic function is not given upon definition, contrarily to CLOS. This will permit methods with different signatures for a given generic function, as we shall see later. The three next lines define methods for the G generic function. Each method uses a sequence of "parameter specializers" that specify when the given method is applicable. A specializer permits to indicate the class a parameter must belong to (directly or indirectly) to be applicable. If no specializer is given, the system defaults it to `'. Thus, the first method definition is equivalent to (define-method (G (a ) (b )) 'integer) Now, let us look at some possible calls to generic function G: (G 2 3) => integer (G 2 #t) => integer (G 1.2 'a) => real (G #t #f) => top (G 1 2 3) => error (since no method exists for 3 parameters) The preceding methods use only one specializer per parameter list. Of course, each parameter can use a specializer. In this case, the parameter list is scanned from left to right to determine the applicability of a method. Suppose we declare now (define-method (G (a ) (b )) 'integer-number) (define-method (G (a ) (b )) 'integer-real) (define-method (G (a ) (b )) 'integer-integer) (define-method (G a (b )) 'top-number) In this case, (G 1 2) => integer-integer (G 1 1.0) => integer-real (G 1 #t) => integer (G 'a 1) => top-number  File: goops.info, Node: Next-method, Next: Example, Prev: Generic functions and methods, Up: Generic functions Next-method ----------- When a generic function is called, the list of applicable methods is built. As mentioned before, the most specific method of this list is applied (see *Note Generic functions and methods::). This method may call the next method in the list of applicable methods. This is done by using the special form `next-method'. Consider the following definitions (define-method (Test (a )) (cons 'integer (next-method))) (define-method (Test (a )) (cons 'number (next-method))) (define-method (Test a) (list 'top)) With those definitions, (Test 1) => (integer number top) (Test 1.0) => (number top) (Test #t) => (top)  File: goops.info, Node: Example, Prev: Next-method, Up: Generic functions Example ------- In this section we shall continue to define operations on the `' class defined in Figure 2. Suppose that we want to use it to implement complex numbers completely. For instance a definition for the addition of two complexes could be (define-method (new-+ (a ) (b )) (make-rectangular (+ (real-part a) (real-part b)) (+ (imag-part a) (imag-part b)))) To be sure that the `+' used in the method `new-+' is the standard addition we can do: (define-generic new-+) (let ((+ +)) (define-method (new-+ (a ) (b )) (make-rectangular (+ (real-part a) (real-part b)) (+ (imag-part a) (imag-part b))))) The `define-generic' ensures here that `new-+' will be defined in the global environment. Once this is done, we can add methods to the generic function `new-+' which make a closure on the `+' symbol. A complete writing of the `new-+' methods is shown in Figure 3. (define-generic new-+) (let ((+ +)) (define-method (new-+ (a ) (b )) (+ a b)) (define-method (new-+ (a ) (b )) (make-rectangular (+ a (real-part b)) (imag-part b))) (define-method (new-+ (a ) (b )) (make-rectangular (+ (real-part a) b) (imag-part a))) (define-method (new-+ (a ) (b )) (make-rectangular (+ (real-part a) (real-part b)) (+ (imag-part a) (imag-part b)))) (define-method (new-+ (a )) a) (define-method (new-+) 0) (define-method (new-+ . args) (new-+ (car args) (apply new-+ (cdr args))))) (set! + new-+) _Fig 3: Extending `+' for dealing with complex numbers_ We use here the fact that generic function are not obliged to have the same number of parameters, contrarily to CLOS. The four first methods implement the dyadic addition. The fifth method says that the addition of a single element is this element itself. The sixth method says that using the addition with no parameter always return 0. The last method takes an arbitrary number of parameters(1). This method acts as a kind of `reduce': it calls the dyadic addition on the _car_ of the list and on the result of applying it on its rest. To finish, the `set!' permits to redefine the `+' symbol to our extended addition. To terminate our implementation (integration?) of complex numbers, we can redefine standard Scheme predicates in the following manner: (define-method (complex? c ) #t) (define-method (complex? c) #f) (define-method (number? n ) #t) (define-method (number? n) #f) ... ... Standard primitives in which complex numbers are involved could also be redefined in the same manner. ---------- Footnotes ---------- (1) The parameter list for a `define-method' follows the conventions used for Scheme procedures. In particular it can use the dot notation or a symbol to denote an arbitrary number of parameters  File: goops.info, Node: Concept Index, Next: Function and Variable Index, Prev: Tutorial, Up: Top Concept Index ============= * Menu: * accessor: Slot description. * class: Class definition. * default slot value: Slot description. * generic function: Generic functions and methods. * getter: Slot description. * instance: Instance creation and slot access. * keyword: Slot description. * loading: Intro. * main module: Intro. * parameter specializers: Generic functions and methods. * preparing: Intro. * setter: Slot description. * slot: Class definition. * top level environment: Slot description.  File: goops.info, Node: Function and Variable Index, Prev: Concept Index, Up: Top Function and Variable Index =========================== * Menu: * #:accessor <1>: Slot description. * #:accessor: Slot Options. * #:allocation <1>: Slot description. * #:allocation: Slot Options. * #:class: Slot description. * #:each-subclass: Slot description. * #:environment: Class Options. * #:getter <1>: Slot description. * #:getter: Slot Options. * #:init-form: Slot Options. * #:init-keyword <1>: Slot description. * #:init-keyword: Slot Options. * #:init-thunk <1>: Slot description. * #:init-thunk: Slot Options. * #:init-value <1>: Slot description. * #:init-value: Slot Options. * #:instance: Slot description. * #:metaclass: Class Options. * #:name: Class Options. * #:setter <1>: Slot description. * #:setter: Slot Options. * #:slot-ref <1>: Slot description. * #:slot-ref: Slot Options. * #:slot-set! <1>: Slot description. * #:slot-set!: Slot Options. * #:virtual: Slot description. * (oop goops): Intro. * =: Object Comparisons. * add-method!: Method Definition Internals. * apply-generic: Determining Which Methods to Apply. * apply-method: Determining Which Methods to Apply. * apply-methods: Determining Which Methods to Apply. * change-class: Changing the Class of an Instance. * class: Class Definition Internals. * class-direct-methods: Classes. * class-direct-slots: Classes. * class-direct-subclasses: Classes. * class-direct-supers: Classes. * class-environment: Classes. * class-methods: Classes. * class-name: Classes. * class-of: Instances. * class-precedence-list: Classes. * class-redefinition: Customizing Class Redefinition. * class-slot-definition: Slots. * class-slot-ref: Class Slots. * class-slot-set!: Class Slots. * class-slots: Classes. * class-subclasses: Classes. * compute-applicable-methods: Determining Which Methods to Apply. * compute-std-cpl: Customizing Class Definition. * deep-clone: Cloning Objects. * default slot value: Slot description. * define-accessor: Basic Generic Function Creation. * define-class <1>: Class definition. * define-class: Basic Class Definition. * define-generic <1>: Generic functions and methods. * define-generic: Basic Generic Function Creation. * define-method <1>: Generic functions and methods. * define-method: Basic Method Definition. * display: Write and Display. * enable-primitive-generic!: Extending Guiles Primitives. * ensure-accessor: Generic Function Internals. * ensure-generic: Generic Function Internals. * ensure-metaclass: Class Definition Internals. * ensure-metaclass-with-supers: Class Definition Internals. * equal?: Object Comparisons. * eqv?: Object Comparisons. * generic-capability?: Extending Guiles Primitives. * generic-function-methods: Generic Functions. * generic-function-name: Generic Functions. * goops-error: Error Handling. * goops-version: Administrative Functions. * instance?: Instances. * is-a?: Instances. * make <1>: Instance creation and slot access. * make: Basic Instance Creation. * make-accessor: Generic Function Internals. * make-class: Class Definition Internals. * make-generic: Generic Function Internals. * make-instance: Basic Instance Creation. * make-method: Method Definition Internals. * method: Method Definition Internals. * method-generic-function: Generic Function Methods. * method-more-specific?: Determining Which Methods to Apply. * method-procedure: Generic Function Methods. * method-source: Generic Function Methods. * method-specializers: Generic Function Methods. * no-applicable-method: Handling Invocation Errors. * no-method: Handling Invocation Errors. * no-next-method: Handling Invocation Errors. * primitive-generic-generic: Extending Guiles Primitives. * set!: Slot description. * shallow-clone: Cloning Objects. * slot-bound-using-class?: Instance Slots. * slot-bound?: Instance Slots. * slot-definition-accessor: Slots. * slot-definition-allocation: Slots. * slot-definition-getter: Slots. * slot-definition-init-form: Slots. * slot-definition-init-keyword: Slots. * slot-definition-init-thunk: Slots. * slot-definition-init-value: Slots. * slot-definition-name: Slots. * slot-definition-options: Slots. * slot-definition-setter: Slots. * slot-exists-using-class?: Instance Slots. * slot-exists?: Instance Slots. * slot-init-function: Slots. * slot-missing: Handling Slot Access Errors. * slot-ref <1>: Instance creation and slot access. * slot-ref: Instance Slots. * slot-ref-using-class: Instance Slots. * slot-set! <1>: Instance creation and slot access. * slot-set!: Instance Slots. * slot-set-using-class!: Instance Slots. * slot-unbound: Handling Slot Access Errors. * sort-applicable-methods: Determining Which Methods to Apply. * standard-define-class: STKlos Compatibility. * write: Write and Display.