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-THE FACTOR GENERIC WORD SYSTEM
-
-Factor's generic word system is a very abstract generalization of
-"object oriented" features found in other programming languges.
-
-To use the generic word system, you must put the following near the
-beginning of your source file:
-
-USE: generic
-
-The key motivation is that sometimes, you want to write a word that has
-differing behavior depending on the class of its argument. For example,
-in a game, a 'draw' word could take different action if given a ship, a
-weapon, a planet, etc.
-
-Duplicating 'type case' logic is undesirable and also results in
-unnecessary coupling -- adding support for a new type of graphical
-object would require modifying the original definition of 'draw', for
-example.
-
-* Types
-
-In Factor, the idea of a 'type' refers to a very concrete concept. The
-type of an object is its representation in runtime object memory. Types
-include fixnums, bignums, cons cells, vectors, strings, and so on. The
-set of available types is fixed; adding a new type requires modifying
-the runtime source written in C.
-
-* Classes
-
-In Factor, a 'class' is just a predicate that categorizes objects as
-being a member of the class or not. To be useful, it must be consistent
--- for a given object, it must always return the same truth value.
-
-Examples of classes might include:
-
-- Cons cells where both elements are integers
-
-- Floating point numbers between -1 and 1
-
-- Hash tables holding a certain key
-
-- Any object that occurs as a member of a certain global variable
-holding a list.
-
-- ... and so on.
-
-As you can see, a class of objects does not need to be a subset or a
-superset of a type of objects.
-
-Classes, unlike types, can be defined by the user.
-
-* Generic words
-
-A generic word is a word whose behavior depends on the class of the
-object at the top of the stack.
-
-Generic words are defined using the following syntax:
-
-GENERIC: draw ( actor -- )
-#! Draw the actor.
-
-A stack effect comment, as above, is not required but recommended.
-
-* Methods
-
-A method associates behavior to a generic word. Methods are defined by
-writing M:, followed by a class name, followed by the name of a
-previously-defined generic word.
-
-One of the main benefits of generic words is that each method definition
-can potentially occur in a different source file. Generic word
-definitions also hide conditionals.
-
-Here are two methods for the generic 'draw' word:
-
-M: ship draw ( actor -- )
- [
- surface get screen-xy radius get color get
- filledCircleColor
- ] bind ;
-
-M: plasma draw ( actor -- )
- [
- surface get screen-xy dup len get + color get
- vlineColor
- ] bind ;
-
-Here, 'ship' and 'class' are user-defined classes.
-
-* Metaclasses
-
-To understand what classes already exist, and how to define your own
-classes, the concept of a 'metaclass' must be grasped first. Roughly
-speaking, a metaclass is a class of classes.
-
-New metaclasses can be defined by the user, but its an involved process
-that requires a deeper understanding of the generic word systsem than
-can be given here.
-
-** The object class
-
-Every object is a member of the object class. The object class is also a
-metaclass, and it is the one and only instance of itself.
-
-Confusing? The idea is pretty simple. If you define a method on
-'object', it will be called when no more specific method is available:
-
-GENERIC: describe
-M: number describe "The number " write . ;
-M: object describe "I don't know anything about " write . ;
-
-Since the only instance of the object metaclass is itself, you cannot
-define new classes in the object metaclass.
-
-** The builtin metaclass
-
-The builtin metaclass contains precisely the following classes; each
-class corresponds to a runtime type:
-
-alien
-array
-bignum
-complex
-cons
-dll
-f
-fixnum
-float
-port
-ratio
-sbuf
-string
-t
-vector
-word
-
-Each builtin class has a corresponding membership test predicate, named
-after the builtin class suffixed with '?'. For example, cons?, word?,
-etc.
-
-Adding new classes to the builtin metaclass requires modifications to
-the C code comprising Factor's runtime.
-
-** The union metaclass
-
-The union metaclass contains classes whose members are defined to be the
-aggregate of the members of a list of existing classes.
-
-For example, the Factor library defines some unions over numeric types:
-
-UNION: integer fixnum bignum ;
-UNION: rational integer ratio ;
-UNION: real rational float ;
-UNION: number real complex ;
-
-Now, the absolute value function can be defined in an efficient manner
-for real numbers, and in a more general fashion for complex numbers:
-
-GENERIC: abs ( z -- |z| )
-M: real abs dup 0 < [ neg ] when ;
-M: complex abs >rect mag2 ;
-
-New unions can be defined by you, and the numerical types example above
-gives the syntax: you write UNION: followed by the name of the union,
-followed by its members. The list of members is terminated with a
-semi-colon.
-
-A predicate named after the union followed by '?' is
-automatically-defined. For example, the following definition of 'real?'
-was automatically created:
-
-: real?
- dup rational? [
- drop t
- ] [
- dup float? [
- drop t
- ] [
- drop f
- ] ifte
- ] ifte ;
-
-** The predicate metaclass
-
-The predicate metaclass contains classes whose membership test is an
-arbitrary expression. To speed up dispatch, each predicate must be
-defined as a subclass of some other class. That way predicates
-subclassing from disjoint builtin classes do not need to be
-simultaenously tested.
-
-The library/strings.factor module defines some subclasses of integer,
-classifying the different types of ASCII characters:
-
-PREDICATE: integer blank " \t\n\r" str-contains? ;
-PREDICATE: integer letter CHAR: a CHAR: z between? ;
-PREDICATE: integer LETTER CHAR: A CHAR: Z between? ;
-PREDICATE: integer digit CHAR: 0 CHAR: 9 between? ;
-PREDICATE: integer printable CHAR: \s CHAR: ~ between? ;
-
-Each predicate defines a corresponding predicate word whose name is
-suffixed with '?'; for example, a 'digit?' word is automatically
-defined:
-
-: digit?
- dup integer? [
- CHAR: 0 CHAR: 9 between?
- ] [
- drop f
- ] ifte ;
-
-For obvious reasons, the predicate definition must consume and produce
-exactly one value on the stack.
-
-** The tuple metaclass
-
-This is quite involved; see tuples.txt.
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