Nyquist / XLISP 2.0 -
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XLISP Object System
- Definitions
- selector - a symbol used to select an appropriate method
- message - a selector and a list of actual arguments
- method - the code that implements a message
- object - the top of the class hierarchy
- class - the class of all object classes
- The 'send' Function
- The 'self' Symbol
- The 'send-super' Function
- The 'object' Class
- object - the top of the class hierarchy
- :show - show an object's instance variables
- :class - return the class of an object
- :isnew - the default object initialization routine
- send-super - send superclass a message
- The 'class' Class
- class - class of all object classes
- :new - create a new instance of a class
- :isnew - initialize a new class
- :answer - add a message to a class
1 Definitions
selector |
— |
a symbol used to select an appropriate method, usually a keyword |
message |
— |
a selector symbol and a list of actual arguments |
method |
— |
the Lisp code that implements a message |
object |
— |
the top of the class hierarchy |
class |
— |
the class of all object classes [including itself] |
Since XLISP was created to provide a simple basis for experimenting with
object-oriented programming, one of the primitive data types
included is object. In
XLISP, an object consists of a data structure containing a pointer to
the object's class as well as an array
containing the values of the object's instance variables.
Officially, there is no way to see inside an object [look at the values
of its instance variables]. The only way to communicate with an object is by
sending it a message.
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2 The 'send' Function
You can send a message to an object using the
send function:
- (send object selector [args])
- object - an object
selector - message selector for object
arg - parameter sent to object method
returns - the object
The send function takes the object as
its first argument, the message selector as its second argument [which must
be a symbol] and the message arguments as its remaining arguments. It
determines the class of the receiving object and attempts to find a
method corresponding to the message selector in the set of messages defined
for that class. If the message is not found in the object's
class and the class has a super-class, the search continues by
looking at the messages defined for the super-class. This
process continues from one super-class to the next until a
method for the message is found. If no method is found, an
error occurs.
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3 The 'self' Symbol
When a method is found, the evaluator binds the receiving object to the
symbol self and evaluates the method
using the remaining elements of the original list as arguments to the
method. These arguments are always evaluated prior to being bound to their
corresponding formal arguments. The result of evaluating the method becomes
the result of the expression.
Within the body of a method, a message can be sent to the current object
by calling:
(send self ... )
The method lookup starts with the object's class regardless of the class
containing the current method.
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4 The 'send-super' Function
Sometimes it is desirable to invoke a general method in a superclass even
when it is overridden by a more specific method in a subclass. This can be
accomplished by calling send-super, which begins the
method lookup in the superclass of the class defining the current method
rather than in the class of the current object:
- (send-super selector [args])
- selector - the message selector
args - the optional message arguments
returns - the result of sending the message
The send-super function
takes a selector as its first argument [which must be a symbol] and the
message arguments as its remaining arguments. Notice that send-super can only be sent from
within a method, and the target of the message is always the current object
self.
(send-super ... )
is similar to:
(send self ... )
except that method lookup begins in the superclass of the class
containing the current method rather than the class of the current
object.
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5 The 'object' Class
object - the top of the class hierarchy.
Messages:
- (send object :show)
- show an object's instance variables.
- returns - the object
- (send object :class)
- return the class of an object
- returns - the class of the object
- (send object :isnew args)
- run the default object initialization routine
- returns - the object
- (send object :isa class)
- test if object inherits from class
- returns - T if object
is an instance of class or a subclass of class, otherwise
NIL
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6 The 'class' Class
class
- class of all object classes (including itself)
Messages:
- (send class :new ivars [cvars [super]])
- create a new instance of a class
- returns - the new class object
- (send class :isnew ivars [cvars [super]])
- initialize a new class
- ivars - list of instance variable symbols
cvars - list of class variable symbols
super - the superclass, default is object
returns - the new class object
- (send class :answer selector fargs body)
- add a message to a class
- selector - a message selector symbol
fargs - the formal argument list, a lambda list
body - a list of executable expressions
returns - the object
When a new instance of a
class is created by sending
the message :new
to an existing class, the
message :isnew followed by
whatever parameters were passed to the
:new message is sent to the
newly created object.
When a new class is created
by sending the :new message to
the object class, an optional
parameter may be specified indicating the superclass of the new
class. If this parameter is
omitted, the new class will be
a subclass of object. A
class inherits all instance
variables, class variables, and methods from its superclass.
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Nyquist / XLISP 2.0 -
Contents |
Tutorials |
Examples |
Reference