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.
You can send a message to an object using the 'send' function. This 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.
The 'send' function 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.
A message can also be sent from the body of a method by using the current object, but the method lookup starts with the object's superclass rather than its class. This allows a subclass to invoke a standard method in its parent class even though it overrides that method with its own specialized version.
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.