Unlike other Lisp-based object-oriented programming languages like
CLOS [#!CLOS:Keene!#],
EusLisp is a Lisp system built on the basis of object-orientation.
In the former approach, Lisp is used as an
implementation language for the object-oriented programming,
and there is apparent distinction between system defined objects
and user defined objects, since system data types do not have
corresponding classes.
On the other hand, every data structure in EusLisp except number is
represented by an object, and there is no inherent difference between
built-in data types, such as cons and symbols,
and user defined classes.
This implies that even the system built-in data types
can be extended (inherited) by user-defined classes.
Also, when a user defines his own class as a subclass of a built-in class,
he can use built-in methods and functions for the new class,
and the amount of description for a new program can be reduced.
For example, you may extend the cons class to have extra field other
than car and cdr to define queues, trees, stacks, etc.
Even for these instances, built-in functions for
built-in cons are also applicable without any loss of efficiency,
since those functions recognize type hierarchy in a constant time.
Thus, EusLisp makes all the system built-in facilities open to programmers
in the form of extensible data types.
This uniformity is also beneficial to the implementation of EusLisp,
because, after defining a few kernel functions
such as defclass, send, and instantiate,
in the implementation language,
most of house-keeping functions to access the internal structure of built-in
data types can be coded in EusLisp itself.
This has much improved the reliability and maintainability of EusLisp.
k-okada
2013-05-21
