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CommonLoops meets Object Lisp
In order to understand what would be involved in implementing ObjectLisp
on top of CommonLoops the CommonLoops group got together with Gary
Drescher to talk about it.
This message is a brief summary of what we discovered. Understanding
this message requires familiarity with both CommonLoops (CLoops) and
The major issues are:
1) Each ObjectLisp object is a hybrid of class and instance: it can be
invoked like an instance, and other objects can inherit from it like
2) In OBL method lookup is based on the "current object" (the value of
the special variable *object*). In Commonloops method lookup is
based on the value of one or more of the arguments to the method.
3) In OBL both method-lookup and slot-access inherit from the global
lisp environment if there is no object-binding.
The key points of the implementation scheme are:
1) Create a new discriminator mixin (discriminate-on-specials-mixin)
which extends the behavior of the standard CommonLoops discriminator
by allowing method lookup based on the value of special variables.
OBL will use this to discriminate based on the value of *OBJECT*.
Even though this discriminator mixin will discriminate on specials
it will continue to use the standard CLoops method caching mechanisms.
2) Create a new metaclass object-lisp-metaclass which merges the CLoops
class-instance relation and the CLoops class-subclass relation to get
the OBL object-specialization relation. In order to do this all OBL
objects will include the object-lisp-metaclass as one of their
a) class-of for any OBL object will return the object itself,
b) so that the superclasses of the class of an object will be the
supers of the object itself,
c) Each OBL-bound function FOO for object BAR will implemented
as a method defined on bar "as a class".
d) The object-lisp-metaclass will also support a new kind of
slot-allocation which is a simple extension of :class
allocation. This allocation will allow objects and their
specializations to share the same slot.
e) The get-slot-using-class method for object-lisp-metaclass will
implement the object lisp semantics of getting the global
binding of a variable if that variable is not bound in the