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From: dave@hpgnd.HP.COM (Dave PENKLER)
Newsgroups: comp.lang.lisp
Subject: Lisp 1.5 functional values as objects
Message-ID: <1950001@hpgnd.HP.COM>
Date: 9 Oct 90 13:45:43 GMT
Organization: Hewlett-Packard, GND
Lines: 43


A recent discussion on whether CL functions are first class objects brought up
a point on the run time creation of functions. This is somthing I missed in CL
and have consequently remained stranded in the ancient world of Ye Old Lisp.

Ye old lisp's abilty to handle functional values can be interpreted as a sort
of support for object oriented development. The beauty of it is its extreme
simplicity in the prinicple of operation permitting at the same time 
arbitrarily complex hierachies and collections of objects. For free you have
the ability to modify methods in a given instance, all instances of a class
or future instances. This is great for modelling and simulations where
you may need to tweak individual instances during the run to see what the
effects on the system might be. Dynamic scoping gives you what I call dynamic
inheritance where free variables/methods in the closure are resolved in the 
invoking environment and captured in the instance.

Here is an ultra simple example to give the basic idea of functional values
as objects:

To create a class you simply define a function that returns itself in a funarg -

 (defun SIMP (X Y) ; inittable instance vars X and Y
    (prog (Z SHOW) ; instance var Z and a method SHOW
	  (setq Z 'some-default) ; initial value of Z 
	  (defun SHOW () (print (list X Y Z))) ; method to print instance vars 
          (fun SIMP))) ; return self in funarg

To create an instance -

(setq INST1 (SIMP 'fred 'joe))

To apply the method SHOW to our instance -

 (send '(SHOW) INST1)  ; show its guts
(fred joe some-default)

The real work here is done by "send" 
(defun send (EXP OBJ) (apply (list 'close (car EXP) (caddr OBJ)) (cdr EXP)))


Is there any way to do this sort of thing in  newer dialects of lisp ?

DaveP