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Path: utzoo!linus!security!genrad!decvax!betz
From: betz@decvax.UUCP (David Betz)
Newsgroups: net.lang.lisp
Subject: LISP evaluator question
Message-ID: <281@decvax.UUCP>
Date: Tue, 15-Nov-83 17:48:24 EST
Article-I.D.: decvax.281
Posted: Tue Nov 15 17:48:24 1983
Date-Received: Wed, 16-Nov-83 01:38:28 EST
Organization: DEC UNIX Engineering Group
Lines: 45

Does anyone know the reasoning behind why LISP keeps a separate function
value cell for each atom?  Why isn't the normal value cell used to store
function values?  If the normal value cell were used, the evaluator could
just 'eval' the first element of a form in order to find the function being
applied.  I used this approach in my XLISP language.  The reason that I
needed to 'eval' the first element of a form was that I wanted to be able
to handle normal function application and object oriented message passing
in a form.

For instance:

	(fun arg1 arg2)		; call function 'fun' with arguments 'arg1'
				; and 'arg2'

	(obj msg arg1 arg2)	; send 'obj' the message 'msg' with arguments
				; 'arg1' and 'arg2'

The problem comes when the object in a message is itself an expression:

	((obj1 msg1 arg1) msg2 arg2)
				; first send the 'obj1' the message 'msg1'
				; with argument 'arg1'.
				; then take the value of that expression
				; and send it the message 'msg2' with
				; argument 'arg2'

If 'eval' doesn't evaluate the first element of a form, the (obj1 msg1 arg1)
message is never evaluated.

I solved this problem by storing the function value of an atom in the same
place as the normal value.  Then whenever I evaluate a form, I first
evaluate the first element to find out if it is a function or an object.
If it is a function, I do a normal function call.  If it is an object,
I send the object a message.

What I am wondering is whether I have violated a basic principle of LISP
in changing the evaluator to work in this way.  Why is it important to
keep function values separate from normal values?  Is this convention just
historical?

If the answer to this question is obvious, please forgive my ignorance.

	Thanks,
	David Betz
	decvax!betz