Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!pasteur!ucbvax!hplabs!otter!psa
From: psa@otter.hple.hp.com (Patrick Arnold)
Newsgroups: comp.lang.lisp
Subject: Re: Functions vs. Procedures in Lisp
Message-ID: <1350017@otter.hple.hp.com>
Date: 16 Jun 88 08:27:16 GMT
References: <34296@linus.UUCP>
Organization: Hewlett-Packard Laboratories, Bristol, UK.
Lines: 34

John Gately is quite right to point out that function has sevral meanings
depending on the context of the conversation. I'm afraid it's one of my
"religious beliefs" that functions are expressions which denote values and
that calling a function with the same actual parameters will always produce
the same result and will have no insidious effects on other parts of the
system (i.e. side effects). This is the mathematical concept of a function.
I am quite happy with the other use provided the meaning is agreed at the
outset. 

I tend to use the term procedure for a parameterised piece of code which
may return a value (or even many values) and may have side effects. This is
sometimes a useful distinction to make. LISP and indeed most other
languages don't support the real notion of a function because there is
always the ability to access (and change destructively) other values or
parameters.

I hope this explanation gives you some hints as to why I don't think LISP
has functions in the true sense (although you can write inefficient Lisp
programs that are functional).

First class procedures are a very powerful programming tool enabling the
use of generic operations over data structures and object oriented style
programming using procedures with local state (closures) as objects. There
is quite heavy use of them in Abelson and Sussman (a must for any serious
Lisp programmer) and the book illustrates their use in a number of
different programming paradigms including an object oriented style.
(Interesting that what Lisp like languages have been able to do for years
is now in fashion in a different guise!!).

By the way I would just like to say that I don't get anything for plugging
Structure and Interpretation of Computer Programs, but it is quite the best
book on Lisp (Scheme actually) in particular and some programing techniques
in general that I have come across.

				Patrick.