Path: utzoo!attcan!uunet!odi!dlw
From: dlw@odi.UUCP (Dan Weinreb)
Newsgroups: comp.lang.lisp
Subject: Re: LET vs. &aux
Message-ID: <116@odi.UUCP>
Date: 21 Dec 88 19:55:58 GMT
References: <4400007@uicslsv> <5727@polya.Stanford.EDU>
Organization: Object Design Inc., Burlington, MA
Lines: 35
In-reply-to: caron@polya.Stanford.EDU's message of 19 Dec 88 01:40:04 GMT

>The question is: why does &AUX even exist? is this another throwback
>to Mac/Inter/Zeta-Lisp? 

&AUX actually predates LET.  &AUX, along with &OPTIONAL and &REST, was
introduced into the MIT Lisp Machine dialect (later known as Zetalisp)
sometime prior to the summer of 1976 (by Richard Greenblatt).  All of
these &-keywords were borrowed from MDL (a.k.a Muddle), a very unusual
dialect of Lisp used by a small group at MIT's Project MAC, in which
they were written with double-quotes instead of the ampersand, e.g.
"OPTIONAL", "AUX".

They were backmigrated into Maclisp some years later.  They were never
in Interlisp or Scheme.

At that time (1976), LET had not been codified as part of the
language; many people at MIT had their own LET-like macros, all
slightly different and incompatible.  Soon thereafter, someone
(probably Dave Moon) picked the current LET syntax and installed it as
part of the basic Lisp system, implemented (originally) as a macro
that expanded into a lambda-combination.  This was one of the first
times a macro had been installed as part of the Lisp system itself.

In my own opinion, &AUX is essentially superseded by LET.  I gave up
using &AUX long ago, and personally don't care for it stylistically.
I don't remember why it was carried into Common Lisp; I don't think it
should have been.

As for the implementation, it could be different in every Lisp, but
for the Symbolics 3600 (and Ivory) compiler now in use, I am pretty
sure that when I wrote that compiler, I did exactly as you say:
transformed it into a LET*.  In any case, I would be very surprised if
there were any performance difference on the Symbolics 3600 family
between &AUX and a top-level LET*.  (I haven't actually tested this.)

Dan Weinreb, Object Design Inc. (formerly of Symbolics, Inc.)