Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!allegra!princeton!caip!seismo!lll-crg!lll-lcc!vecpyr!amd!pesnta!hplabs!tektronix!tekcrl!tekchips!willc
From: willc@tekchips.UUCP (Will Clinger)
Newsgroups: net.lang.lisp
Subject: Re: Abusing "define" in Scheme... (car crashes?)
Message-ID: <328@tekchips.UUCP>
Date: Fri, 23-May-86 14:05:52 EDT
Article-I.D.: tekchips.328
Posted: Fri May 23 14:05:52 1986
Date-Received: Thu, 29-May-86 02:31:07 EDT
References: <13508@ucla-cs.ARPA> <2048@mit-eddie.MIT.EDU> <2049@mit-eddie.MIT.EDU>
Reply-To: willc@tekchips.UUCP (Will Clinger)
Distribution: net
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 56
Keywords: Scheme
Summary: Reserved words are allowed

In article <2049@mit-eddie.MIT.EDU> jbs@mit-eddie.UUCP (Jeff Siegal) writes:
>Actually, redefining lambda or define would not change the meaning of
>expressions like (define a b) or (lambda (x) (* x 2)).  These are are
>"special forms" and (if I remember my 6.001 correctly) the evaulator
>does not look for their definitions in the current context before
>actually doing the work involved (binding or procedure creation).  So
>actually the list of indentifiers which can not be redefined is the
>list of special forms.
>
>Note however, that you can do the following:
>
>--> (DEFINE LAMBDA 1)
>LAMBDA
>
>--> LAMBDA
>1
>
>--> ((LAMBDA (X) (+ X 1)) 1)
>2
>
>LAMBDA has been defined as a symbol bound to 1, but this does not
>affect the normal usage of LAMBDA, which still works as expected(?).
>
>Jeff Siegal

Whether you can do this or not is implementation-dependent.  The
Revised Revised Report on Scheme (MIT AI Memo 848) says "Some
implementations [reserve] the identifiers that serve as keywords
of special forms, while other implementations allow the keyword
meaning of an identifier to be shadowed by lexical bindings."
The code that Jeff wrote illustrates yet a third implementation,
in which the shadowing is context-dependent.

To write portable Scheme code, you have to assume that the keywords
of special forms are reserved.  That's one reason people are so
fanatic about having the number of special forms be as small as
possible.

I don't know how Common Lisp stands on this issue.  To judge by the
program-analyzing algorithm on page 57 of Steele's "Common Lisp: the
Language", the names of special forms are reserved when they appear
in the car of a form, but I don't know what happens when they appear
in other contexts.  Common Lisp distinguishes between special forms
and macros, however, and it is clear that macro names can be shadowed
because they live in the "function" environment, which is lexical
except for some special treatment of the global function environment.
On the other hand, implementations of Common Lisp are explicitly
allowed to implement certain macros as special forms, and I don't
know how their names can be simultaneously reserved (because they
are special forms) and shadowable (because they are macros).  I'd
appreciate any pointers to authoritative explanations of these two
matters.

Peace, William Clinger
Tektronix Computer Research Laboratory
willc%tekchips@tektronix