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From: willc@tekchips.UUCP (Will Clinger)
Newsgroups: net.lang.lisp
Subject: Re: Abusing "define" in Scheme... (car crashes?)
Message-ID: <315@tekchips.UUCP>
Date: Mon, 19-May-86 14:27:18 EDT
Article-I.D.: tekchips.315
Posted: Mon May 19 14:27:18 1986
Date-Received: Sat, 24-May-86 02:22:42 EDT
References: <13508@ucla-cs.ARPA>
Reply-To: willc@tekchips.UUCP (Will Clinger)
Distribution: net
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 34
Keywords: Scheme

In article <13508@ucla-cs.ARPA> hydar@ucla-cs.ARPA (Dan Hydar) writes:
>
>
>	Does anyone know if Scheme is defined such that ANY binding can be
>   changed or is there some set of functions/identifiers that can be (or
>   MUST be) prevented from being re-defined?
>
Certainly any identifier that is not the keyword of a special form can
be *bound* as in (LET ((CAR 5) (CDR LOG)) (CDR CAR)); the question is
whether all variables can be *assigned*.  By my reading of the idealized
semantics of Scheme, the answer is yes.  Some implementations of Scheme,
however, prevent you from assigning a new value to variables like CAR;
though deliberate, I consider this behavior to be a (tolerable) bug
introduced in exchange for simpler open-coding of important operations.

>	For instance --- are you able to do blatantly stupid things like:
>
>			(define car  5)
>
An implementation of Scheme that lets you do this should be civil enough
not to crash when you do it.  (The system code should have been compiled
in a different environment from the one in which you are wreaking your
havoc.  I confess, however, that MacScheme isn't entirely bulletproof
in this regard.)

Compare this to the status quo in other languages, where assignments such
as (SETF (SYMBOL-FUNCTION 'CDR) #'LOG) may be permitted but ineffective,
or have effect only when #'CAR appears in an argument position, or have
effect only in code that is interpreted or code that is compiled
following the assignment.

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