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From: psi@mit-eddie.MIT.EDU (Joseph J. Mankoski)
Newsgroups: net.lang.lisp
Subject: Re: Abusing "define" in Scheme... (car crashes?)
Message-ID: <2048@mit-eddie.MIT.EDU>
Date: Wed, 21-May-86 05:57:47 EDT
Article-I.D.: mit-eddi.2048
Posted: Wed May 21 05:57:47 1986
Date-Received: Sat, 24-May-86 02:38:09 EDT
References: <13508@ucla-cs.ARPA>
Distribution: net
Organization: Massachusetts Institute of Technology
Lines: 45
Keywords: Scheme
Summary: Scheme Allows Redefinitions, or You're Responsible for Your Own Car.

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?	

	I do  not  know off hand of  any prohibition on redefining any
identifiers  in Scheme.   Not having a manual on  hand,  I resorted to
testing the readily accessible Scheme implementaions here at MIT.
	On both the VAX and PDP-10 versions of Scheme, I was allowed
to do the following:

	==> (define car 8)
	CAR
	
	==> car
	8

	==> (car '(foo baz bar))
	ERROR: Inapplicable procedure object ...*mumble*...

(The last should be expected, due to the fact that identifiers only
have one value in Scheme.)
	I see  nothing inherently wrong  in letting the  user redefine
whatever he wants in the Scheme environment.  This is  in keeping with
the Lisp(hacker) philosophy that one should be able to frob the system
at will, just as long as  you are willing to  do it responsibly enough
that you don't hose yourself.
	I have seen Scheme programs written  by first-time programmers
which redefined T to be some random thing.  Having learned (true) Lisp
before Scheme,  I was first  horrified when I   saw them  doing  this.
There is no problem with redefining T, however, as the actual value of
Truth is  kept elsewhere(#!true),  and  final  cond-clauses  are often
written with Else in the place of T.
	Whether or not such redefinions should be  allowed is a matter
of personal taste, though  prohibiting some redefitions  detracts from
the uniformity of  the language.  Personally,  I prefer the freedom to
redefine anything, though  I still avoid redefining  T, Lambda, and of
course, Define.

				Lisp is True Beauty,
					Joseph J. Mankoski ***PSI***
					...!mit-eddie!psi(UUCP)
					psi@ai.ai.mit.edu(ARPA)