Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!mit-eddie!barmar
From: barmar@mit-eddie.UUCP
Newsgroups: comp.lang.lisp
Subject: Re: Against the TIde of Common LISP
Message-ID: <4934@mit-eddie.MIT.EDU>
Date: Tue, 24-Feb-87 03:17:52 EST
Article-I.D.: mit-eddi.4934
Posted: Tue Feb 24 03:17:52 1987
Date-Received: Thu, 26-Feb-87 23:48:20 EST
References: <2624@well.UUCP> <2626@well.UUCP>
Reply-To: barmar@eddie.MIT.EDU (Barry Margolin)
Organization: MIT, EE/CS Computer Facilities, Cambridge, MA
Lines: 43

In article <2626@well.UUCP> jjacobs@well.UUCP (Jeffrey Jacobs) writes:

Describing a way for SET to assign to lexical variables:

>(DEFUN FOO (X Y Z) (SET X (CONS Y Z)))
>
>The compiler would have to generate code that would effectively
>be equal to
>
>(COND	((EQ X 'X)  (SETQ X (CONS Y Z)))
>	((EQ X 'Y)  (SETQ Y (CONS Y Z)))
>	((EQ X 'Z)  (SETQ Z (CONS Y Z)))
>	(T (SET-SYMBOL-VALUE X (CONS Y Z)))))

That is the wrong thing, though.  Consider FOO being used in the
following:

(DEFUN FOO-CALLER (ARG1 ARG2)
  (LET ((X 3))
    (FOO 'X ARG1 ARG2)
    (PRINT X)))

The X that is passed to FOO is in the lexical scope of FOO-CALLER, so
it is the one that one would expect to be assigned.  One of the goals
of lexical scoping is that it should not make any difference to the
caller what the names of locals are in a function; if a lexical
variable is renamed, the only places you have to look for references
to the variable is within the lexical scope of that variable.  The
parameters to FOO are not lexical variables because they can be
referenced outside the function.

I will admit that there are uses for this type of thing; for example,
in an object-oriented programming system implemented using lexical
variables, one might have a SET-INSTANCE-VARIABLE function that takes
the name of an instance variable.  However, this would probably differ
from FOO because it wouldn't have the T clause, since it is ONLY
interested in lexical variables.  I doubt that there is a use for the
generality of the FOO example, in which SET will set either a lexical
or special.
-- 
    Barry Margolin
    ARPA: barmar@MIT-Multics
    UUCP: ..!genrad!mit-eddie!barmar