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From: steele@unc.UUCP (Oliver Steele)
Newsgroups: comp.lang.lisp
Subject: Re: A small brush fire ...
Message-ID: <269@unc.unc.UUCP>
Date: Thu, 20-Nov-86 18:16:42 EST
Article-I.D.: unc.269
Posted: Thu Nov 20 18:16:42 1986
Date-Received: Fri, 21-Nov-86 20:30:15 EST
References: <8868@duke.duke.UUCP>
Reply-To: steele@unc.UUCP (Oliver Steele)
Organization: CS Dept, U. of N. Carolina, Chapel Hill
Lines: 46

In article <8868@duke.duke.UUCP> jds@duke.UUCP (Joseph D. Sloan) writes:
>
>	So how would you answer the following:  Since "good"
>programmers know better than using free (== global) variables,
>the argument between lexical and dynamic scoping is really a
>mute point.  The real issue this: Can you exhibit a piece of code
>that depends on the scoping rules that can't be coded AS WELL
>in a way that doesn't depend on the scoping rules?
>
>				duke!jds


(defun mymapcar (myfunction list)
  (cond ((null list) nil)
	(t (cons (myfunction (car list))		;or funcall, etc.
		 (mymapcar myfunction (cdr list))))))

(defun consall (l list)
  (mymapcar '(lambda (x) (cons x list))
	    l))

lexical:
(consall '(a b c) '(d)) ==> ((a d) (b d) (c d))

dynamic:
(consall '(a b c) '(d)) ==> ((a a b c) (b b c) (c c))


The problem isn't so much that you don't get what you expected with dynamic
scoping, but that what you get depends on the lambda variables used in the
function you're calling.  This is the same problem as using global
variables:  you have to know what variables the procedures you're calling
use, instead of just what they do.

e.g.:
(defun consall2 (l llist)
  (mymapcar '(lambda (x) (cons x llist))
	    l))
ought to do the same thing as consall, but will give different results
under dynamic scoping.

-- 

Oliver Steele----------------------------------steele@unc
When a tree dies,	  ...!{decvax,ihnp4}!mcnc!unc!steele
plant another in its place.	steele%unc@csnet-relay.csnet