Path: utzoo!attcan!uunet!husc6!rice!titan!dorai
From: dorai@titan.rice.edu (Dorai Sitaram)
Newsgroups: comp.lang.scheme
Subject: Re: Re : set in Scheme
Summary: set is not possible (perhaps rightly so) in your favorite language
Keywords: set, set!, eval, Scheme
Message-ID: <683@thalia.rice.edu>
Date: 10 May 88 15:41:48 GMT
References: <12073@shemp.CS.UCLA.EDU>
Sender: usenet@rice.edu
Reply-To: dorai@titan.UUCP (Dorai Sitaram)
Distribution: na
Organization: Rice University, Houston
Lines: 55

Here's my invaluable ;-) comment about _set_ in Scheme. As it is fairly easy
to build up a case for loss of program readability with the addition of _set_
(as opposed to _set!_), we should perhaps be pleased that it is probably 
impossible (with macros, extend-syntax, what-not) to define _set_ in Scheme.

The most "correct" version of _set_ in terms of _set!_ given on the net,
(herein transliterated to extend-syntax) is probably

(extend-syntax (set)
  [(set x y) (eval (list 'set! x (quote y)))]).

However, Scheme does not offer 'eval' to the user. The most it does is offer
a *global* eval, which ain't the same thing. So,

(define x 0)
==> x
(define y 'x)
==> y
(let ([x 1])
  (let ([y 'x])
    (set y 2)
    x))
==> 1 {instead of 2}
x
==> 2 {instead of 0}

Continuing { with global x = 2, y = x }

(define z 3)
==> z
(let ([z 4])
  (set y z)
  x)
==> 3 {instead of 4}

The second problem can be appeased by evaluating the settend (to coin a 
word) beforehand as in,

(extend-syntax (set)
  [(set x y) (begin (set! |weird-identifier| y)
		    (eval (list 'set! x (quote |weird-identifier|))))])

{|weird-identifier| HAS to be a global variable, because of the globalness
of eval.}

But the first problem remains. A modified version of the second problem can
occur if there are _set_'s inside the settee (to coin a not-so-new word), as 
in

(set (set <blah> <foo>) <hukares>)

Any amount of tweaking the extend-syntax for set, seems destined to lead
nowhere. That, as they (and Magnum) say, is "the hell of it".

--dorai