Path: utzoo!attcan!uunet!snorkelwacker!bloom-beacon!RPAL.COM!shaff
From: shaff@RPAL.COM (Mike Shaff)
Newsgroups: comp.lang.scheme
Subject: Counter in SIOD
Message-ID: <9008151504.AA15562@rpal.com>
Date: 15 Aug 90 15:04:45 GMT
References: <1990Aug15.005252.8122@agate.berkeley.edu>
Sender: daemon@athena.mit.edu (Mr Background)
Reply-To: shaff@rpal.com
Organization: The Internet
Lines: 37

ciao,

ericco@ssl.berkeley.edu wrote:

	(define (cnt x) (lambda () (let ((y x)) (set! y (+ y 1)))))

Eric is looking for a function that saves state with respect to y (actually he
probably has something totally different in mind, but he did not want to
complicate things).  There are two problems with this code.  First, you are not
spawning a new closure (combination of state and lexical body).  Second the use
of set! to return a result leads to unspecified execution.  Below is an example
of how to obtain the functionality you want.

(define spawn-counter
  (lambda (x)				;Take in our starting point
    (let ((y x))			;capture it
      (lambda ()			;return a new  closure that will...
	(set! y (+ y 1))		;bang on state each time called
	y))))				;and return the new status
;Value: spawn-counter

(define counter (spawn-counter 10))
;Value: counter

(counter)
;Value: 11

(counter)
;Value: 12

Enjoy, anonymous closures are one of Scheme's most wonderful things.  The other
things?  Macros?  No, just dreaming...

	mas

Once in a while you get shown the light
in the strangest of places if you look at it right.