Re: fluid-let

[hjstein@bfr.co.il (Harvey J. Stein)]

Marius Vollmer <mvo@zagadka.ping.de> writes:

 > Russ McManus <mcmanr@eq.gs.com> writes:
 > 
 > > > Binding temp-var with `let' might be cleaner here.
 > > 
 > > Since I was using 'set!' willy-nilly, I just kept going.  See below
 > > for version with 'let'.
 > 
 > Yeah, no big deal.  The point was not to get rid of one `set!' because
 > it isn't functional or something, but to avoid putting so much code
 > between the declaration and use of temp-var.

For the hell of it, here's my fluid-let.  First of all, from the
wrappers package I posted, I have:

(define-macro (with-wrappers wrapper starter ender symlist form)
  (let ((oldsym (map (lambda (sym) (gensym)) symlist)))
    `(let ,(map (lambda (old sym)
		  (list old sym))
		oldsym
		symlist)
       (dynamic-wind (lambda ()
		       (,starter)
		       ,@(map (lambda (sym)
				`(set! ,sym (,wrapper ',sym ,sym)))
			      symlist))
		     (lambda ()
		       ,form)
		     (lambda ()
		       ,@(map (lambda (sym old)
				`(set! ,sym ,old))
			      symlist oldsym)
		       (,ender))))))
	       
Then, fluid-let is:
(define-macro (fluid-let args . body)
  `(with-wrappers (lambda (sym symval) (cadr (assoc sym ',args)))
		  (lambda () #t)
		  (lambda () #t)
		  ,(map car args)
		  (begin ,@body)))

Of course, it'd be better to gut with-wrappers to generate fluid-let:

(define-macro (fluid-let args . body)
  (let ((oldsyms (gensym)))
    `(let ((,oldsyms ,(map car 'args))
		  (list old sym))
		oldsym
		args)
       (dynamic-wind (lambda ()
		       (,starter)
		       ,@(map (lambda (sym)
				`(set! ,sym (,wrapper ',sym ,sym)))
			      symlist))
		     (lambda ()
		       ,form)
		     (lambda ()
		       ,@(map (lambda (sym old)
				`(set! ,sym ,old))
			      symlist oldsym)
		       (,ender))))))

-- 
Harvey J. Stein
BFM Financial Research
hjstein@bfr.co.il