Re: lazy-let & loop

[<Joe...@t-...>]

Hi,

Sam wrote:
>"lazy-binding": bind the variable but not initialize its value. until it is explicitly set,
>accessing it reaches up in the environment.
Interesting challenge :-)

> This would require something like `lazy-let` with the following test case:
--8<---------------cut here---------------start------------->8---
(let ((a 1))
  (lazy-let (a b)
    (print a) ; prints 1
    (print b) ; error - unbound variable
    (setq a 10) ; inner binding of a is activated
    (print a) ; prints 10
    (setq b 22)) ; binding of b is activated
  (print b) ; error - unbound variable
  (print a)) ; prints 1
--8<---------------cut here---------------end--------------->8---

>Alternatively, `lazy-let` can be implemented as something like
> (let ((a (if (boundp 'a) a (sys::%unbound))))  ...)
Certainly not, BOUNDP would only work with variables declared special.

Perhaps SYMBOL-MACROLET?
But that won't work either, because the problem remains that once `a becomes a symbol-macro,
one cannot express anymore "let me access `a from the outer environment". Didn't somebody once
post a bug report about similar infinite macro expansion about regular macros?

This makes me remember that there have been several past attempts at trying to make
the environment-access functions of CLtL2 fly. But Google turned up few references,
I must have been dreaming. :-(
http://clast.sourceforge.net/

Hmm, there's perhaps nothing that another level of delayed evaluation
in order to get the desired scoping couldn't solve, so here it is:

(defmacro lazy-let1 ((var) &body body)
  (let ((shadow (gensym (symbol-name var))))
    `(let ((,shadow 'unbound));(sys::%unbound)
       (flet ((access-a () (if (eq ,shadow 'unbound) ,var ,shadow))
	      ((setf access-a) (x) (setf ,shadow x)))
	 (symbol-macrolet ((,var (access-a))) .,body)))))

; bonus point 0: Add more required gensyms
; bonus point 1: Support a list of variables as per Sam's specification
; bonus point 2: See if LET can work with clisp's %unbound trick
; bonus point 3.5: See if you can make it work with special variables...

(let ((a 1))
  (lazy-let1 (a)
    (print a) ; prints 1
    (setq a 10) ; inner binding of a is activated
    (print a)) ; prints 10
  (print a)) ; prints 1

Regards,
	Jörg