Re: lazy-let & loop

[Sam S. <sd...@gn...>]

Hi,

> *  <Wbret-Plevy.Ubruyr@g-flfgrzf.pbz> [2017-12-20 17:35:44 +0000]:
>
>> This would require something like `lazy-let` with the following test case:
>
> (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
>
>>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.

of course - that's the point

> 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

Wow, I am impressed.
However, I am not sure CLISP will compile that to good code.

I think the first use of (setf access-a) should modify both (setf
access-a) and access-a to be trivial operations.

This is why I think a new special form TAGLET is necessary.

-- 
Sam Steingold (http://sds.podval.org/) on darwin Ns 10.3.1504
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