Scheme like question

[Marco Antoniotti]

Thanks.

My brain was not working AND I did not do the necessary RTFM of SICP.  The

(let ((somestuff nil))
   (setf somestuff (something-with-somestuff-closed-over-and-delayed)))

works of course.

The rest is easy macrology.

All the best

Marco

On Tue, Dec 14, 2021 at 6:44 AM Vibhu Mohindra <vibhu.mohindra at gmail.com>
wrote:

> Hi Marco,
>
> The last function below is the Common Lisp equivalent. The function
> before that contains the essence of it. Because cons-stream's second arg
> is evaluated only much later, by the time it is evaluated the setf has
> completed and int has been fully installed. (SICP section 4.1.6 shows
> that Scheme's inner defines are really just syntactic sugar for the same
> kind of thing.)
>
>
> ;SICP doesn't delay cars
> (defstruct my-stream
>   delayed-car delayed-cdr)
>
> (defvar *to-init* (gensym))
>
> ;SICP's memo-proc approach avoids creating memo symbols
> (defmacro delay (x)
>   (let ((memo (gensym)))
>     `(let ((,memo *to-init*))
>        #'(lambda ()
>            (cond ((eql ,memo *to-init*) (setf ,memo ,x))
>                  (t ,memo))))))
> (defun force (delayed-val)
>   (funcall delayed-val))
>
> (defmacro cons-stream (x y)
>   `(make-my-stream :delayed-car (delay ,x) :delayed-cdr (delay ,y)))
>
> (defvar *stream-nil* (gensym))
>
> (defun stream-car (s) (force (my-stream-delayed-car s)))
> (defun stream-cdr (s) (force (my-stream-delayed-cdr s)))
> (defun stream-null (s) (eql s *stream-nil*))
>
> (defun example1 (x)
>   (let ((int nil))
>     (setf int (cons-stream x int))))
> ; (stream-car (stream-cdr (stream-cdr (rec-stream 'a))))
> ; -> A
>
> ; omitting defs of add-streams scale-stream
> (defun integral (integrand initial-value dt)
>   (let ((int nil))
>     (setf int (cons-stream initial-value
>                            (add-streams (scale-stream integrand dt)
>                                         int)))))
>
>
> --
> Vibhu
>
>

-- 
Marco Antoniotti, Professor                           tel. +39 - 02 64 48
79 01
DISCo, Università Milano Bicocca U14 2043   http://dcb.disco.unimib.it
Viale Sarca 336
I-20126 Milan (MI) ITALY
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mailman.common-lisp.net/pipermail/pro/attachments/20211214/fc410dea/attachment.html>