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From: "B. Pym" <Nobody447095@here-nor-there.org>
Newsgroups: comp.lang.lisp,comp.lang.scheme
Subject: Re: Lisp newbie needs help
Date: Thu, 29 Aug 2024 11:38:11 -0000 (UTC)
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B. Pym wrote:

> B. Pym wrote:
> 
> > > (defun my-test ()
> > >    (loop for number = (1+ (random 6))
> > >          as sum = number then (+ sum number)
> > >          until (= number 1)
> > >          do (format t "~&~D thrown. Sum: ~D" number sum)
> > >          finally (format t "~&One thrown.")))
> > 
> > Gauche Scheme
> > 
> > (use srfi-1)  ;; unfold
> > (use srfi-27) ;; random-integer
> > 
> > (define (my-test)
> >   (fold
> >     (^(n sum) (print n " thrown.  Sum: " (inc! sum n)) sum)
> >     0
> >     (cdr (unfold zero? ($ + 1 $) (^_ (random-integer 6)) -1)))
> >   (print "One thrown."))
> > 
> > 
> > gosh> (my-test)
> > 2 thrown.  Sum: 2
> > 2 thrown.  Sum: 4
> > 6 thrown.  Sum: 10
> > One thrown.
> > 
> > gosh> (my-test)
> > One thrown.
> > 
> > 
> > Explanation of "unfold":
> > 
> > Function: unfold end-test key gen-next-seed seed :optional tail-gen
> > 
> > (unfold zero? (lambda(n) (+ 800 n)) (lambda(n) (- n 1)) 7)
> >   ===>
> > (807 806 805 804 803 802 801)
> > 
> > (unfold zero? (lambda(n) (+ 800 n)) (lambda(n) (- n 1)) 7
> >   (lambda(n) (list "The number" n "ended the unfolding.")))
> >   ===>
> > (807 806 805 804 803 802 801 "The number" 0 "ended the unfolding.")
> 
> Using "do" is a bit tricky, but the result is shorter.
> 
> (define (my-test)
>   (do ((n #f (+ 1 (random-integer 6)))
>        (sum 0))
>     ((eqv? 1 n) (print "One thrown."))
>     (if n (print n " thrown.  Sum: " (inc! sum n)))))

It seems to me that "do*" is more appropriate for this,
but this version is 2 characters longer!

(define (my-test)
  (do* ((n #f (+ 1 (random-integer 6)))
        (sum 0 (+ n sum)))
    ((eqv? 1 n) (print "One thrown."))
    (if n (print n " thrown.  Sum: " sum))))

I don't know why Common Lisp has "do*", but Gauche, Racket,
and Chicken Scheme don't.

Here's a version that I cobbled together.  (A macro guru
may give us a better one.)  "do" is to "do*" as "let" is
to "let*".

(define-syntax do*-aux
  (syntax-rules ()
    [(do* (inits ...) ((var update) ...) (test expr ...) stuff ...)
     (let* (inits ...)
       (if test
         (begin expr ...)
         (begin
           (begin stuff ...)
           (let loop ()
             (begin (set! var update) ...)
             (if test
               (begin expr ...)
               (begin  stuff ...
                 (loop)))))))]))

(define-syntax do*
  (syntax-rules (!!!)
    [(do* !!! (inits ...) (updates ...)
       ((var init update) more ...) until body ...)
     (do* !!! (inits ... (var init)) (updates ... (var update))
       (more ...) until body ...)]
    [(do* !!! (inits ...) (updates ...)
       ((var init) more ...) until body ...)
     (do* !!! (inits ... (var init)) (updates ... )
       (more ...) until body ...)]
    [(do* !!! inits updates () until body ...)
     (do*-aux inits updates until body ...)]
    [(do* inits-updates until stuff ...)
     (do* !!! () () inits-updates until stuff ...)]))

(do* ((x 0 (+ 1 x))
      (y 922))
  ((= 9 x) (print 'ok))
  (print x " " y))

0 922
1 922
2 922
3 922
4 922
5 922
6 922
7 922
8 922
ok