Hi,

In dryads-wake I need selection of the element in a list in a macro from
user-input. Currently I have multiple macros, and the correct one (which
strips the non-selected choices) is selected in a simple cond:

(define-syntax-rule (Choose resp . choices)
   "Ask questions, apply consequences"
   (cond
    ((equal? resp 1) ;; resp is user-input. It is a natural number.
     (Respond1 choices))
    ((equal? resp 2)
     (Respond2 choices))
    ((equal? resp 3)
     (Respond3 choices))
    (else
     #f)))

For this however I have three syntax-case macros:

(define-syntax Respond1
  (lambda (x)
    (syntax-case x ()
      ((_ ((question consequences ...) choices ...))
        #`(begin
           (respond consequences ...)))
      ((_ (choices ...))
        #`(begin #f)))))

(define-syntax Respond2
  (lambda (x)
    (syntax-case x ()
      ((_ (choice choices ...))
        #`(begin
           (Respond1 (choices ...))))
      ((_ (choices ...))
        #`(begin #f)))))

(define-syntax Respond3
  (lambda (x)
    (syntax-case x ()
      ((_ (a b choices ...))
        #`(Respond1 (choices ...)))
      ((_ (choices ...))
        #`(begin #f)))))


I would like to get rid of those three definitions and replace them by
at most two (one that strips N initial list entries, and Respond1).

I cannot move to procedures, because I have code that must be executed
only during final processing, and when I evaluate any of the
consequences (as it happens with procedure-arguments), then the timing
of the code execution does not match anymore. So I must absolutely do
this in macros.


I’ve tried to get that working, but all my tries failed. Is there a way
and can you show it to me?

This is a minimal working example. The output should stay the same,
except for part 4, which needs this change to work (see at the bottom),
but I would like to:

- replace Respond2 and Respond3 by something recursive, so resp can have
  arbitrary high values (not infinite: max the length of the options) and
- replace the cond-clause by a call to the recursive macro.

(define-syntax-rule (respond consequence consequence2 ...)
  (begin
    (write consequence)
    (when (not (null? '(consequence2 ...)))
      (write (car (cdr (car `(consequence2 ...))))))))

(define-syntax Respond1
  (lambda (x)
    (syntax-case x ()
      ((_ ((question consequences ...) choices ...))
        #`(begin
           (respond consequences ...)))
      ((_ (choices ...))
        #`(begin #f)))))

(define-syntax Respond2
  (lambda (x)
    (syntax-case x ()
      ((_ (choice choices ...))
        #`(begin
           (Respond1 (choices ...))))
      ((_ (choices ...))
        #`(begin #f)))))

(define-syntax Respond3
  (lambda (x)
    (syntax-case x ()
      ((_ (a b choices ...))
        #`(Respond1 (choices ...)))
      ((_ (choices ...))
        #`(begin #f)))))


(define-syntax-rule (Choose resp . choices)
   "Ask questions, apply consequences"
   (cond
    ((equal? resp 1)
     (Respond1 choices))
    ((equal? resp 2)
     (Respond2 choices))
    ((equal? resp 3)
     (Respond3 choices))
    (else
     #f)))


(display "Choose 1: should be bar:")
(Choose 1 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar))
(newline)
(display "Choose 2: should be warhar:")
(Choose 2 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar))
(newline)
(display "Choose 3: should be mar:")
(Choose 3 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar))
(newline)
(display "Choose 4: should be tar:")
(Choose 4 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar))
(newline)
(display "Choose 5: should be #f:")
(Choose 5 (foo 'bar) (foo 'war 'har) (foo 'mar) (foo 'tar))
(newline)


Best wishes,
Arne
-- 
Unpolitisch sein
heißt politisch sein
ohne es zu merken