Implement yield and send in Scheme

Question

I\'m trying to port yield and yield from from Python to Scheme.

Here is an implementation I\'ve done:

(define (coroutine ro         


        

Answer 1

Something like this:

(define (make-generator procedure)
  (define last-return values)
  (define last-value #f)
  (define (last-continuation _) 
    (let ((result (procedure yield))) 
      (last-return result)))

  (define (yield value)
    (call/cc (lambda (continuation)
               (set! last-continuation continuation)
               (set! last-value value)
               (last-return value))))

  (lambda args
    (call/cc (lambda (return)
               (set! last-return return)
               (if (null? args)
                   (last-continuation last-value)
                   (apply last-continuation args))))))

Used like this:

(define test 
 (make-generator
   (lambda (collect)
     (collect 1)
     (collect 5)
     (collect 10)
     #f)))

(test) ; ==> 1
(test) ; ==> 5
(test) ; ==> 10
(test) ; ==> #f (procedure finished)

Now we can wrap the internals into a macro:

(define-syntax (define-coroutine stx)
  (syntax-case stx ()
    ((_ (name . args) . body )
     #`(define (name . args)
         (make-generator 
          (lambda (#,(datum->syntax stx 'yield))
            . body))))))

Notice that define-coroutine is implemented using syntax-case since we need to make yield unhygienic.

(define-coroutine (countdown-from n)
  (let loop ((n n))
    (if (= n 0)
        0
        (loop (- (yield n) 1)))))

(define countdown-from-10 (countdown-from 10))

(define (ignore procedure)
  (lambda ignore
    (procedure)))

(map (ignore countdown-from-10) '(1 1 1 1 1 1)) ; ==> (10 9 8 7 6 5)

;; reset
(countdown-from-10 10)  ; ==> 9
(countdown-from-10)     ; ==> 8
;; reset again
(countdown-from-10 100) ; ==> 99