(define-syntax generator-lambda
  (lambda (stx)
    (syntax-case stx ()
      [(generator-lambda body body* ...)
       (with-syntax ([yield (datum->syntax #'stx 'yield)])
         #'(lambda (yield) body body* ...))])))
(define a
  (generator-lambda
   (let loop ([i 0])
     (when (< i 3)
       (yield i)
       (loop (+ i 1))))))

(a (lambda (s) (display s)))