shift rest in R6RS

partcont.scm

(library (partcont)
    (export shift reset)
    (import (rnrs) (srfi :39))
  (define *meta-continuation*
    (make-parameter
     (lambda value
       (error "No top-level RESET" value))))

  (define-syntax reset
    (syntax-rules ()
      ((reset body ...)
       (let ((mc (*meta-continuation*)))
         (call-with-current-continuation
	  (lambda (k)
	    (*meta-continuation*
	     (lambda (value)
	       (*meta-continuation* mc)
	       (k value)))
	    (let-values ((result (begin body ...)))
	      ;;** do not beta-substitute!!
	      (apply (*meta-continuation*) result))))))))

  (define-syntax shift
    (syntax-rules ()
      ((shift var body ...)
       (call-with-current-continuation
	(lambda (k)
	  (let-values ((result (let ((var (lambda value
					    (reset (apply k value)))))
				 body ...)))
	    ;;** do not beta-substitute!!
	    (apply (*meta-continuation*) result)))))))
)