(define-record-type :rule
  (really-make-rule prereqs wants-build? build-func)
  is-rule?
  (prereqs rule-prereqs)
  (wants-build? rule-wants-build?)
  (build-func rule-build-func))

(define rules (list))
(define lock-rules (make-lock))

(define (rule-make rule init-state)
  (let* ((res-pres (map (lambda (prereq) 
			  (rule-make prereq init-state))
			(rule-prereqs rule)))
	 (res-wants-build? (call-with-values 
			       (lambda () 
				 (apply values (append res-pres 
						       (list init-state))))
			     (rule-wants-build? rule)))
	 (build? (car res-wants-build?))
	 (cooked-state (cdr res-wants-build?)))
    (if build?
	(call-with-values 
	    (lambda () 
	      (apply values (append (list build?)
				    res-pres
				    (list cooked-state))))
	  (rule-build-func rule))
	res-wants-build?)))

(define (make-rule prereqs wants-build? build-func)
  (let ((rule (really-make-rule prereqs wants-build? build-func)))
    (with-lock lock-rules 
      (lambda () 
	(if (not (find (lambda (r) (eq? r rule)) rules))
	    (set! rules (cons rule rules))
	    (error "make-rule: rule already exists."))))
    rule))