추상 데이터에서 명시적인 기계 상태까지 생각 하나를 이어 가세요.

(begin
  (define (make-sum left right) (list 'sum left right))
  (define (left-operand expression) (cadr expression))
  (define (right-operand expression) (caddr expression))
  (define (lookup name environment) (cdr (assoc name environment)))
  (define (evaluate expression environment)
    (if (symbol? expression)
        (lookup expression environment)
        (+ (evaluate (left-operand expression) environment)
           (evaluate (right-operand expression) environment))))
  (define (compile expression)
    (if (symbol? expression)
        (list (list 'load expression))
        (append (compile (left-operand expression))
                (append (compile (right-operand expression)) '(add)))))
  (define (step code environment state)
    (let ((pc (car state)) (stack (cadr state)))
      (let ((instruction (list-ref code pc)))
        (if (pair? instruction)
            (list (+ pc 1) (cons (lookup (cadr instruction) environment) stack))
            (list (+ pc 1) (cons (+ (cadr stack) (car stack)) (cddr stack)))))))
  (define (run code environment state)
    (if (= (car state) (length code)) state
        (run code environment (step code environment state))))
  (define (make-pipeline expression)
    (let ((code (compile expression)) (runs 0))
      (lambda (environment)
        (set! runs (+ runs 1))
        (list (evaluate expression environment)
              (run code environment (list 0 '())) runs))))
  (let ((pipeline (make-pipeline (make-sum 'x 'y))))
    (list (pipeline '((x . 4) (y . 5)))
          (pipeline '((x . 2) (y . 10))))))