Question

我试图在方案中解释lambda。这是我的代码：

(define get-operator (lambda (op-symbol)
  (cond
   ((equal? op-symbol '+) +)
   ((equal? op-symbol '-) -)
   ((equal? op-symbol '*) *)
   ((equal? op-symbol '/) /)
   (else (error "interpret: operator not implemented -->" op-symbol)))))

(define (apply-lambda clos actuals)
  (let* ((lam (cadr clos))
         (def-env (caddr clos))
         (formals (cadr lam))
         (body (caddr lam))
         (new-env (bind-all formals actuals def-env)))
    (val-body new-env)))


(define interpret (lambda (e env)
  (cond
   ((number? e) e)
   ((symbol? e) (get-value e env))
   ((not (list? e)) (error "interpret: cannot evaluate -->" e))
   ((if-stmt? e) (if (eq? (cadr e) 0)
                     (interpret (cadddr e) env)
                     (interpret (caddr e) env)))
   ((let-stmt? e) ;;GoZoner's part of code
    (let ((names (map car  (cadr e)))
          (inits (map cadr (cadr e))))
      ;; Evaluate inits in env
      (let ((vals (map (lambda (init) (interpret init env)) inits)))
        ;; Extend env with names+vals
        (let ((new-env (append (map cons names vals) env)))
          ;; Eval body in new env
          (interpret (caddr e) new-env)))))

   ((lambda-stmt? e) (apply-lambda e env))
   (else
    (let ((operands (map interpret (cdr e) (make-list (length (cdr e)) env)))
          (operator (get-operator (car e))))
      (apply operator operands))))))

当我尝试输入((lambda (n) (+ n 2)) 5)时，我收到一条错误消息“作为()的参数传递的对象safe-car不是一对”。为什么会出现这种情况？我确信lambda-stmt?函数运行良好，所以我没有在这里写，对apply-lambda的解释有问题，但我找不到它。

Answer 1

这似乎不对：

(lambda-stmt? e)

应该是：

(lambda-stmt? (car e))

Answer 2

解释lambda本身是微不足道的。

(define (make-closure formals body env)
  `(CLOSURE ,formals ,body ,env))
(define closure-formals cadr)
(define closure-body    caddr)
(define closure-env     cadddr)

(define (procedure? thing)
  (and (pair? thing) (eq? 'CLOSURE (car thing))))

然后在翻译中：

((lambda-stmt? e) (make-closure (cadr e) (caddr) env))

然后，您必须更改函数/运算符（您的'else'子句）的应用程序来处理过程（除了数学运算之外）。并且，您需要在调用时将该过程应用于其参数 - 但这是很多，如let（将lambda形式绑定到参数，解释{{1在新环境中的身体）。

[添加...]

我开始这么想：

lambda

在解释方案中的lambda时出错

2 个答案: