Define a procedure make-prev that can be used to create the procedure prev described in part a. make-prev should take as an argument the value that should be returned by the first call to prev. That is, if we (define prev (make-prev ’first-call )) prev should behave as shown above.(use lisp piease)
时间: 2024-02-22 16:57:10 浏览: 18
以下是用 Lisp 实现的 `make-prev` 过程,它可以用来创建 `prev` 过程:
```lisp
(defun make-prev (init-value)
(let ((prev-value init-value))
(lambda ()
(let ((current-value prev-value))
(setf prev-value (read))
current-value))))
```
这个过程接受一个初始值 `init-value` 作为参数,并返回一个新过程。当调用这个新过程时,它会先返回初始化时传入的值,然后每次调用时返回上一次传入的参数。具体可以参考题目中给出的 `prev` 过程的行为。
例如,我们可以使用以下代码定义 `prev` 过程:
```lisp
(defvar prev (make-prev 'first-call))
```
现在,我们可以通过调用 `prev` 过程来获取上一个传入的值,例如:
```lisp
(funcall prev) ; => FIRST-CALL
(funcall prev) ; => NIL
(funcall prev) ; => HELLO
(funcall prev) ; => WORLD
```
相关问题
Define a procedure make-prev that can be used to create the procedure prev described in part a. make-prev should take as an argument the value that should be returned by the first call to prev. That is, if we (define prev (make-prev ’first-call )) prev should behave as shown above.
Here is a possible implementation of the `make-prev` procedure in Scheme:
```scheme
(define (make-prev initial-value)
(let ((prev-value initial-value))
(lambda (x)
(let ((result prev-value))
(set! prev-value x)
result))))
```
The `make-prev` procedure takes one argument `initial-value`, which is the value that should be returned by the first call to the `prev` procedure. The procedure returns a new anonymous procedure that behaves like the `prev` procedure described in the previous question. The `prev-value` variable is initialized to `initial-value`, and the anonymous procedure returned by `make-prev` takes one argument `x`. On the first call to the anonymous procedure, it returns `prev-value`, which is the initial value of `prev-value`. On subsequent calls, it returns the previous value of `x` that was passed to it, which is stored in `prev-value`, and updates `prev-value` to the current value of `x`. Here are some examples of using `make-prev`:
```scheme
(define prev (make-prev 'first-call))
(prev 'a) ;=> first-call
(prev 3) ;=> a
(prev (+ 1 5)) ;=> 3
```
Note that the `make-prev` procedure creates a new `prev` procedure each time it is called with a different initial value. This allows you to create multiple `prev` procedures with different initial values if needed.
Define a one-parameter procedure prev that returns as its value the argument that was passed to it the previous time it was called. The first time prev is called it should return the symbol first-call. (prev ’a) ’first-call (prev 3) ’a (prev (+ 1 5)) 3
Here is a possible implementation of the `prev` procedure in Scheme:
```scheme
(define (prev)
(let ((prev-value 'first-call))
(lambda (x)
(let ((result prev-value))
(set! prev-value x)
result))))
```
The `prev` procedure returns a new anonymous procedure that takes one argument `x`. The first time the anonymous procedure is called, it returns the symbol `first-call`, which is the initial value of the `prev-value` variable. On subsequent calls, the anonymous procedure returns the previous value of `x` that was passed to it, which is stored in `prev-value`. The `set!` special form is used to update the value of `prev-value` to the current value of `x`. Here are some examples of using `prev`:
```scheme
(define p (prev))
(p 'a) ;=> first-call
(p 3) ;=> a
(p (+ 1 5)) ;=> 3
```
Note that the `prev` procedure has no parameters, as requested in the problem statement. The parameter `x` is passed to the anonymous procedure returned by `prev`.