SICP 实践 3.51 应用错误类型：#<syntax-transformer cons-stream>

Question

在SICP的实践3.51中，定义了一个过程"show"，并使用stream-map创建一个流：

(add-to-load-path ".")
(load "stream.scm")

(define (show x)
  (display-line x)
  x)

(define x0 (stream-enumerate-interval 0 3))

(display-stream x0)   ;succ, no error

(stream-map show x0)  ;all element printed, but interpreter report error at last

stream.scm中关于流的其他工作人员：

#!/usr/bin/guile
!#

(define (stream-null? s)
  (null? s))

(define (stream-ref s n)
  (if (= n 0)
      (stream-car s)
      (stream-ref (stream-cdr s) (- n 1))))

(define (stream-map proc s)
  (if (stream-null? s)
      the-empty-stream
      (cons-stream (proc (car s))
                   (stream-map proc (stream-cdr s)))))

(define (stream-for-each proc s)
  (if (stream-null? s)
      'done
      (begin
        (proc (stream-car s))
        (stream-for-each proc (stream-cdr s)))))

(define (display-stream s)
  (stream-for-each display-line s))

(define (display-line x)
  (display x))

(define (stream-car stream) (car stream))
(define (stream-cdr stream) (force (cdr stream)))

(define (stream-enumerate-interval low high)
  (if (> low high)
      the-empty-stream
      (cons-stream
        low
        (stream-enumerate-interval (+ low 1) high))))

(define (stream-filter pred stream)
  (cond
    ((stream-null? stream) the-empty-stream)
    ((pred (stream-car stream))
     (cons-stream (stream-car stream)
                  (stream-filter pred (stream-cdr stream))))
    (else
      (stream-filter pred (stream-cdr stream)))))

(define-syntax cons-stream
  (syntax-rules ()
    ((_ a b) (cons a (delay b)))))

(define the-empty-stream '())

(define (stream-enumerate-interval low high)
  (if (> low high)
      the-empty-stream
      (cons-stream low
                   (stream-enumerate-interval (+ low 1) high))))

错误是这样的：

0123Backtrace:
           8 (apply-smob/1 #<catch-closure 557e16ae8c20>)
In ice-9/boot-9.scm:
    705:2  7 (call-with-prompt ("prompt") #<procedure 557e16aef6a0 …> …)
In ice-9/eval.scm:
    619:8  6 (_ #(#(#<directory (guile-user) 557e16b9e140>)))
In ice-9/boot-9.scm:
   2312:4  5 (save-module-excursion #<procedure 557e16b24330 at ice-…>)
  3822:12  4 (_)
In stream.scm:
     25:8  3 (stream-map #<procedure show (x)> (0 . #<promise (1 . …>))
     25:8  2 (stream-map #<procedure show (x)> (1 . #<promise (2 . …>))
     25:8  1 (stream-map #<procedure show (x)> (2 . #<promise (3 . …>))
In unknown file:
           0 (_ 3 ())

我不知道为什么 display-stream 成功了，但是 stream-map "show" 失败了。代码与 SICP 中的示例相同。方案解释器是 'guile'.

有什么想法吗？谢谢

Answer 1

移动后错误消失

(define-syntax cons-stream
  (syntax-rules ()
    ((_ a b) (cons a (delay b)))))

到文件的顶部。

显然，在 Guile 中，它必须在文件中的第一个使用点之上定义。

您没有看到 stream-enumerate-interval 的错误，因为它被定义两次 - 最后一次低于定义cons-stream.

在使用 "guile 2.0.13".

的 https://ideone.com 中测试

SICP 实践 3.51 应用错误类型：#<syntax-transformer cons-stream>

SICP practise 3.51 Wrong type to apply: #<syntax-transformer cons-stream>

scheme

sicp

guile