使用球拍ISL定义函数使用列表抽象

Question

我试图创建一个函数，接受一个数字，然后产生文本，他们可以临时复制到他们的纸作为一个节省空间。以下是几个例子：

(define
  CITATION-SPACE-2
  (string-append
   "[1] author information\n"
   "title information\n"
   "venue information\n"
   "year & page information\n"
   "[2] author information\n"
   "title information\n"
   "venue information\n"
   "year & page information"))

(define
  CITATION-SPACE-3
  (string-append
   "[1] author information\n"
   "title information\n"
   "venue information\n"
   "year & page information\n"
   "[2] author information\n"
   "title information\n"
   "venue information\n"
   "year & page information\n"
   "[3] author information\n"
   "title information\n"
   "venue information\n"
   "year & page information"))

(check-expect (citation-space 2) CITATION-SPACE-2)
(check-expect (citation-space 3) CITATION-SPACE-3)

我已经能够使用cond和string-append来定义函数了，但是我想不出如何在结果的每一行之间都有一个\n (虽然不是在最后一行之后)。另外，我不确定如何使用foldr列表抽象来定义和简化函数。

这就是我迄今尝试过的：

(define (citation-space num)
  (local [; data-string : Nat -> String
          ; produces a space saver
          ; for a supplied number
          (define (data-string num)
            (foldl (λ (s1 s2)
                     (string-append s2 "\n" s1))
                   (string-append
                    "[" (number->string num) "]"
                    " author information")
                   (list "title information"
                         "venue information"
                         "year & page information")))]
    (cond
      [(zero? num) ""]
      [(= num 1) (data-string 1)]
      [else (string-append
             (citation-space (sub1 num))
             "\n"
             (data-string num))])))

Answer 1

如果您不必在ISL中进行此练习，则可以使用由string-join提供的racket/string。

如果不必使用foldl或foldr，则可以编写如下内容：

(define (my-join strs sep)
  (cond ((null? strs) "")
        ((= (length strs) 1) (first strs))
        (else (string-append (first strs)
                             sep
                             (my-join (rest strs) sep)))))

但是我认为用一对foldl也可以解决这个问题。使用的语言是中间学生与Lambda：

(define
  CITATION-SPACE-2
  (string-append
   "[1] author information\n"
   "title information\n"
   "venue information\n"
   "year & page information\n"
   "[2] author information\n"
   "title information\n"
   "venue information\n"
   "year & page information"))

(define
  CITATION-SPACE-3
  (string-append
   "[1] author information\n"
   "title information\n"
   "venue information\n"
   "year & page information\n"
   "[2] author information\n"
   "title information\n"
   "venue information\n"
   "year & page information\n"
   "[3] author information\n"
   "title information\n"
   "venue information\n"
   "year & page information"))

(define (data-string number)
  (foldl (lambda (s result)
           (string-append result "\n" s))
         (format "[~a] author information" number)
         (list "title information"
               "venue information"
               "year & page information")))

(define (citation-space number)
  (if (zero? number) ""
      (let ((strs (map data-string (range 1 (add1 number) 1))))
        (foldl (lambda (s result) (string-append result "\n" s))
               (first strs)
               (rest strs)))))

(check-expect (citation-space 2) CITATION-SPACE-2)
(check-expect (citation-space 3) CITATION-SPACE-3)

编辑:一个foldl的解决方案

(define (data-string number)
  (foldl (lambda (s result)
           (string-append result "\n" s))
         (format "[~a] author information" number)
         (list "title information"
               "venue information"
               "year & page information")))

(define (citation-space num)
  (cond
    [(zero? num) ""]
    [(= num 1) (data-string 1)]
    [else (string-append
           (citation-space (sub1 num))
           "\n"
           (data-string num))]))