如何在R5RS方案中“显示"多个参数 [英] How to 'display' multiple parameters in R5RS Scheme

问题描述

在R5RS方案中，如何通过一次调用显示多个参数?我在下面的实现有效，但添加了额外的括号和空格.

In R5RS Scheme how do you display multiple parameters, with a single call? my implementation below works, but adds extra parentheses and spaces.

#!/usr/bin/env racket
#lang r5rs
(define (display-all . rest) (display rest))
(display-all "I " "have " "a " "lovely " "bunch " "of " "coconuts\n")

产生

owner@K53TA:~$ ./Template.ss
(I  have  a  lovely  bunch  of  coconuts
)

推荐答案

最简单:

(define (display-all . vs)
  (for-each display vs))

请注意，使用for-each代替map-for-each是相同的事情，但是假定您只是出于副作用而调用该函数，因此不要返回结果列表(使用map与display一起将返回void的列表)，它仅返回void.

Note the use of for-each instead of map - for-each is the same thing but assumes you're only calling the function for side-effects, so instead of returning a list of results (using map with display would return a list of voids) it just returns void.

如果要显示非字符串内容并在它们之间留有间距，这可能会很烦人，例如，如果您想(显示全部12个香蕉")显示字符串"12个香蕉"，则必须手动将数字成字符串，然后自己添加空格.仅在列表的元素之间添加空格会更容易:

This can get annoying if you want to display non-string things and have spacing between them, for instance if you want (display-all 12 "bananas") to display the string "12 bananas" you have to manually turn the number into a string and add the space yourself. It would be easier to just add spaces in-between elements of the list:

(define (insert-between v xs)
  (cond ((null? xs) xs)
        ((null? (cdr xs)) xs)
        (else (cons (car xs)
                    (cons v (insert-between v (cdr xs)))))))

(define (display-all . vs)
  (for-each display (insert-between " " vs)))

现在称呼这个:

(display-all "blah" 4 "bloo")

符合您的期望.如果不想自动插入空格，则可以指定另一个参数作为分隔符对象，并根据需要使用它.这是一个接受分隔符对象的版本:

does what you'd expect. If you don't want the spaces inserted automatically, you can specify another argument as the separator object and use it however you need. Here's a version that accepts a separator object:

(define (display-all sep . vs)
  (for-each display (insert-between sep vs)))

但是，在支持可选参数和关键字参数的方案版本中，这种方法更有意义，因此您可以将其默认为空格或空字符串，而不会干扰rest-args.

This approach would make more sense in a version of scheme that supports optional and keyword arguments however, so you could default it to either a space or the empty string and not interfere with the rest-args.

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