function - 方案 lambda 函数中的默认参数？

Question

我最近开始使用 ChickenScheme，现在我想声明一个带有默认参数(如果未指定)的函数。我在 Racket 网站上找到了这个例子，我知道 Racket 和 ChickenScheme 是不同的，但我认为这些基本的东西是相同的。

(define greet
  (lambda (given [surname "Smith"])
    (string-append "Hello, " given " " surname)))

这是来自 ChickenScheme 解释器的错误:

Error: during expansion of (lambda ...) - in `lambda' - lambda-list expected: (lambda (given (surname "Smith")) (string-append "Hello, " given " " surname))

Call history:

<syntax>      (define greet (lambda (given (surname "Smith")) (string-append "Hello, " given " " surname)))
<syntax>      (##core#set! greet (lambda (given (surname "Smith")) (string-append "Hello, " given " " surname)))
<syntax>      (lambda (given (surname "Smith")) (string-append "Hello, " given " " surname))    <--

Answer 1

在普通方案中，您可以使用 dotted tail notation获取附加参数的列表，然后检查此列表以查看是否提供了附加参数:

(define greet
   (lambda (given . rest)
     (let ((surname (if (pair? rest) (car rest) "Smith")))
       (string-append "Hello, " given " " surname))))

因为这不是很方便，不同的Scheme系统为此提供了不同的替代方案。在CHICKEN，我们支持DSSSL annotations ，所以你可以这样做:

 (define greet
   (lambda (given #!optional (surname "Smith"))
     (string-append "Hello, " given " " surname)))