方案“不是函数"错误 [英] Scheme "Not a function" error

问题描述

我正在学习 Scheme 并且我不断收到此错误:错误:20 不是函数"来自以下代码:

I am learning Scheme and I keep getting this error: "Error: 20 is not a function" from the following code:

(define myFunction (lambda (x y)
(* x y)))
(define (higherOrder func x y)
(
    func x y))

(display ((higherOrder myFunction 4 5)))

我试图传递一个函数作为参数之一.它通过数学运算，因为它在错误消息中说20"和 (5 * 4 = 20) 但它认为它是一个函数.问题是什么?我想不明白.我在 https://repl.it/languages/Scheme 上运行此代码.

I am trying to pass a function as one of the arguments. It goes through with the math since it says "20" in the error message and (5 * 4 = 20) but then it thinks it is a function. What is the problem? I cannot figure it out. I am running this code on https://repl.it/languages/Scheme.

推荐答案

你的括号对太多了，表达式 (higherOrder myFunction 4 5) 求值为整数 20，然后 repl尝试计算 (20)，它不能，因为 20 不是函数.当 Scheme 计算一个列表(其中列表是括号中没有被引用的任何内容)时，列表中的第一个条目被假定为一个函数.

You have one too many pairs of parens, the expression (higherOrder myFunction 4 5) evaluates to the integer 20, then the repl tries to evaluate (20), which it can't because 20 isn't a function. When Scheme evaluates a list (where a list is anything in parens that isn't quoted) the first entry in the list is assumed to be a function.

将最后一行改为

(display (higherOrder myFunction 4 5))

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