什么是“回报"?在计划? [英] What is the "return" in scheme?

问题描述

我正在尝试将一些 Python 代码转换为方案.

I am trying to translate some python code to scheme.

def test(x):
    if x > 1:
        do something
    if x > 10:
        return
    if x == 4: 
        do something
test(11)

我没有在方案中找到回报".当 x > 10 时，我想退出测试功能.如何模拟方案中的回报"?(我正在使用诡计)谢谢

I did not find the "return" in scheme. I want to quit from the test function when x >10. How can I simulate the "return" in scheme? (I am using guile) Thanks

推荐答案

在 Scheme 中没有明确的 return 关键字 - 它比这简单得多，最后一个表达式的值表达式序列是返回的表达式.例如，您的 Python 代码将转换为这样，并注意 (> x 10) 案例必须移动到底部，因此如果为真，它可以立即退出函数 (> x 10)代码>#f 值.

In Scheme there isn't an explicit return keyword - it's a lot simpler than that, the value of the last expression in a sequence of expressions is the one that gets returned. For example, your Python code will translate to this, and notice that the (> x 10) case had to be moved to the bottom, so if it's true it can exit the function immediately with a #f value.

(define (test x)
  (if (> x 1)
      (do-something))
  (if (= x 4)
      (do-something))
  (if (> x 10)
      #f))

(test 11)
=> #f

事实上，在对条件重新排序后，我们可以删除最后一个，但要注意:如果 x 不是 4，根据 Guile 的 文档 - 换句话说，您应该始终在每个case 和 if 表达式应该同时包含后续部分和替代部分.

In fact, after reordering the conditions we can remove the last one, but beware: an unspecified value will be returned if x is not 4, according to Guile's documentation - in other words, you should always return a value in each case, and an if expression should have both consequent and alternative parts.

(define (test x)
  (if (> x 1)
      (do-something))
  (if (= x 4)
      (do-something)))

(test 11)
=> unspecified

顺便说一句，我认为 Python 代码中的逻辑有点偏离.每当传递大于 1 的 x 值时，将始终评估第一个条件，但如果它小于或等于 1Python 中的返回值将是 None 并且在 Scheme 中是未指定的.此外，原始函数没有明确返回值 - 在 Python 中这意味着 None 将被返回，在 Scheme 中，返回值将是 (do-something) 如果x 恰好是 4，或者在任何其他情况下未指定.

And by the way, I believe the logic in the Python code is a bit off. The first condition will always be evaluated whenever a value of x greater than 1 is passed, but if it is less than or equal to 1 the returned value in Python will be None and in Scheme is unspecified. Also the original function isn't explicitly returning a value - in Python this means that None will be returned, in Scheme the returned value will be either (do-something) if x happens to be 4, or unspecified in any other case.

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