SML中绑定的价值？

问题描述

有人可以解释为什么ans在这里经过评估后的价值是16吗？这是一个正确的答案吗？

我认为答案3自我们调用函数f并发送值1和2作为函数f也不会看到值5和10，但我想我错了。

  val x = 1 
 
 val y = 2 
 
 val f = fn y => x + y 
 
 val x = 5 
 
 val y = 10 
 
 val ans = fx + y 
  

解决方案

您所看到的有时称为词法作用域。函数 f 是在 x 的特定绑定范围内定义的，该范围是唯一重要的范围了解 f 在调用 f 时会做什么。 x 在调用 f 的范围内具有不同含义的事实不会影响 f 本身。在函数式编程的情况下，其他任何内容都会违反。在一个绑定的范围内，如

  val x = 1 
  

应该可以自由地用 1 >。因此，您定义 f 应该与定义相同：

  def fy = 1 + y 
  

的确如此。 p>

can someone please explain why is "ans" is bound to value of 16 in here after evaluation - this is a correct answer?

I thought the answer 3 since we're calling function f and sending values 1 and 2 as function f doesn't also see the values 5 and 10 but I guess I am wrong.

val x = 1

val y = 2

val f = fn y => x + y

val x = 5

val y = 10

val ans = f x + y

解决方案

What you are seeing is sometimes called lexical scoping. The function f was defined in the scope of a certain binding for x, that scope is the only scope that matters in understanding what f does when f is invoked. The fact that x has a different meaning in the scope in which f is invoked doesn't affect the meaning of f itself. In the context of functional programming anything else would violate referential transparency. In the scope of a binding such as

val x = 1

it should be possible to freely replace x by 1. Thus your definition of f should be equivalent to the definition:

def f y = 1 + y

as, indeed, it is.

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