“等于（=）”的区别是什么？和“相同（==）”。在ocaml？ [英] What's the difference between "equal (=)" and "identical (==)" in ocaml?

问题描述

在 OCaml 中，我们有两种权益比较：

x = y 和 x == y ，

那么它们之间的确切区别是什么？

在ocaml中是 x = y 就像Java中的 x.equals（y）
$ b 和 x == y 就像 x == y（比较地址） 在Java中？

解决方案

我不知道 x是如何。等于（y）在Java中工作。如果它进行了深入的比较，那么类比就相当接近。有一点需要注意的是，物理平等在OCaml中是一个滑溜的概念（一般而言是泛函）。编译器和运行时系统将会移动值，并且可以随意合并和取消纯（非可变）值。所以你应该只使用 == ，如果你真的知道你在做什么。在某种程度上，它需要熟悉实现（除非必要，这是需要避免的）。

OCaml为创建的具体保证==> 很弱。可变值在物理上与您所期望的相同（即，如果两个变异中的一个实际上会使另一个变异）。但对于不可变的值，唯一的保证是比较物理上相等的值（ == ）也会相等为（ = ）。请注意，相反是 not true，因为sepp2k指出了浮动值。

从本质上讲，语言规范告诉你什么 - 可变值是您可以使用 == 作为快速检查来确定两个非可变值是否相等（ = ）。如果他们比较物理上相等，他们在价值上是平等的。如果他们没有比较物理上的平等，你不知道他们的价值是否相等。您仍然必须使用 = 来决定。

In OCaml, we have two kinds of equity comparisons:

x = y and x == y,

So what's exact the difference between them?

Is that x = y in ocaml just like x.equals(y) in Java?

and x == y just like x == y (comparing the address) in Java?

解决方案

I don't know exactly how x.equals(y) works in Java. If it does a "deep" comparison, then the analogy is pretty close. One thing to be careful of is that physical equality is a slippery concept in OCaml (and functional languages in general). The compiler and runtime system are going to move values around, and may merge and unmerge pure (non-mutable) values at will. So you should only use == if you really know what you're doing. At some level, it requires familiarity with the implementation (which is something to avoid unless necessary).

The specific guarantees that OCaml makes for == are weak. Mutable values compare as physically equal in the way you would expect (i.e., if mutating one of the two will actually mutate the other also). But for non-mutable values, the only guarantee is that values that compare physically equal (==) will also compare as equal (=). Note that the converse is not true, as sepp2k points out for floating values.

In essence, what the language spec is telling you for non-mutable values is that you can use == as a quick check to decide if two non-mutable values are equal (=). If they compare physically equal, they are equal value-wise. If they don't compare physically equal, you don't know if they're equal value-wise. You still have to use = to decide.

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