“等于 (=)" 和有什么不一样?和“相同(==)"在ocaml? [英] What's the difference between "equal (=)" and "identical (==)" in ocaml?

问题描述

在OCaml中，我们有两种股权比较:

x = y 和 x == y,

那么它们之间的确切区别是什么?

So what's exact the difference between them?

ocaml 中的 x = y 和 Java 中的 x.equals(y) 一样吗?

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

and x == y 就像Java中的x == y(比较地址)?

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

推荐答案

我不知道 x.equals(y) 在 Java 中究竟是如何工作的.如果它进行深度"比较，则类比非常接近.需要注意的一件事是，物理平等在 OCaml(以及一般的函数式语言)中是一个模糊的概念.编译器和运行时系统将移动值，并且可以随意合并和取消合并纯(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，否则应避免这种情况).

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).

OCaml 为 == 做出的具体保证是弱的.可变值在物理上以您期望的方式进行比较(即，如果改变两者之一实际上也会改变另一个).但是对于非可变值，唯一的保证是比较物理上相等的值 (==) 也将比较相等 (=).请注意，正如 sepp2k 指出的浮动值所指出的那样，相反的情况不为真.

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.

本质上，对于非可变值，语言规范告诉您的是，您可以使用 == 作为快速检查来确定两个非可变值是否相等(=).如果它们在物理上比较相等，则它们在价值方面是相等的.如果它们在物理上不相等，则您不知道它们在价值方面是否相等.您仍然必须使用 = 来决定.

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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