() 在 Haskell 中是什么意思 [英] What does () mean in Haskell

问题描述

在我遇到的一些 Haskell 代码中:

In some Haskell code I came across:

put :: s -> m ()

这里的 () 是什么意思?

What does the () mean here?

我会使用搜索引擎，但找不到正确处理 () 的搜索引擎.

I'd use a search engine, but I can't find one that handles () correctly.

推荐答案

如果不是奇怪的特殊语法，可以定义为

If not for the weird special syntax, it could be defined as

data () = ()

这是最无聊的类型.语法应该让你想到元组:(a,b) 是一对，(a,b,c) 是一个三元组，等等，() 是一个 0 元组.唯一缺少的是一个 1 元组，它不能有这种语法，因为它会与通常使用的括号冲突.

It's about the most boring type there is. The syntax is supposed to make you think of tuples: (a,b) is a pair, (a,b,c) is a triple, etc., and () is a 0-tuple. The only thing missing is a 1-tuple, which can't have that syntax because it would clash with the usual use of parentheses.

() 经常被用作没有有趣结果的结果.例如，一个 IO 操作应该执行一些 I/O 并终止而不产生结果，它通常具有类型 IO ().当你需要一些无趣的输入时也可以使用它；GHC 对此有特殊的语法，但在 Haskell 98 中模仿 Lisp 的 cond 的方式是这样的:

() is very often used as the result of something that has no interesting result. For example, an IO action that is supposed to perform some I/O and terminate without producing a result will typically have type IO (). It can also be used when you need some uninteresting input; GHC has special syntax for this, but in Haskell 98 the way to mimic Lisp's cond is like this:

case () of
  () | c1 -> e1
     | c2 -> e2
     ...
     | otherwise -> e3

询问您拥有 () 类型的值是完全有效的，但也非常无聊；你只能拥有一个合法的.

It's perfectly valid, but also perfectly boring, to ask what value of type () you have; there's only one legitimate one that you could have.

从Haskell-as-almost-category"的角度来看，() 是一个最终对象.也就是说，对于任何类型X，恰好有一个类型为X->的合法函数.()，即const().从另一个方向看，pigworker 提到的 Void 类型是一个初始对象.对于任何类型 X，恰好有一个类型为 Void -> 的合法函数.X，即荒谬.如果您想进行分类推理，那么使用初始对象和最终对象会很有用.

From the "Haskell-as-almost-category" standpoint, () is a final object. That is, for any type X, there is exactly one legitimate function of type X -> (), namely const (). From the other direction, the Void type pigworker mentions is an initial object. For any type X, there is exactly one legitimate function of type Void -> X, namely absurd. If you're in the mood for categorical reasoning, initial and final objects can be useful to have around.

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