一对如何与箭头函数的类型统一

发布于 2024-10-14 12:08:19 字数 584 浏览 3 评论 0原文

一些使用箭头的函数在成对使用时非常方便。但我无法理解这些函数的类型如何与一对统一。总的来说，我发现与 Arrow 相关的函数的类型非常混乱。

例如，我们有 first :: abc -> a (b, d) (c, d)，这对我来说意义不大。但它可以用来增加一对中的第一个数字：

Prelude Control.Arrow> :t first (+1)
first (+1) :: (Num b) => (b, d) -> (b, d)

有人可以

Prelude Control.Arrow> :t (&&&)
(&&&) :: (Arrow a) => a b c -> a b c' -> a b (c, c')

Prelude Control.Arrow> :t (pred &&& succ)
(pred &&& succ) :: (Enum b) => b -> (b, b)

解释一下这是如何工作的吗？

原文

Some of the functions for working with Arrows are quite handy to use on pairs. But I can't understand how the types of these functions unify with a pair. In general, I find the types of the Arrow related functions to be quite confusing.

For example, we have first :: a b c -> a (b, d) (c, d), which means little to me. But it can be used to, say, increment the first number in a pair:

Prelude Control.Arrow> :t first (+1)
first (+1) :: (Num b) => (b, d) -> (b, d)

And

Prelude Control.Arrow> :t (&&&)
(&&&) :: (Arrow a) => a b c -> a b c' -> a b (c, c')

Prelude Control.Arrow> :t (pred &&& succ)
(pred &&& succ) :: (Enum b) => b -> (b, b)

Could someone please explain how this works?

分享到QQ

分享到微博

如果你对这篇内容有疑问，欢迎到本站社区发帖提问参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。

发布评论

需要登录才能够评论，你可以免费注册一个本站的账号。

掩于岁月 2024-10-21 12:08:19

有一个箭头 (->) 的实例。如此

(&&&) :: (Arrow a) => a b c -> a b c' -> a b (c,c')

实例化也是

(&&&) :: (->) b c -> (->) b c' -> (->) b (c,c')

，或者用更传统的符号编写，

(&&&) :: (b -> c) -> (b -> c') -> (b -> (c,c'))

其余的应该由此而来。

我在 上一直使用箭头函数（尤其是 (***) 和 (&&&)） >(->) 实例。我很少将这些组合器用于任何其他 Arrow 实例。因此，每当您看到 ab c 时，请考虑“从 b 到 c 的（广义）函数”，这也适用于常规函数。

There is an instance for Arrow (->). So

(&&&) :: (Arrow a) => a b c -> a b c' -> a b (c,c')

has the instantiation

(&&&) :: (->) b c -> (->) b c' -> (->) b (c,c')

or, written in more conventional notation,

(&&&) :: (b -> c) -> (b -> c') -> (b -> (c,c'))

The rest should follow from that.

I use the arrow functions (especially (***) and (&&&)) all the time on the (->) instance. My usage of those combinators for any other instance of Arrow is very rare. So whenever you see a b c, think "(generalized) function from b to c", which works for regular functions too.

回复收藏 0 原文

饮惑 2024-10-21 12:08:19

第一个箭头采用普通箭头，并将其更改为对元组中的第一个元素执行操作，并将结果输出为箭头，

a b c -> a (b, d) (c, d)

a b c -- is the input arrow, an operation that maps type b to c
a (b, d) (c, d) -- is the output arrow, an operation that maps a tuple (b, d) to (c, d)

它使用 d 作为元组中未知的第二类型的虚拟变量

&&&接受两个接受相同输入的箭头，并创建一个接受该输入的箭头，将其复制到一个元组中，并在元组的每个部分上运行其中一个箭头，返回更改后的元组。

对于一些扎实的教程，请查看：
http://www.vex.net/~trebla/haskell /hxt-arrow/lesson-0.xhtml

The first arrow takes a normal arrow, and changes it to perform its operation on the first element in a tuple and outputs the result as an arrow

a b c -> a (b, d) (c, d)

a b c -- is the input arrow, an operation that maps type b to c
a (b, d) (c, d) -- is the output arrow, an operation that maps a tuple (b, d) to (c, d)

it uses d as a dummy for the unknown second type in the tuple

&&& takes two arrows that take the same input and creates an arrow that takes that input, duplicates it into a tuple and runs one of the arrows on each part of the tuple, returning the altered tuple.

for some solid tutorial, check out:
http://www.vex.net/~trebla/haskell/hxt-arrow/lesson-0.xhtml

回复收藏 0 原文