Scalaz7中的“序列"在哪里 [英] Where is `sequence` in Scalaz7

问题描述

我正在学习 Scalaz，并且我有一个已经使用 Scalaz7 的项目.按照这个问题我想使用该功能

I am learning Scalaz and I have a project that already makes use of Scalaz7. Following this question I would like to use the function

sequence[T](l: List[Option[T]]): Option[List[T]]

(并不是说自己写起来很难).但上述问题提到了Scalaz6.

(not that it is hard to write it myself). But the aforementioned question mentions Scalaz6.

Scalaz7 中的序列函数在哪里?

Where to find the sequence function in Scalaz7?

推荐答案

它定义在 scalaz.Traverse 类型类，如下所示:

It's defined in the scalaz.Traverse type class, where it looks like this:

def sequence[G[_]:Applicative,A](fga: F[G[A]]): G[F[A]] =
  traversal[G].run[G[A], A](fga)(ga => ga)

scalaz.syntax.TraverseOps 提供了一个被拉到 List 的版本，因为 List 有一个 Traverse 实例.

scalaz.syntax.TraverseOps provides a version that gets pimped onto List, since List has a Traverse instance.

您可以只导入您需要的内容:

You can either import just what you need:

import scalaz._, std.list._, std.option._, syntax.traverse._

或者所有东西和厨房水槽:

Or everything and the kitchen sink:

import scalaz._, Scalaz._

然后你可以像这样使用它:

And then you can use it like this:

scala> val xs: List[Option[Int]] = Some(1) :: Some(2) :: Nil
xs: List[Option[Int]] = List(Some(1), Some(2))

scala> xs.sequence
res0: Option[List[Int]] = Some(List(1, 2))

或者，如果您想要问题中的确切表述:

Or if you want exactly the formulation in your question:

scala> def sequence[T](l: List[Option[T]]): Option[List[T]] = l.sequence
sequence: [T](l: List[Option[T]])Option[List[T]]

scala> sequence(xs)
res1: Option[List[Int]] = Some(List(1, 2))

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