Scalaz：找不到幺半群

Question

我正在学习这个 scalaz 教程：http://eed3si9n.com/learning-scalaz/FoldLeft.html：

scala> object FoldLeftList {
         def foldLeft[A, B](xs: List[A], b: B, f: (B, A) => B) = xs.foldLeft(b)(f)
       }
defined module FoldLeftList

scala> def sum[A: Monoid](xs: List[A]): A = {
         val m = implicitly[Monoid[A]]
         FoldLeftList.foldLeft(xs, m.mzero, m.mappend)
       }
sum: [A](xs: List[A])(implicit evidence: Monoid[A])A

显示已进入 scala sbt 控制台，但我正在尝试运行在独立的 scala 文件中：

object main extends App {

  trait FoldLeft[F[_]] {
    def foldLeft[A, B](xs: F[A], b: B, f: (B, A) => B): B
  }
  object FoldLeft {
    implicit val FoldLeftList: FoldLeft[List] = new FoldLeft[List] {
      def foldLeft[A, B](xs: List[A], b: B, f: (B, A) => B) = xs.foldLeft(b)(f)
    }
  }

  def sum[M[_]: FoldLeft, A: Monoid](xs: M[A]): A = {
    val m = implicitly[Monoid[A]]
    val fl = implicitly[FoldLeft[M]]
    fl.foldLeft(xs, m.mzero, m.mappend)
  }

}

但我收到编译器错误：

[error] \src\main\scala\custom.scala:12: not found: type Monoid
[error]   def sum[M[_]: FoldLeft, A: Monoid](xs: M[A]): A = {
[error]                              ^
[error] \src\main\scala\custom.scala:13: not found: type Monoid
[error]     val m = implicitly[Monoid[A]]
[error]                        ^

是否应该导入 Monoid，如果是，位于何处？

更新：

似乎我错过了 Monoid def :

  trait Monoid[A] {
    def mappend(a1: A, a2: A): A
    def mzero: A
  }

Answer 1

scalaz 进口的标准是 import scalaz._, Scalaz._。您稍后可以删除 Scalaz._ 部分，然后只选择您需要的部分。

Scalaz：找不到幺半群

Scalaz : monoid not found

scala

scalaz