Question

通过FP in Scala的Monoid章节的第一次练习，尝试使用“listMonoid”创建特征的新实现时发生了编译时错误。

object MonoidTesting1 {
    trait Monoid[A] {
        def op(a1: A, a2: A): A
        def zero: A
    }

    val listMonoid = new Monoid[List[A]] {
        def op(a1: List[A], a2: List[A]) = a1 ++ a2
        val zero = Nil
    }
}


C:\Users\Kevin\Workspace\side-work\Monoid>scalac MonoidTesting.scala
MonoidTesting.scala:12: error: not found: type A
        val listMonoid = new Monoid[List[A]] {
                                         ^
MonoidTesting.scala:13: error: not found: type A
                def op(a1: List[A], a2: List[A]) = a1 ++ a2
                                ^
MonoidTesting.scala:13: error: not found: type A
                def op(a1: List[A], a2: List[A]) = a1 ++ a2
                                             ^
three errors found

如何创建listMonoid以使用未指定的泛型类型？

Answer 1

列表实际上是一个免费的幺半群。您需要在列表A范围内明确创建Monoid：

object MonoidTest {
  trait Monoid[A] {
    def op(a1: A, a2: A): A
    def zero: A
  }

  def listMonoid[A] = new Monoid[List[A]] {
    def op(a1: List[A], a2: List[A]) = a1 ++ a2
    val zero = List.empty[A]
  }
}

通用类型＆＃34; A＆＃34;未找到

1 个答案: