gistfile1.scala

object SemiGroupApp {

  trait SemiGroup[T] {

    def append(a: T, b: T): T
  }

  /*object IntSemiGroup extends SemiGroup[Int] {
    def append(a: Int, b: Int) = a + b
  }*/

  trait Monoid[T] extends SemiGroup[T] {
    // 定义单位元
    def zero: T
  }

  object IntMonoid extends Monoid[Int] {
    // 二元操作
    override def append(a: Int, b: Int) = a + b

    // 单位元
    override def zero = 0
  }

  /*object StringMonoid extends Monoid[String] {
    override def append(a: String, b: String) = a + b

    override def zero = ""
  }*/

  implicit val strMonoid = new Monoid[String] {
    override def append(a: String, b: String) = a + b

    override def zero = ""
  }

  implicit val intMonoid = new Monoid[Int] {
    def append(a: Int, b: Int) = a + b
    def zero = 0
  }

  def listMonoid[T] = {
    new Monoid[List[T]] {
      override def append(a: List[T], b: List[T]) = a ++ b

      override def zero = Nil
    }
  }

  def acc[T](list: List[T], m: Monoid[T]) = {
    list.foldLeft(m.zero)(m.append)
  }

  def accImpl[T](list: List[T])(implicit m: Monoid[T]) = {
    list.foldLeft(m.zero)(m.append)
  }

  def accImply[T](list: List[T]) = {
   
    val m = implicitly[Monoid[T]]
    list.foldLeft(m.zero)(m.append)
  }

  def main(args: Array[String]) {

    val lm = accImply(List(1, 2, 3))
    
    println(lm)
  }
  
  第60行提示：could not find implicit value for parameter e: SemiGroupApp.Monoid[T]
  是什么原因？

你的方法声明错了，应该是：

def accImply[T:Monoid](list: List[T]) = {
  val m = implicitly[Monoid[T]]
  list.foldLeft(m.zero)(m.append)
}

注意： T:Monoid