Scala隐式转换

Question

我很难让这个隐式转换正常工作。我一直收到这些错误：

[error]  

found: (scala.collection.immutable.Map[O,scala.collection.immutable.Seq[D]], O) => scala.collection.immutable.Map[O,scala.collection.immutable.Seq[D]]
required: 
(Object, Object) => Object  
     at (operToDocsMap: Map[O, Seq[D]], operator: O) =>

[error] type mismatch;
found   : Object
required: scala.collection.immutable.Map[O,scala.collection.immutable.Seq[D]]
 at .fold(operatorToDocsMap){

我的代码：

object ModelTypes {
  trait Document

  trait DocumentOperator {
    def operatesOn[D <: Document](document: D): Boolean
  }

  class Documents[D <: Document](docs: Seq[D]) {
    def groupByOperator[O <: DocumentOperator](operators: Seq[O])
      : Map[O, Seq[D]] = {
      docs.foldLeft(Map[O, Seq[D]]()) {
        (operatorToDocsMap: Map[O, Seq[D]], document: D) =>
          operators
            .filter(_.operatesOn(document))
            .fold(operatorToDocsMap){
              (operToDocsMap: Map[O, Seq[D]], operator: O) => 
                {operToDocsMap + (operator -> (document +: operToDocsMap.getOrElse(operator, Seq())))}
            }
      }
    }
  }
  implicit def documentsConverter[D <: Document](docs: Seq[D]) =
    new Documents(docs)
}

是不是类型边界有问题？任何帮助都是非常感谢的。

Answer 1

下面是实现您的需求的更常用的方法。这个逻辑应该在不使用复杂的嵌套foldJoins的情况下给出操作符和文档之间的分组。

  class Documents[D <: Document](docs: Seq[D]) {
    def groupByOperator[O <: DocumentOperator](operators: Seq[O]): Map[O, Seq[D]] = {
      val operatorDoc =
        for {
          doc <- docs
          operator <- operators if operator.operatesOn(doc)
        } yield (operator -> doc)
      operatorDoc
          .groupBy({ case (x, _) => x })
          .mapValues(_.map({ case (_, x) => x }))
    }
  }