Scala隱式轉換

Question

我無法讓此隱式轉換正常工作。 我不斷收到這些錯誤：

[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){ 

我的代碼：

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) 
} 

它說的是錯的類型限制？ 任何幫助，不勝感激。

Answer 1

下面是更加慣用的方式來實現您的要求。這個邏輯應該給你在運算符和文檔之間的分組，而不使用複雜的嵌套foldJoins。

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 })) 
    } 
    }