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これが私のコードです。Casbah を使用してタイプセーフな MongoDB クエリを作成できます。

trait TypesafeQuery[ObjectType, BuildType] {

  def build: BuildType

}

trait TypesafeMongoQuery[ObjectType] extends TypesafeQuery[ObjectType, DBObject]

case class TypesafeMongoQueryConjunction[ObjectType](queries: Seq[TypesafeMongoQuery[ObjectType]]) extends TypesafeMongoQuery[ObjectType] {
  override def build(): DBObject = $and(queries.map(_.build))
}

case class TypesafeMongoQueryDisjunction[ObjectType](queries: Seq[TypesafeMongoQuery[ObjectType]]) extends TypesafeMongoQuery[ObjectType] {
  override def build(): DBObject = $or(queries.map(_.build))
}

object TypesafeMongoQuery {

  // TODO could probably be reworked? see http://stackoverflow.com/questions/23917459/best-way-to-create-a-mongo-expression-that-never-matches
  val AlwaysMatchingQuery: DBObject = $()
  val NeverMatchingQuery: DBObject = $and($("_id" -> 1), $("_id" -> -1))

  def AlwaysMatchingTypesafeQuery[ObjectType] = new TypesafeMongoQuery[ObjectType] { override def build(): DBObject = AlwaysMatchingQuery }
  def NeverMatchingTypesafeQuery[ObjectType] = new TypesafeMongoQuery[ObjectType] { override def build(): DBObject = NeverMatchingQuery }

  def and[ObjectType](queries: TypesafeMongoQuery[ObjectType]*) = TypesafeMongoQueryConjunction(queries)
  def or[ObjectType](queries: TypesafeMongoQuery[ObjectType]*) = TypesafeMongoQueryDisjunction(queries)

  // TODO maybe define Scalaz Monoids

  def foldAnd[ObjectType](queries: Seq[TypesafeMongoQuery[ObjectType]]) = {
    queries.foldLeft(AlwaysMatchingTypesafeQuery[ObjectType]) { (currentQuery, queryInList) =>
      TypesafeMongoQuery.and(currentQuery, queryInList)
    }
  }
  def foldOr[ObjectType](base: TypesafeMongoQuery[ObjectType], queries: Seq[TypesafeMongoQuery[ObjectType]]) = {
    queries.foldLeft(NeverMatchingTypesafeQuery[ObjectType]) { (currentQuery, queryInList) =>
      TypesafeMongoQuery.or(currentQuery, queryInList)
    }
  }

}

これらの行に満足していないことを除いて、正常に動作します。

  def AlwaysMatchingTypesafeQuery[ObjectType] = new TypesafeMongoQuery[ObjectType] { override def build(): DBObject = AlwaysMatchingQuery }
  def NeverMatchingTypesafeQuery[ObjectType] = new TypesafeMongoQuery[ObjectType] { override def build(): DBObject = NeverMatchingQuery }

フォールディング操作ごとにこれら2つのオブジェクトの新しいインスタンスを作成するのではなく、TypesafeMongoQuery[Nothing]構築される基礎となるDBObjectが常に同じであるため、タイプのval /シングルトンを使用することは可能だと思います。

署名を至る所で置き換えるなど、いくつかのことを試しました[ObjectType,T <% ObjectType]が、大きな成功はありませんでした。

私の問題を解決する方法について何か考えはありますか?

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