尝试为参数化的半身像创建通用零时使用Nothing bottom类型的问题

我已经尝试了一些方法，例如用[ObjectType,T <% ObjectType]替换我的签名，但都没有成功.关于如何解决我的问题的任何想法?解决方案可以使ObjectType协变吗?trait TypesafeQuery[+ObjectType, BuildType] { def build: BuildType}trait TypesafeMongoQuery[+ObjectType] extends TypesafeQuery[+ObjectType, DBObject]object AlwaysMatchingTypesafeQuery extends TypesafeMongoQuery[Nothing] { override def build(): DBObject = AlwaysMatchingQuery }object NeverMatchingTypesafeQuery extends TypesafeMongoQuery[Nothing] { override def build(): DBObject = NeverMatchingQuery }Here's my code. It permits to create typesafe MongoDB queries using Casbahtrait 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) } }}It works fine, except I'm not satisfied with these lines: def AlwaysMatchingTypesafeQuery[ObjectType] = new TypesafeMongoQuery[ObjectType] { override def build(): DBObject = AlwaysMatchingQuery } def NeverMatchingTypesafeQuery[ObjectType] = new TypesafeMongoQuery[ObjectType] { override def build(): DBObject = NeverMatchingQuery }I think It would be possible to not create a new instance of these 2 objects for each folding operation, but rather using a val / singleton of type TypesafeMongoQuery[Nothing] since the underlying DBObject being built would always be the same.I've tried some things, like replacing my signatures everywhere by [ObjectType,T <% ObjectType] but with no great success.Any idea on how to solve my problem? 解决方案 Can you make ObjectType covariant?trait TypesafeQuery[+ObjectType, BuildType] { def build: BuildType}trait TypesafeMongoQuery[+ObjectType] extends TypesafeQuery[+ObjectType, DBObject]object AlwaysMatchingTypesafeQuery extends TypesafeMongoQuery[Nothing] { override def build(): DBObject = AlwaysMatchingQuery }object NeverMatchingTypesafeQuery extends TypesafeMongoQuery[Nothing] { override def build(): DBObject = NeverMatchingQuery } 这篇关于尝试为参数化的半身像创建通用零时使用Nothing bottom类型的问题的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！