Scala Option返回类型

Question

我是Scala编程世界的新手,但喜欢它.最近我开始将我的研究应用程序移植到Scala中,而我仍在努力的一件事就是返回关键字.例如,在下面的代码中

def readDocument(dbobj:MongoDBObject) = Option[ContainerMetaData] 
{
  for(a <- dbobj.getAs[String]("classname");
      b <- dbobj.getAs[Long]("id"); 
      c <- dbobj.getAs[Long]("version");
      d <- dbobj.getAs[String]("description");
      e <- dbobj.getAs[String]("name");
      f <- dbobj.getAs[String]("tag");
      g <- dbobj.getAs[Int]("containertype");
      h <- dbobj.getAs[Date]("createddate")
  )
  {
      val ctype = ContainerType(g)
      val jodadt = new DateTime(h) 
      val data = new ContainerMetaData(a,b,c,d,e,f,ctype,jodadt)
      Some(data)
  }
  None
}

在上面的代码中我收到错误消息:

type mismatch;  found   : None.type  required: om.domain.ContainerMetaData  

因此,如果我删除显式返回类型代码工作,但没有显式返回关键字我无法终止我的代码Some(data).

def readDocument(dbobj:MongoDBObject)= 
{
  for(a <- dbobj.getAs[String]("classname");
      b <- dbobj.getAs[Long]("id"); 
      c <- dbobj.getAs[Long]("version");
      d <- dbobj.getAs[String]("description");
      e <- dbobj.getAs[String]("name");
      f <- dbobj.getAs[String]("tag");
      g <- dbobj.getAs[Int]("containertype");
      h <- dbobj.getAs[Date]("createddate")
  )
  {
      val ctype = ContainerType(g)
      val jodadt = new DateTime(h) 
      val data = new ContainerMetaData(a,b,c,d,e,f,ctype,jodadt)
      Some(data)
  }
  None
}

如果添加一个return关键字,那么编译器会抱怨

method `readDocument` has return statement; needs result tye

更多的额外信息,这是我扩展的特点

trait MongoDAOSerializer[T] {
    def createDocument(content:T) :  DBObject
    def readDocument(db:MongoDBObject) : Option[T]
}

Answer 1

问题是,你yield在for-comprehension 中缺少关键字.并且None最后也是不必要的,因为None如果其中一个值缺失并且Some不需要在理解中明确创建a,那么for-comprehension将会产生,因为它将创建一个Option无论如何.您的代码看起来像这样(未经测试)

def readDocument(dbobj: MongoDBObject): Option[ContainerMetaData] = {
  for {
      a <- dbobj.getAs[String]("classname")
      b <- dbobj.getAs[Long]("id")
      c <- dbobj.getAs[Long]("version")
      d <- dbobj.getAs[String]("description")
      e <- dbobj.getAs[String]("name")
      f <- dbobj.getAs[String]("tag")
      g <- dbobj.getAs[Int]("containertype")
      h <- dbobj.getAs[Date]("createddate")
  } yield {
      val ctype = ContainerType(g)
      val jodadt = new DateTime(h) 
      new ContainerMetaData(a,b,c,d,e,f,ctype,jodadt)
  }
}