Scala Option返回类型

Question

I am newbie in Scala programming world but loving it. Recently I have started porting my research App into Scala and one of thing I am still struggling is the return keyword. For example in below code

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
}

In above code I get the error message:

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

So if I remove the explicit return type the code works but then without explicit return keyword I am not able to terminate my code at 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
}

And if add a return keyword then compiler complains

method `readDocument` has return statement; needs result tye

Few more additional info, this is the trait I am extending

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

Answer 1

The problem is, that you are missing the yield keyword in the for-comprehension. And also the None at the end is unnecessary, as the for-comprehension will yield None , if one of the values is missing and also the explicit creation of a Some in the comprehension is not needed, as it will create an Option anyway. Your code hase to look like this (not tested)

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)
  }
}