Scala Option return type

scala

Solution

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)
  }
}

Problem

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] } ```

Original source