How to return different collection with yield statement

Question

Based on my limited knowledge i know compiler automatically inherits the collection return type and based on that it determines the type of collection to return so in below code i want to return Option[Vector[String]].

I tried to experiment with below code and i get compilation error

type mismatch;  found   : scala.collection.immutable.Vector[String]  required: Option[Vector[String]]

Code:

def readDocument(v:Option[Vector[String]]) : Option[Vector[String]] =    
{
   for ( a <- v;  
   b <- a )
     yield 
     {
        b
     }
}

score 2 · Answer 1 · edited May 23 '17 at 12:04

2

scala> for (v <- Some(Vector("abc")); e <- v) yield e
<console>:8: error: type mismatch;
 found   : scala.collection.immutable.Vector[String]
 required: Option[?]
              for (v <- Some(Vector("abc")); e <- v) yield e
                                               ^

scala> for (v <- Some(Vector("abc")); e = v) yield e
res1: Option[scala.collection.immutable.Vector[String]] = Some(Vector(abc))

A nested x <- xs means flatMap and will work only when the returned type is the same type as the most outer one.

edited May 23 '17 at 12:04

Community

1
1

answered Mar 19 '13 at 13:39

kiritsuku

52,967
18
114
136

or `for (v <- Some(Vector("abc"))) yield v` – sourcedelica Mar 20 '13 at 14:11

score 0 · Answer 2 · answered Mar 19 '13 at 13:38

0

The for comprehension already unboxes for you Option so this should work

def readDocument(v:Option[Vector[String]]) : Option[Vector[String]] =    
{
  for ( a <- v )
  yield 
  {
    a
  }
}

answered Mar 19 '13 at 13:38

korefn

955
6
17

score 0 · Accepted Answer · answered Mar 19 '13 at 14:20

0

How about this?

  def readDocument(ovs: Option[Vector[String]]): Option[Vector[String]] =
    for (vs <- ovs) yield for (s <- vs) yield s

answered Mar 19 '13 at 14:20

john sullivan

1,898
2
13
21

How to return different collection with yield statement

3 Answers3