Scala for-comprehension type inference

Question

The next code

  def f(chars: List[Char]): List[List[Char]] = chars match {
    case Nil => List(Nil)
    case x :: xs => for {
      v <- f(xs)
    } yield List(x) :: v
  }

gives the error message

- type mismatch;  found   : List[List[Any]]  required: List[List[Char]]

Please help me understand why 'for' chooses the most general Any instead of Char here? What topic in language spec should I read? Thanks.

Answer 1

The result, you are yielding is a mix of List[List[List[Char]]] and List[List[Char]]. Scala upcasts that to List[List[Any]]. For your case either of the following will do the job:

scala>  def f(chars: List[Char]): List[List[Char]] = chars match {
     |     case Nil => List(Nil)
     |     case x :: xs => for {
     |       v <- f(xs)
     |     } yield x :: v
     |   }
f: (chars: List[Char])List[List[Char]]

scala>  def f(chars: List[Char]): List[List[Char]] = chars match {
     |     case Nil => List(Nil)
     |     case x :: xs => for {
     |       v <- f(xs)
     |     } yield List(x) ++ v
     |   }
f: (chars: List[Char])List[List[Char]]