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]]