Converting nested for loops into for comprehension

Question

I can collect the results at the inner-most for body into a List[Output] and return them. But I want to use yield. How can this method be converted into using for-yield pattern:

def useForComprehension(input : Input): List[Output] = {        
  for (o <- splitInputIntoPieces(input)) {
    for (restResults <- useForComprehension(subtract(input, o)) ) {
        for (w <- f3(o)) {
          yield w::restResults // !!!!! Error
        }
    }           
  }    
}

Answer 1

In Scala, nested iteration is handled by adding additional <- clauses.

For example, let's say we have two lists l1 and l2 and we want generate over every pair of elements (x,y) where x is in l1 and y is in l2. The syntax in Scala is:

for { x <- l1
      y <- l2
    } yield (x,y)

When no yield keyword follows the for then the entire expression results in a type of Unit, which is the source of your type error. This is useful for performing side effects on the iteration, for example

for { x <- l1
      y <- l2
    } println((x,y))

For more information on for comprehensions see What is Scala's yield?