Converting nested for loops into for comprehension - scala

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?

Answer 2

Your error may be because you are surrounding yield in {}.
for {stuff} {yield otherstuff}
The form should be:
for {stuff} yield otherstuff
You can of course replace "otherstuff" with a block if you want it to contain multiple expressions so you have:
for {stuff} yield {otherstuff}
Using your example I suspect you want something like:
def useForComprehension(input: Input): List[Output] =
for {
o <- splitInputIntoPieces(input)
restResults <- useForComprehension(subtract(input, o))
w <- f3(o)
} yield w :: restResults