yield in for

yield in for

Adding yield to a for comprehension, for (x <- collection) yield transform(x), transforms each element and collects the results into a new collection of the same type as the source, instead of just running side effects. This is functionally equivalent to calling .map (and with filters/multiple generators, roughly equivalent to .flatMap/.filter), but often reads more clearly for complex, multi-generator logic. The resulting collection's type matches the type being iterated over.

Note: for (x <- xs) yield f(x) is syntactic sugar that the compiler translates into a chain of map/flatMap/filter calls.

Example: yield in for

object Main extends App {
  val numbers = 1 to 5

  val squares = for (n <- numbers) yield n * n
  println(squares)

  val evens = for (n <- numbers if n % 2 == 0) yield n
  println(evens)
}