For Comprehensions

Scala offers a lightweight notation for expressing sequence comprehensions. Comprehensions have the form for (enumerators) yield e, where enumerators refers to a list of enumerators. An enumerator is either a generator, or it is a guard (see: Control Structures). A comprehension evaluates the body e for each binding generated by the enumerators and returns a sequence of these values.

Here’s an example:

case class User(name: String, age: Int)

val userBase = List(
  User("Travis", 28),
  User("Kelly", 33),
  User("Jennifer", 44),
  User("Dennis", 23))

val twentySomethings =
  for (user <- userBase if user.age >=20 && user.age < 30)
  yield user.name  // i.e. add this to a list

twentySomethings.foreach(println)  // prints Travis Dennis

case class User(name: String, age: Int)

val userBase = List(
  User("Travis", 28),
  User("Kelly", 33),
  User("Jennifer", 44),
  User("Dennis", 23))

val twentySomethings =
  for user <- userBase if user.age >=20 && user.age < 30
  yield user.name  // i.e. add this to a list

twentySomethings.foreach(println)  // prints Travis Dennis

A for loop with a yield statement returns a result, the container type of which is determined by the first generator. user <- userBase is a List, and because we said yield user.name where user.name is a String, the overall result is a List[String]. And if user.age >=20 && user.age < 30 is a guard that filters out users who are not in their twenties.

Here is a more complicated example using two generators. It computes all pairs of numbers between 0 and n-1 whose sum is equal to a given value v:

def foo(n: Int, v: Int) =
   for (i <- 0 until n;
        j <- 0 until n if i + j == v)
   yield (i, j)

foo(10, 10).foreach {
  case (i, j) =>
    println(s"($i, $j) ")  // prints (1, 9) (2, 8) (3, 7) (4, 6) (5, 5) (6, 4) (7, 3) (8, 2) (9, 1)
}

def foo(n: Int, v: Int) =
   for i <- 0 until n
       j <- 0 until n if i + j == v
   yield (i, j)

foo(10, 10).foreach {
  (i, j) => println(s"($i, $j) ")  // prints (1, 9) (2, 8) (3, 7) (4, 6) (5, 5) (6, 4) (7, 3) (8, 2) (9, 1)
}

Here n == 10 and v == 10. On the first iteration, i == 0 and j == 0 so i + j != v and therefore nothing is yielded. j gets incremented 9 more times before i gets incremented to 1. Without the if guard, this would simply print the following:

(0, 0) (0, 1) (0, 2) (0, 3) (0, 4) (0, 5) (0, 6) (0, 7) (0, 8) (0, 9) (1, 0) ...

Note that comprehensions are not restricted to lists. Every datatype that supports the operations withFilter, map, and flatMap (with the proper types) can be used in sequence comprehensions.

You can omit yield in a comprehension. In that case, comprehension will return Unit. This can be useful in case you need to perform side-effects. Here’s a program equivalent to the previous one, but without using yield:

def foo(n: Int, v: Int) =
   for (i <- 0 until n;
        j <- 0 until n if i + j == v)
   println(s"($i, $j)")

foo(10, 10)

def foo(n: Int, v: Int) =
   for i <- 0 until n
       j <- 0 until n if i + j == v
   do println(s"($i, $j)")

foo(10, 10)

More resources

• Other examples of “For comprehension” in the Scala Book