How to combine two lists into tuple list (like Cartesian Products) in Scala?

Question

Suppose list1 = [1, 2, 3] and list2 = ["a", "b"]

I want to combine them such that I get a tuple list like so:

list3 = [(1, "a"), (1, "b"), (2, "a"), (2, "b"), (3, "a"), (3, "b")]

Is there any way to do this concisely?

Answer 1

You can do it fairly easily with Scala's for ... yield expression.

for {
  x <- list1
  y <- list2
} yield {
  (x, y)
}

This desugars to flatMap and map calls, so it's equivalent to

list1.flatMap { x => list2.map { y => (x, y) } }

If you're using Scalaz, then the Cartesian product is just an application of the Applicative instance for lists. With appropriate Scalaz imports, this will suffice

^(list1, list2) { (_, _) }

cats has similar functionality with different syntax:

import cats.syntax.all._
(list1, list2).tupled