Scala Cookbook

10.16. Combining map and flatten with flatMap

Problem

When you first come to Scala, the flatMap method can seem very foreign, so you’d like to understand how to use it and see where it can be applied.

Solution

Use flatMap in situations where you run map followed by flatten. The specific situation is this:

• You’re using map (or a for/yield expression) to create a new collection from an existing collection.

• The resulting collection is a list of lists.

• You call flatten immediately after map (or a for/yield expression).

When you’re in this situation, you can use flatMap instead.

The next example shows how to use flatMap with an Option. In this example, you’re told that you should calculate the sum of the numbers in a list, with one catch: the numbers are all strings, and some of them won’t convert properly to integers. Here’s the list:

val bag = List("1", "2", "three", "4", "one hundred seventy five")

To solve the problem, you begin by creating a “string to integer” conversion method that returns either Some[Int] or None, based on the String it’s given:

def toInt(in: String): Option[Int] = {
  try {
    Some(Integer.parseInt(in.trim))
  } catch {
    case e: Exception => None
  }
}

With this method in hand, the resulting solution is surprisingly simple:

scala> bag.flatMap(toInt).sum
res0: Int = 7

Discussion

To see how this works, break the problem down into smaller steps. First, here’s what happens when you use map on the initial collection of strings:

scala> bag.map(toInt) res0: List[Option[Int]] = List(Some(1), ...