Chaining Options in Scala

Two dependent lookups and a test, with no null check anywhere.

val owners: Map[String, String] = Map("orders" -> "ada", "billing" -> "grace")
val inboxes: Map[String, String] = Map("ada" -> "Ada@Example.COM")

// flatMap threads one Option-returning step into the next, and map
// transforms the value inside. Two nested null checks become one line.
def inboxFor(team: String): Option[String] =
  owners.get(team).flatMap(inboxes.get).map(_.toLowerCase)

// filter keeps the value only when the test passes, else gives None.
def companyInbox(team: String): Option[String] =
  inboxFor(team).filter(_.endsWith("@example.com"))

// fold unwraps both cases at once: the default first, then the transform.
def contactLine(team: String): String =
  companyInbox(team).fold("no company contact")(mail => s"write to $mail")

How it works

1. owners.get(team).flatMap(inboxes.get) feeds one lookup straight into the next.
2. .map(_.toLowerCase) reaches inside the Option, and filter(_.endsWith("@example.com")) throws the value away when the test fails.
3. fold unwraps both cases at once, taking the default no company contact before the transform.

Keywords and builtins used here

• Map
• Option
• String
• companyInbox
• contactLine
• def
• inboxFor
• val

The run, in numbers

Lines

15

Characters to type

747

Tokens

128

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 118 seconds.

Type this snippet

Step 2 of 3 in Option & Either, step 20 of 27 in Language basics.

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