Scala's Option Monad

December 10, 2013

Recently I’ve been playing with Monads and making an effort to understand how they work and their usefulness in the greater context of writing good software. A common example given is the Maybe monad in Haskell. I believe the same concept is called Option in Scala.

Commonly I would use Option as follows:

val hash = HashMap("one" -> 1, "two" -> 2)
val one = hash.get("one") match {
    case Some(n) => n // this case is used
    case None => 0
}

val three = hash.get("three") match {
    case Some(n) => n
    case None => 0 // this case is used
}

This works, but doesn’t leverage the fact that Option is a Monad. Since it’s a Monad, we can easily extract the value and perform simple compositions on the Option object.

val hash = HashMap("one" -> 1, "two" -> 2)

// simply grab the value
hash.get("one") getOrElse 0 // 1

// or we can perform operations on it inside the Option Monad
hash.get("one").map(_*10).filter(_ <= 10).map(_ / 3) // Some(3)
hash.get("one").map(_*20).filter(_ <= 10).map(_ / 3) // None, filter(_ <= 10) results in None
hash.get("three").map(_*10).filter(_ <= 10).map(_ / 3) // None, "three" doesn't exist

According to the Scala documentation, this is the more idiomatic approach when dealing with Option.

Discussion, links, and tweets

I am learning to be programmer and math.

Follow @gregghz