# Covariance in Scala

August 14, 2014

Starting with the `Option` implementation from the option monad exercise:

``````sealed trait Option[A] { ... }
case class Some[A](a: A) extends Option[A]
case class None[A]() extends Option[A]``````

Make `Option` covariant in its type `A` so that only a single `None` instance is needed:

``case object None extends Option[???]``
``````def quotient(divisor: Int)(dividend: Int): Int =
dividend / divisor

def remainder(divisor: Int)(dividend: Int): Option[Int] =
(dividend % divisor) match {
case 0 => None
case r => Some(r)
}

val x3 =
for {
x <- Some(42)
q  = quotient(2)(x)
r <- remainder(4)(x)
} yield (x,q,r)

println(x3) // Some((42,21,2))``````

# Solution

``````sealed trait Option[+A] {

def map[B](f: A => B): Option[B] =
this match {
case Some(a) => Some(f(a))
case None  => None
}

def flatMap[B](f: A => Option[B]): Option[B] =
this match {
case Some(a) => f(a)
case None  => None
}

override def toString(): String =
this match {
case Some(a) => "Some(" + a + ")"
case None  => "None"
}

}

case class Some[A](a: A) extends Option[A]
case object None extends Option[Nothing]``````

## Demo

This file is literate Scala, and can be run using Codedown:

``````\$ curl https://earldouglas.com/posts/exercises/scala-covariance.md |
codedown scala |
xargs -0 scala -nc -e
Some((42,21,2))``````