Implementing map on Option - scala

Question

Working more on FP in Scala examples, I tried to implement the Option trait's map function as follows:
sealed trait MyOption[+A] {
def map[B](f: A => B): Option[B] = this match {
case Some(a) => Some(f(a))
case _ => None
}
}
However, compile-time errors show, if I understand correctly, that I'm not pattern-matching correctly for the case of Some(A). Using pattern matching, how can I write the first case to get Some(A) values to match?
>scalac MyOption.scala
MyOption.scala:3: error: constructor cannot be instantiated to expected type;
found : Some[A(in class Some)]
required: MyOption[A(in trait MyOption)]
case Some(a) => Some(f(a))
^
MyOption.scala:3: error: not found: value a
case Some(a) => Some(f(a))
^
two errors found

Answer 1

You are trying to define map in terms of Some and None which are subclasses of the Scala-provided Option trait, rather than in terms of subclasses of your own trait. Try something like:
sealed trait MyOption[+A] {
import MyOption._
def map[B](f: A => B): MyOption[B] = this match {
case MySome(a) => MySome(f(a))
case _ => MyNone
}
}
object MyOption {
case class MySome[+A](a: A) extends MyOption[A]
case object MyNone extends MyOption[Nothing]
}