flatMap implementation in Scala

Question

In Functional Programming in Scala chapter 4 we are provided the following definitions for map, getOrElse and flatMap for the Option type:

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

def getOrElse[B>:A](default: => B): B = this match {
  case Some(x) => x
  case _ => default
}

def flatMap[B](f: A => Option[B]): Option[B] = 
  map(f) getOrElse None

I get what flatMap is doing, but I don't get how the call map(f) in flatMap's definition works. f has A => Option[B] as its type in flatMap, yet we seem to be able to call map, which expects a function of type A => B, with f. The call getOrElse None is obviously the key, but I don't understand how it allows us to call map(f) in flatMap.

Answer 1

When you call flatMap and pass the function A => Option[B], flatMap calls map and passes the same function, but the B in flatMap is not same B as in map. For example, if you pass some

Int => Option[String]

Then for map, B = Option[String] and will return Option[Option[String]].

Thus the getOrElse in flatMap to get the will either retrieve the inner Option[B] or return None.