Understanding Option.flatten in Scala

Question

I noticed that Option.flatten is defined as follows:

def flatten[B](implicit ev: A <:< Option[B]): Option[B] =
    if (isEmpty) None else ev(this.get)

What is ev here ? What does A <:< Option[B] mean ? What is it used for ?

Answer 1

This is common practice to constrain some methods to be executed against particular types. Actually, <:< is a type defined in scala.Predef as follows:

@implicitNotFound(msg = "Cannot prove that ${From} <:< ${To}.")
sealed abstract class <:<[-From, +To] extends (From => To) with Serializable
...
implicit def conforms[A]: A <:< A = ...

So that, implicit of type <:<[A, B] can be resolved only if A is a subtype of B.

In this case, it can be resolved only if Option in wrapped in another Option. In any other cases, compile error will occur:

scala> Option(42).flatten
<console>:8: error: Cannot prove that Int <:< Option[B].
          Option(42).flatten
                     ^

Answer 2

As ScalaDoc says - An instance of A <:< B witnesses that A is a subtype of B.

So in this case an Option can be flattened only if it contains another Option inside.

An Option[Option[String]] flattens to an Option[String]