https://newcircle.com/s/post/1696/an_Introduction_to_pattern_matching_in_scala_brian_clapper_tutorial

val f: String => String = { case "ping" => "pong" }


val f: PartialFunction[String, String] = { case ... }
f.isDefinedAt("ping") // true
f.isDefinedAt("pong") // false

trait PartialFunction[-A, +R] extends Function1[-A, +R] {
    def apply(x: A) : R
    def isDefinedAt(x: A): Boolean
}

the compiler will expand { case "ping" => "pong" } as

new PartialFunction[String, String] {
    def apply(x: String) = x match {
        case "ping" => "pong"
    }
    def isDefinedAt(x: String) = x match {
        case "ping" => true
        case _      => false
    }
}