Inferring the return type of a partial function

partial-function-return-inference.scala

class PF[A] {
  def apply[B](pf: PartialFunction[A, B]) = pf
}

def pf[A] = new PF[A]

pf[Any] { case 5 => true }



def applyPf[A, B](a: A)(pf: PartialFunction[A, B]): Option[B] =
   if (pf.isDefinedAt(a)) Some(pf(a)) else None


applyPf(1) { case 5 => true }