Example showing unexpected behaviour from a PartialFunction

PartialFunctionSpec.scala

class ScalaSpec extends FlatSpec with MustMatchers {
  behavior of "a PartialFunction"

  it should "chain" in {

    var pf: PartialFunction[String, String] = {
      case "1" => "2"
    }

    pf = pf andThen {
      case "2" => "3"
    }

    pf("1") must be ("3")

    var pf2: PartialFunction[String, Option[String]] = {
      case "1" => Some("2")
    }

    val pf3 = pf2 andThen {
      case None => "3"
    }

    pf3.isDefinedAt("1") must be(false) // this is true
    pf3("1")                            // which means this fails with a MatchError
  }
}