Composing partial functions in Scala

Question

I have two partial functions f1 and f2 which I want to compose into a new partial function f so that f.isDefinedAt(x) iff f1.isDefinedAt(x) || f2.isDefinedAt(x). I mean

        --
       | f1(x) iff f1.isDefinedAt(x)  
       |
f(x) = |
       |
       | f2(x) iff  !f1.isDefinedAt(x) && f2.isDefinedAt(x)  
        --

Is there a way to compose f1 and f2 that way?

Answer 1

Use method .orElse on PartialFunction:

scala> val f1: PartialFunction[Int, Unit] = { case x if x > 0 => println(s"called f1 with $x") }
f1: PartialFunction[Int,Unit] = <function1>

scala> val f2: PartialFunction[Int, Unit] = { case x if x < 0 => println(s"called f2 with $x") }
f2: PartialFunction[Int,Unit] = <function1>

scala> (f1 orElse f2)(1)
called f1 with 1

scala> (f1 orElse f2)(-1)
called f2 with -1

scala> (f1 orElse f2)(0)
scala.MatchError: 0 (of class java.lang.Integer)
  at scala.PartialFunction$$anon$1.apply(PartialFunction.scala:253)
  at scala.PartialFunction$$anon$1.apply(PartialFunction.scala:251)
  at $anonfun$1.applyOrElse(<console>:11)
  at $anonfun$1.applyOrElse(<console>:11)
  at scala.runtime.AbstractPartialFunction$mcVI$sp.apply$mcVI$sp(AbstractPartialFunction.scala:36)
  at scala.runtime.AbstractPartialFunction$mcVI$sp.apply(AbstractPartialFunction.scala:36)
  at scala.runtime.AbstractPartialFunction$mcVI$sp.apply(AbstractPartialFunction.scala:28)
  at $anonfun$1.applyOrElse(<console>:11)
  at $anonfun$1.applyOrElse(<console>:11)
  at scala.PartialFunction$OrElse.apply(PartialFunction.scala:167)
  at scala.Function1$class.apply$mcVI$sp(Function1.scala:36)
  at scala.PartialFunction$OrElse.apply$mcVI$sp(PartialFunction.scala:164)
  ... 32 elided