Chaining PartialFunctions with andThen in Scala

Question

Let us reuse examples from Daily scala :

type PF = PartialFunction[Int,Int]

val pf1 : PF = {case 1 => 2}                      

val pf2 : PF = {case 2 =&         


        

Answer 1

You need the equivalent of flatMap for PartialFunctions.

implicit class CollectPartial[A, B](f: PartialFunction[A, B]) {
    def collect[C](g: PartialFunction[B, C]) = Function.unlift { a: A =>
        f.lift(a).flatMap(g.lift)
    }
}

Use it like

val a: PartialFunction[String, Int] = ...
val b: PartialFunction[Int, Char] = ...
val c: PartialFunction[String, Char] = a collect b

This works as expected even with side-effects.