Partial functions

A partial function is a function that does not provide a result for every possible input value it can be given. It supports only a subset of possible inputs.

In Scala, we can use scala.PartialFunction to define partial functions.

In Scala source code, PartialFunction is defined as follows:

trait PartialFunction[-A, +B] extends (A) ⇒ B 

Consider the following example:

scala> val numRange = 1 to 5 
numRange: scala.collection.immutable.Range.Inclusive = Range(1, 2, 3, 4, 5) 
 
scala> val isEven: PartialFunction[Int, String] = { 
     |   case num if num % 2 == 0 => num +" is even" 
     | } 
isEven: PartialFunction[Int,String] = <function1> 
 
scala> val evenNumbers = numRange collect isEven 
evenNumbers: scala.collection.immutable.IndexedSeq[String] = Vector(2 is even, 4 is even)