Scala Cookbook

9.8. Creating Partial Functions

Problem

You want to define a function that will only work for a subset of possible input values, or you want to define a series of functions that only work for a subset of input values, and combine those functions to completely solve a problem.

Solution

A partial function is a function that does not provide an answer for every possible input value it can be given. It provides an answer only for a subset of possible data, and defines the data it can handle. In Scala, a partial function can also be queried to determine if it can handle a particular value.

As a simple example, imagine a normal function that divides one number by another:

val divide = (x: Int) => 42 / x

As defined, this function blows up when the input parameter is zero:

scala> divide(0)
java.lang.ArithmeticException: / by zero

Although you can handle this particular situation by catching and throwing an exception, Scala lets you define the divide function as a PartialFunction. When doing so, you also explicitly state that the function is defined when the input parameter is not zero:

val divide = new PartialFunction[Int, Int] {
  def apply(x: Int) = 42 / x
  def isDefinedAt(x: Int) = x != 0
}

With this approach, you can do several nice things. One thing you can do is test the function before you attempt to use it:

scala> divide.isDefinedAt(1)
res0: Boolean = true

scala> if (divide.isDefinedAt(1)) divide(1)
res1: AnyVal = 42

scala> divide.isDefinedAt(0)
res2: Boolean = false

This isn’t all you can do ...