Writing functions and methods that process the input data and produce some output is the very core of any type of programming. When diving deeper into different types of functions in Scala, there is one type that differs from others — partial function. As the element of Scala’s standard library, it is appropriate to know what it is and where it should be used. Let’s dive in!

What is Partial Function?

A partial function of type PartialFunction[A, B] is a unary function where the domain does not necessarily include all values of type A.

Scala Standard Library

The definition above, taken straight from the Scala lang standard library docs may seem quite obfuscated. Fear not though, as the concept of partial function is very easy to understand.

In essence, a partial function is a function that accepts only a specific set of data as the input. This doesn’t mean that the function takes only integers and not strings — this behaviour is a standard element of any function. The partial function that accepts integer parameter as the input, can specify what exact integers are accepted.

To better understand the concept, let’s see some code examples.

How to write Partial Function

First, let’s write a standard function that accepts any input of type Int:

val standardFunction = (x: Int) => x % 10

The standardFunction is like any other - takes a parameter and returns the remainder when dividing by 10. Now, what if a developer passes zero as the parameter? Or if we need even more refined restrictions regarding the parameter? One way would be to write a long block of ifs and elses, but that is not what you would want to do. Instead, let’s write a partial function, like this:

val partialFunction: PartialFunction[Int, Int] = {
    case 1 => 100
    case 2 => 120
    case 3 => 130
  }

The partialFunction above is defined using the PartialFunction[Int, Int] trait. The type parameters say that both input and the output will be of type Int. As you can see, this function accepts only three parameters: either 1, 2 or 3. For any other parameter, an exception will be thrown.

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