Scala Cookbook

9.3. Defining a Method That Accepts a Simple Function Parameter

Problem

You want to create a method that takes a simple function as a method parameter.

Solution

This solution follows a three-step process:

1. Define your method, including the signature for the function you want to take as a method parameter.

2. Define one or more functions that match this signature.

3. Sometime later, pass the function(s) as a parameter to your method.

To demonstrate this, define a method named executeFunction, which takes a function as a parameter. The method will take one parameter named callback, which is a function. That function must have no input parameters and must return nothing:

def executeFunction(callback:() => Unit) {
  callback()
}

Two quick notes:

• The callback:() syntax defines a function that has no parameters. If the function had parameters, the types would be listed inside the parentheses.

• The => Unit portion of the code indicates that this method returns nothing.

I’ll discuss this syntax more shortly.

Next, define a function that matches this signature. The following function named sayHello takes no input parameters and returns nothing:

val sayHello = () => { println("Hello") }

In the last step of the recipe, pass the sayHello function to the executeFunction method:

executeFunction(sayHello)

The REPL demonstrates how this works:

scala> def executeFunction(callback:() => Unit) { callback() }
executeFunction: (callback: () => Unit)Unit

scala> val sayHello = () => { println("Hello") } sayHello: () => Unit = <function0> ...