Scala Cookbook

9.4. More Complex Functions

Problem

You want to define a method that takes a function as a parameter, and that function may have one or more input parameters, and may also return a value.

Solution

Following the approach described in the previous recipe, define a method that takes a function as a parameter. Specify the function signature you expect to receive, and then execute that function inside the body of the method.

The following example defines a method named exec that takes a function as an input parameter. That function must take one Int as an input parameter and return nothing:

def exec(callback: Int => Unit) {
  // invoke the function we were given, giving it an Int parameter
  callback(1)
}

Next, define a function that matches the expected signature. The following plusOne function matches that signature, because it takes an Int argument and returns nothing:

val plusOne = (i: Int) => { println(i+1) }

Now you can pass plusOne into the exec function:

exec(plusOne)

Because the function is called inside the method, this prints the number 2.

Any function that matches this signature can be passed into the exec method. To demonstrate this, define a new function named plusTen that also takes an Int and returns nothing:

val plusTen = (i: Int) => { println(i+10) }

Now you can pass it into your exec function, and see that it also works:

exec(plusTen)   // prints 11

Although these examples are simple, you can see the power of the technique: you can easily swap in interchangeable algorithms. As long as ...