Passing Functions Around

While every programming language ever created probably lets you write pure functions, a second great FP feature of Scala is that you can create functions as variables, just like you create String and Int variables. This feature has many benefits, the most common of which is that it lets you pass functions as parameters into other functions. You saw that earlier in this book when the map and filter methods were demonstrated:

val nums = (1 to 10).toList

val doubles = nums.map(_ * 2)
val lessThanFive = nums.filter(_ < 5)

In those examples, anonymous functions are passed into map and filter. In the lesson on anonymous functions we demonstrated that this example:

val doubles = nums.map(_ * 2)

is the same as passing a regular function into map:

def double(i: Int): Int = i * 2   //a method that doubles an Int
val doubles = nums.map(double)

As those examples show, Scala clearly lets you pass anonymous functions and regular functions into other methods. This is a powerful feature that good FP languages provide.

If you like technical terms, a function that takes another function as an input parameter is known as a Higher-Order Function (HOF). (And if you like humor, as someone once wrote, that’s like saying that a class that takes an instance of another class as a constructor parameter is a Higher-Order Class.)

Function or method?

Scala has a special “function” syntax, but as a practical matter the def syntax seems to be preferred. This may be because of two reasons:

• The def syntax is more familiar to people coming from a C/Java/C# background
• You can use def methods just like they are val functions

What that second statement means is that when you define a method with def like this:

def double(i: Int): Int = i * 2

you can then pass double around as if it were a variable, like this:

val x = ints.map(double)
                 ------

Even though double is defined as a method, Scala lets you treat it as a function.

The ability to pass functions around as variables is a distinguishing feature of functional programming languages. And as you’ve seen in map and filter examples in this book, the ability to pass functions as parameters into other functions helps you create code that is concise and still readable.

A few examples

If you’re not comfortable with the process of passing functions as parameters into other functions, here are a few more examples you can experiment with in the REPL:

List("foo", "bar").map(_.toUpperCase)
List("foo", "bar").map(_.capitalize)
List("adam", "scott").map(_.length)
List(1,2,3,4,5).map(_ * 10)
List(1,2,3,4,5).filter(_ > 2)
List(5,1,3,11,7).takeWhile(_ < 6)

Remember that any of those anonymous functions can also be written as “regular” functions, so you can write a function like this:

def toUpper(s: String): String = s.toUpperCase

and then pass it into map like this:

List("foo", "bar").map(toUpper)

or this:

List("foo", "bar").map(s => toUpper(s))

Those examples that use a “regular” function are equivalent to these anonymous function examples:

List("foo", "bar").map(s => s.toUpperCase)
List("foo", "bar").map(_.toUpperCase)