First-Class Functions

Scala has most features you’d expect in a functional programming language, including:

• Lambdas (anonymous functions)
• Higher-order functions (HOFs)
• Immutable collections in the standard library

Lambdas, also known as anonymous functions, are a big part of keeping your code concise but readable.

The map method of the List class is a typical example of a higher-order function—a function that takes a function as parameter.

These two examples are equivalent, and show how to multiply each number in a list by 2 by passing a lambda into the map method:

val a = List(1, 2, 3).map(i => i * 2)   // List(2,4,6)
val b = List(1, 2, 3).map(_ * 2)        // List(2,4,6)

Those examples are also equivalent to the following code, which uses a double method instead of a lambda:

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

val a = List(1, 2, 3).map(i => double(i))   // List(2,4,6)
val b = List(1, 2, 3).map(double)           // List(2,4,6)

If you haven’t seen the map method before, it applies a given function to every element in a list, yielding a new list that contains the resulting values.

Passing lambdas to higher-order functions on collections classes (like List) is a part of the Scala experience, something you’ll do every day.

Immutable collections

When you work with immutable collections like List, Vector, and the immutable Map and Set classes, it’s important to know that these functions don’t mutate the collection they’re called on; instead, they return a new collection with the updated data. As a result, it’s also common to chain them together in a “fluent” style to solve problems.

For instance, this example shows how to filter a collection twice, and then multiply each element in the remaining collection:

// a sample list
val nums = (1 to 10).toList   // List(1,2,3,4,5,6,7,8,9,10)

// methods can be chained together as needed
val x = nums.filter(_ > 3)
            .filter(_ < 7)
            .map(_ * 10)

// result: x == List(40, 50, 60)

In addition to higher-order functions being used throughout the standard library, you can also create your own.