Higher-Order Functions

This chapter is from the book

Scala for the Impatient, 3rd Edition

Learn More Buy

This chapter is from the book

This chapter is from the book 

Scala for the Impatient, 3rd Edition

Learn More Buy

Topics in This Chapter L1

• 12.1 Functions as Values

• 12.2 Anonymous Functions

• 12.3 Parameters That Are Functions

• 12.4 Parameter Inference

• 12.5 Useful Higher-Order Functions

• 12.6 Closures

• 12.7 Interoperability with Lambda Expressions

• 12.8 Currying

• 12.9 Methods for Composing, Currying, and Tupling

• 12.10 Control Abstractions

• 12.11 The return Expression

• Exercises

Scala mixes object orientation with functional features. In a functional programming language, functions are first-class citizens that can be passed around and manipulated just like any other data types. This is very useful whenever you want to pass some action detail to an algorithm. In a functional language, you just wrap that detail into a function that you pass as an argument. In this chapter, you will see how to be productive with functions that use or return functions.

Highlights of the chapter include:

• Functions are “first-class citizens” in Scala, just like numbers.

• You can create anonymous functions, usually to give them to other functions.

• A parameter that is a function specifies behavior that should be executed later.

• Many collection methods take function parameters, applying a function to the values of the collection.

• There are syntax shortcuts that allow you to express function parameters in a way that is short and easy to read.

• You can create functions that operate on blocks of code and look much like the built-in control statements.

12.1 Functions as Values

In Scala, a function is a first-class citizen, just like a number. You can store a function in a variable:

import scala.math.*
val num = 3.14
val fun = ceil

This code sets num to 3.14 and fun to the ceil function.

When you try this code in the REPL, the type of num is, not surprisingly, Double. The type of fun is reported as Double => Double—that is, a function receiving and returning a Double.

What can you do with a function? Two things:

• Call it.

• Pass it around, by storing it in a variable or giving it to a function as a parameter.

Here is how to call the function stored in fun:

fun(num) // 4.0

As you can see, the normal function call syntax is used. The only difference is that fun is a variable containing a function, not a fixed function.

Here is how you can give fun to another function:

Array(3.14, 1.42, 2.0).map(fun) // Array(4.0, 2.0, 2.0)

The map method accepts a function, applies it to all values in an array, and returns an array with the function values. In this chapter, you will see many other methods that accept functions as arguments.