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Scala 3 — Book

Creating a Method That Returns a Function


Thanks to Scala’s consistency, writing a method that returns a function is similar to everything you’ve seen in the previous sections. For example, imagine that you want to write a greet method that returns a function. Once again we start with a problem statement:

I want to create a greet method that returns a function. That function will take a string parameter and print it using println. To simplify this first example, greet won’t take any input parameters; it will just build a function and return it.

Given that statement, you can start building greet. You know it’s going to be a method:

def greet()

You also know this method will return a function that (a) takes a String parameter, and (b) prints that string using println. Therefore that function has the type, String => Unit:

def greet(): String => Unit = ???

Now you just need a method body. You know that the method needs to return a function, and that function takes a String and prints it. This anonymous function matches that description:

(name: String) => println(s"Hello, $name")

Now you just return that function from the method:

// a method that returns a function
def greet(): String => Unit = 
  (name: String) => println(s"Hello, $name")

Because this method returns a function, you get the function by calling greet(). This is a good step to do in the REPL because it verifies the type of the new function:

scala> val greetFunction = greet()
val greetFunction: String => Unit = Lambda....

Now you can call greetFunction:

greetFunction("Joe")   // prints "Hello, Joe"

Congratulations, you just created a method that returns a function, and then executed that function.

Improving the method

Our method would be more useful if you could pass in a greeting, so let’s do that. All you have to do is pass the greeting in as a parameter to the greet method, and use it in the string inside println:

def greet(theGreeting: String): String => Unit = 
  (name: String) => println(s"$theGreeting, $name")

Now when you call your method, the process is more flexible because you can change the greeting. This is what it looks like when you create a function from this method:

scala> val sayHello = greet("Hello")
val sayHello: String => Unit = Lambda.....

The REPL type signature output shows that sayHello is a function that takes a String input parameter and returns Unit (nothing). So now when you give sayHello a String, it prints the greeting:

sayHello("Joe")   // prints "Hello, Joe"

You can also change the greeting to create new functions, as desired:

val sayCiao = greet("Ciao")
val sayHola = greet("Hola")

sayCiao("Isabella")   // prints "Ciao, Isabella"
sayHola("Carlos")     // prints "Hola, Carlos"

A more real-world example

This technique can be even more useful when your method returns one of many possible functions, like a factory that returns custom-built functions.

For instance, imagine that you want to write a method that returns functions that greet people in different languages. We’ll limit this to functions that greet in English or French, depending on a parameter that’s passed into the method.

A first thing you know is that you want to create a method that (a) takes a “desired language” as an input, and (b) returns a function as its result. Furthermore, because that function prints a string that it’s given, you know it has the type String => Unit. With that information you write the method signature:

def createGreetingFunction(desiredLanguage: String): String => Unit = ???

Next, because you know that the possible functions you’ll return take a string and print it, you can write two anonymous functions for the English and French languages:

(name: String) => println(s"Hello, $name")
(name: String) => println(s"Bonjour, $name")

Inside a method it might be a little more readable if you give those anonymous functions some names, so let’s assign them to two variables:

val englishGreeting = (name: String) => println(s"Hello, $name")
val frenchGreeting = (name: String) => println(s"Bonjour, $name")

Now all you need to do is (a) return englishGreeting if the desiredLanguage is English, and (b) return frenchGreeting if the desiredLanguage is French. One way to do that is with a match expression:

def createGreetingFunction(desiredLanguage: String): String => Unit =
  val englishGreeting = (name: String) => println(s"Hello, $name")
  val frenchGreeting = (name: String) => println(s"Bonjour, $name")
  desiredLanguage match
    case "english" => englishGreeting
    case "french" => frenchGreeting

And that’s the final method. Notice that returning a function value from a method is no different than returning a string or integer value.

This is how createGreetingFunction builds a French-greeting function:

val greetInFrench = createGreetingFunction("french")
greetInFrench("Jonathan")   // prints "Bonjour, Jonathan"

And this is how it builds an English-greeting function:

val greetInEnglish = createGreetingFunction("english")
greetInEnglish("Joe")   // prints "Hello, Joe"

If you’re comfortable with that code—congratulations—you now know how to write methods that return functions.

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