Scala Cookbook

9.1. Using Function Literals (Anonymous Functions)

Problem

You want to use an anonymous function—also known as a function literal—so you can pass it into a method that takes a function, or to assign it to a variable.

Solution

Given this List:

val x = List.range(1, 10)

you can pass an anonymous function to the List’s filter method to create a new List that contains only even numbers:

val evens = x.filter((i: Int) => i % 2 == 0)

The REPL demonstrates that this expression indeed yields a new List of even numbers:

scala> val evens = x.filter((i: Int) => i % 2 == 0)
evens: List[Int] = List(2, 4, 6, 8)

In this solution, the following code is a function literal (also known as an anonymous function):

(i: Int) => i % 2 == 0

Although that code works, it shows the most explicit form for defining a function literal. Thanks to several Scala shortcuts, the expression can be simplified to this:

val evens = x.filter(_ % 2 == 0)

In the REPL, you see that this returns the same result:

scala> val evens = x.filter(_ % 2 == 0)
evens: List[Int] = List(2, 4, 6, 8)

Discussion

In this example, the original function literal consists of the following code:

(i: Int) => i % 2 == 0

When examining this code, it helps to think of the => symbol as a transformer, because the expression transforms the parameter list on the left side of the symbol (an Int named i) into a new result using the algorithm on the right side of the symbol (in this case, an expression that results in a Boolean).

As mentioned, this example shows the long form ...