Scala Language Functions Anonymous Functions


Anonymous functions are functions that are defined but not assigned a name.

The following is an anonymous function that takes in two integers and returns the sum.

(x: Int, y: Int) => x + y

The resultant expression can be assigned to a val:

val sum = (x: Int, y: Int) => x + y

Anonymous functions are primarily used as arguments to other functions. For instance, the map function on a collection expects another function as its argument:

// Returns Seq("FOO", "BAR", "QUX")
Seq("Foo", "Bar", "Qux").map((x: String) => x.toUpperCase)

The types of the arguments of the anonymous function can be omitted: the types are inferred automatically:

Seq("Foo", "Bar", "Qux").map((x) => x.toUpperCase)

If there is just one argument, the parentheses around that argument can be omitted:

Seq("Foo", "Bar", "Qux").map(x => x.toUpperCase)

Underscores shorthand

There is an even shorter syntax that doesn't require names for the arguments. The above snippet can be written:

Seq("Foo", "Bar", "Qux").map(_.toUpperCase)

_ represents the anonymous function arguments positionally. With an anonymous function that has multiple parameters, each occurrence of _ will refer to a different argument. For instance, the two following expressions are equivalent:

// Returns "FooBarQux" in both cases
Seq("Foo", "Bar", "Qux").reduce((s1, s2) => s1 + s2)
Seq("Foo", "Bar", "Qux").reduce(_ + _)

When using this shorthand, any argument represented by the positional _ can only be referenced a single time and in the same order.

Anonymous Functions with No Parameters

To create a value for an anonymous function that does not take parameters, leave the parameter list blank:

val sayHello = () => println("hello")