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 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)
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.
To create a value for an anonymous function that does not take parameters, leave the parameter list blank:
val sayHello = () => println("hello")