Function

Lambda expressions

Kotlin allows you to write even more concise code for functions by using lambda expressions.

For example, the following uppercaseString() function:

fun uppercaseString(text: String): String {
    return text.uppercase()
}
fun main() {
    println(uppercaseString("hello"))
    // HELLO
}

Can also be written as a lambda expression:

fun main() {
    val upperCaseString = { text: String -> text.uppercase() }
    println(upperCaseString("hello"))
    // HELLO
}

Pass to another function

A great example of when it is useful to pass a lambda expression to a function, is using the .filter() function on collections:

fun main() {
    val numbers = listOf(1, -2, 3, -4, 5, -6)
 
 
    val positives = numbers.filter ({ x -> x > 0 })
 
    val isNegative = { x: Int -> x < 0 }
    val negatives = numbers.filter(isNegative)
 
    println(positives)
    // [1, 3, 5]
    println(negatives)
    // [-2, -4, -6]
 
}

Info

If a lambda expression is the only function parameter, you can drop the function parentheses ():

val positives = numbers.filter { x -> x > 0 }

This is an example of a trailing lambda, which is discussed in more detail at the end of this chapter.