What does "trailing lambda" mean?

When the last parameter of a function is a function type, Kotlin allows you to pass the lambda outside the parentheses. myFun(arg1) { lambdaBody } instead of myFun(arg1, { lambdaBody }). This makes DSL-style code more readable.

What is a function type in Kotlin?

(Int, Int) -> Int describes a function that takes two Ints and returns Int. Variables and parameters can hold functions, enabling callbacks and higher-order APIs.

Intermediate 8 min readKotlin 2.0

Kotlin Higher-Order Functions — Passing Functions as Parameters

A higher-order function accepts a function as a parameter or returns a function. This is central to Kotlin's functional programming style.

What You Will Learn

What a function type is
Pass a function as a parameter
Use lambda syntax at the call site
Return a function from a function
Understand the trailing lambda convention

Function Types

A function type describes a function's signature. (Int, Int) -> Int means: takes two Ints, returns an Int.

Accepting a Function

kotlin
fun calculate(a: Int, b: Int, operation: (Int, Int) -> Int): Int {
    return operation(a, b)
}

fun main() {
    val result1 = calculate(10, 5) { x, y -> x + y }
    val result2 = calculate(10, 5) { x, y -> x * y }
    println(result1)
    println(result2)
}

Output

15 50

operation is a parameter with function type (Int, Int) -> Int. At the call site, we pass a lambda { x, y -> x + y }. The trailing lambda syntax places the lambda outside the parentheses.

Beginner Tip: When the last parameter is a function, you can pass it as a trailing lambda outside the (): calculate(10, 5) { ... }

Returning a Function

A function can return another function.

Function Returning a Function

kotlin
fun multiplier(factor: Int): (Int) -> Int {
    return { number -> number * factor }
}

fun main() {
    val triple = multiplier(3)
    val double = multiplier(2)
    println(triple(7))
    println(double(7))
}

Output

21 14

multiplier returns a lambda that captures factor from its outer scope. triple and double are now functions you can call.

Practice Exercise

Exercisepredict output

What prints? fun apply(n: Int, f: (Int) -> Int) = f(n) fun main() { println(apply(5) { it * it }) }

Quick Quiz

What is a higher-order function?

Kotlin Higher-Order Functions — Passing Functions as Parameters

What You Will Learn

Function Types

Accepting a Function

Returning a Function

Function Returning a Function

Practice Exercise

Quick Quiz

Frequently Asked Questions

Related Tutorials