High-Order Functions vs. Inline Functions in Kotlin

Table Of Contents

One of Kotlin’s standout features is its robust support for functions, including high-order functions and inline functions. In this blog post, we will delve into the world of functions in Kotlin, exploring the differences between high-order functions and inline functions and understanding when to leverage each for optimal code efficiency.

High-Order Functions

High-order functions are a fundamental concept in functional programming and play a crucial role in Kotlin’s functional paradigm. A high-order function is a function that takes one or more functions as parameters or returns a function. This ability to treat functions as first-class citizens allows for more modular and flexible code.

The primary advantage of high-order functions lies in their ability to promote code reuse. By passing functions as parameters, developers can create generic functions that operate on various data types or behaviors. This enhances the readability and maintainability of the code by isolating specific functionalities into separate functions.

Consider the following example of a simple high-order function in Kotlin:

fun <T> List<T>.customFilter(predicate: (T) -> Boolean): List<T> {
    val result = mutableListOf<T>()
    for (item in this) {
        if (predicate(item)) {
    return result

fun main() {
    val numbers = listOf(1, 2, 3, 4, 5, 6)
    val evenNumbers = numbers.customFilter { it % 2 == 0 }
    println(evenNumbers) // Output: [2, 4, 6]

In this example, customFilter is a high-order function that takes a predicate function as a parameter. This allows us to filter a list based on various conditions without duplicating filtering logic.

Inline Functions

While high-order functions provide modularity and reusability, they may introduce some runtime overhead due to the creation of function objects. This is where inline functions come into play. Inline functions, as the name suggests, are a way to instruct the compiler to replace the function call site with the actual body of the function during compilation. This process eliminates the overhead associated with function calls and can lead to more efficient code execution.

Consider the following example:

inline fun executeOperation(a: Int, b: Int, operation: (Int, Int) -> Int): Int {
    return operation(a, b)

fun main() {
    val result = executeOperation(5, 3) { x, y -> x + y }
    println(result) // Output: 8

In this example, the executeOperation function is declared as inline. When the compiler encounters a call to this function, it replaces the call site with the actual body of the function, avoiding the creation of additional function objects. This can be particularly beneficial in scenarios where performance is a critical factor.

When to use High-Order vs. Inline Functions?

High order functions are useful when we want to abstract over actions, parameterize behavior or create more flexible and reusable code.

Inline functions are used when we want to eliminate the overhead of function calls and improve performance. The inline keyword suggests to the compiler that it should insert the function’s code directly at the call site avoiding the overhead of function invocation.

Each higher-order function will build a new object and assign it to memory. This will increase the amount of time spent running. We use inline functions to solve this problem. We can use the inline keyword to prevent creating a new object for each higher-level function.

However, inline functions might increase the number of cache misses. Inlining might cause an inner loop to span across multiple lines of the memory cache and that might cause thrashing of the memory-cache.

Arguments for High-Order Functions

  • Modularity and Reusability: We want to create modular and reusable code by abstracting certain behaviors into functions.

  • Flexibility: We need the flexibility to pass different functions dynamically.

  • Code Readability: We prioritize readability and maintainability, as high-order functions contribute to cleaner and more organized code.

Arguments for Inline Functions

  • Performance: Performance is a critical concern, and we want to eliminate the overhead associated with function calls.

  • Code Size: We aim to reduce the size of our compiled code by inlining small functions.

  • Lambda Expressions: We frequently work with lambda expressions and want to minimize the overhead introduced by function objects.


Kotlin’s support for high-order functions and inline functions provides developers with powerful tools to write expressive and efficient code. High-order functions enhance code modularity and readability, while inline functions offer performance improvements by eliminating the overhead of function calls. Choosing between the two depends on the specific needs of our application, striking a balance between readability and performance for optimal code efficiency. As we navigate the world of Kotlin development, understanding when to leverage each type of function will empower us to write clean, maintainable and performant code.

Written By:

Ezra Kanake

Written By:

Ezra Kanake

Ezra is a passionate Kotlin developer and technical writer. He loves working on open-source projects and sharing knowledge across the globe.

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