What is a higher-order function in Kotlin?

Short answer

It is a function that takes another function as a parameter or returns a function as a result.

Explanation

Kotlin treats functions as first-class citizens, allowing functions to be stored in variables, passed, and returned using function types such as (T) -> R. When the lambda is the last parameter of the function, it can be written outside the parentheses of the call (trailing lambda syntax).

Code example

fun filterTransactions(
    transactions: List<Double>,
    predicate: (Double) -> Boolean
): List<Double> {
    val result = mutableListOf<Double>()
    for (t in transactions) {
        if (predicate(t)) result.add(t)
    }
    return result
}

// Trailing lambda
val highValue = filterTransactions(transactions) { it > 1000.0 }

Common mistakes

• Ignoring the cost of creating function objects in memory when the lambda call is repeated in a loop without using inline.

Follow-up questions

• What is the difference between inline, noinline, and crossinline in Kotlin?

Sources

• Kotlin Docs — Higher-order functions and lambdas