Lambdas and higher-order functions

A teacher wants to apply different score corrections without copying the list-processing loop. A higher-order function accepts another function as a value, so the traversal and the correction policy can be expressed separately. A lambda supplies the policy used for each score.

A higher-order function accepts another function or returns one. A lambda supplies behaviour as a value. It can capture surrounding values, so changing captured mutable state can change behaviour later. A trailing lambda can appear outside parentheses when it is the final argument.

Save the complete source as Main.kt and use the Kotlin/JVM environment. No separate input file is needed.

Kotlin/JVM · complete program

fun adjust(scores: List<Int>, rule: (Int) -> Int): List<Int> = scores.map(rule)
fun main() {
  val bonus = 5
  println(adjust(listOf(70, 80)) { score -> score + bonus })
}

Expected output

[75, 85]

Follow the execution

The function type takes one Int and returns an Int. Each score is passed to the lambda, which captures bonus five and returns a corrected mark. Adjust returns [75, 85] without changing the input list.

Try this in the Kotlin compiler by pasting the complete program.

Your task: Why can the lambda use bonus even though it is not its parameter?

Kotlin/JVM · complete program

fun adjust(scores: List<Int>, rule: (Int) -> Int): List<Int> = scores.map(rule)
fun main() {
  val bonus = 5
  println(adjust(listOf(70, 80)) { score -> score + bonus })
}

Worked answer

The lambda captures the surrounding value. Its parameter supplies each score, while bonus comes from the enclosing scope.

Lambda return rules differ between local labelled returns and non-local returns permitted by particular inline calls. Use clear labels when needed. Captured mutable state is not automatically safe for concurrent use.

The lambda captures the surrounding value. Its parameter supplies each score, while bonus comes from the enclosing scope. Lambda return rules differ between local labelled returns and non-local returns permitted by particular inline calls.

Further reading: official Kotlin documentation for this topic.