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Kotlin Lambdas

Also known as anonymous functions - name deriving from lambda calculus of Alonzo Church where all functions are anonymous.

Also synonymous with closures.

// declaration of a var that can hold a lambda var multiplyLambda: (Int, Int) -> Int multiplyLambda = { a: Int, b: Int -> Int a*b } val lambdaResult = multiplyLambda(4, 2) // 8

Shorthand Syntax

multiplyLambda = { a, b -> a*b } // using it keyword for argument var doubleLambda = { a: Int -> 2*a } doubleLambda = { 2 * it } // in declaration val square: (Int) -> Int = { it * it }

Lambdas As Arguments

fun operateOnNumbers(a: Int, b: Int, operation: (Int, Int) -> Int): Int { val result = operation(a, b) println(result) return result } // in use val addLambda = { a: Int, b: Int -> a+b } operateOnNumbers(4, 2, operation = addLambda) // 6 // more usual fun addFunction(a: Int, b:Int) = a + b operateOnNumbers(4, 2, operation = ::addFunction) // 6 // or even operateOnNumbers(4, 2, operation = { a: Int, b: Int -> a+b })

Lambdas with no meaningful return

var unitLambda: () -> Unit = { println("Kotlin Apprentice is awesome!") } unitLambda()

If you literally want no value returnedm you need to use the Nothing type:

var nothingLambda: () -> Nothing = { throw NullPointerException() }

Iterating over collections with lambdas

val values = listOf(1, 2, 3, 4, 5, 6) values.forEach { println("$it: ${it * it}") } // > 1: 1 // > 2: 4 // > 3: 9 // > 4: 16 // > 5: 25 // > 6: 36

To filter some of these out:

var prices = listOf(1.5, 10.0, 4.99, 2.30, 8.19) val largePrices = prices.filter { it > 5.0 } // the above function looks like so public inline fun <T> Iterable<T>.filter(predicate: (T) -> Boolean): List<T> val userInput = listOf("0", "11", "haha", "42") val numbers = userInput.map { it.toIntOrNull() } println(numbers) // > [0, 11, null, 42] // array [1,2,3,4] sum = prices.reduce { a, b -> a + b } println(sum) // 10 // similar but takes initial value var sum = prices.fold(0.0) { a, b -> a + b }

Repository

https://github.com/okeeffed/developer-notes-nextjs/content/kotlin/kotlin-lambdas

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