Kotlin Extensions And Pipe

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Introduction

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fun MutableList<Int>.swap(index1: Int, index2: Int) {
    val tmp = this[index1] // 'this' corresponds to the list
    this[index1] = this[index2]
    this[index2] = tmp
}

val list = mutableListOf(1, 2, 3)
list.swap(0, 2) // 'this' inside 'swap()' will hold the value of 'list'

Extensions

Example - Pipe

Arithmetics Version 1

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fun multiplyByTwo(x: Int): Int = x * 2

fun addOne(x: Int): Int = x + 1

fun addTwo(x: Int) : Int = x + 2

fun performOperations(x : Int) : Int {
    return multiplyByTwo(addTwo(addOne(x)))
}

Arithmetics Version 2

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fun Int.addOne(): Int = this + 1

x.addOne()
 .addTwo()
 .multiplyByTwo()
//  or
x.let { addOne(this) }
 .let { addTwo(this) }
 .let { multiplyByTwo(this) }

Arithmetics Version 3

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infix fun <T> T.then(func: (T) -> T): T = func(this)

x then { it + 1 } then { it + 2 }

Arithmetics Version 4

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fun <T> T.pipe(vararg functions: (T) -> T): T {
    return functions.fold(this) { value, f ->
        f(value)
    }
}

fun main() {
    val result = 100.pipe(::addOne, ::addTwo, ::multiplyByTwo)
    println(result)
}
//  result is 206

Reference