Bunting - A nano command-line flags library extracted from http4k

bunting.kt

import kotlin.properties.ReadOnlyProperty
import kotlin.reflect.KProperty

abstract class Bunting(val args: Array<String>) {
    fun requiredFlag() = BuntingFlag()
    fun defaultedFlag(default: String?) = BuntingFlag(default)
}

class BuntingFlag internal constructor(private val default: String? = null) : ReadOnlyProperty<Bunting, String> {
    override fun getValue(thisRef: Bunting, property: KProperty<*>): String {
        val windowed = thisRef.args.toList().windowed(2).map { it[0] to it[1] }.toMap()
        return windowed["--${property.name}"] ?: default
        ?: throw IllegalArgumentException("no --${property.name} passed")
    }
}

fun <T : Bunting> T.use(fn: T.() -> Unit): Unit = fn(this)

Main.kt

class MyGreatFlags(args: Array<String>) : Bunting(args) {
    val user by requiredFlag()
    val password by requiredFlag()
    val version by defaultedFlag("0.0.0")
}

// run the main with: java (...) Mainkt --user foo --password bar
fun main(args: Array<String>) = MyGreatFlags(args).use {
    println(user) // foo
    println(password) // bar
    println(version) // 0.0.0
}