Yoneda.kt

typealias HomList<A, B> = ((A) -> B) -> List<B>

fun <A, B> List<A>.yoneda(): HomList<A, B> {
    return { f -> this.map(f) }
}

fun <A, B> HomList<A, B>.lower(): List<A> {
    return buildList {
        this@lower { a ->
            add(a)
            Unit as B
        }
    }
}

fun main() {
    val numbers: List<Int> = listOf(1, 2, 3)
    val yoneda = numbers.yoneda<_, Int>()
    val yonedaString = numbers.yoneda<_, String>()
    println(yoneda { it + 1 }) // [2, 3, 4]
    println(yonedaString { "Number: $it" }) // [Number: 1, Number: 2, Number: 3]
    println(yoneda.lower()) // [1, 2, 3]
    println(yonedaString.lower()) // [1, 2, 3]
}