Implement a map using only functions

functionalMap.kt

// This is our map type
typealias FunctionalMap<K, V> = (K) -> V?

// An empty map never has the value
fun <K, V> emptyMap(): FunctionalMap<K, V> = { _ -> null }

// Match the key to the value or fallback
fun <K, V> FunctionalMap<K, V>.put(key: K, value: V): FunctionalMap<K, V> = { k ->
    if (k == key) value else this(k)
}

// Retrieval is just calling the function
operator fun <K, V> FunctionalMap<K, V>.get(key: K): V? = this(key)

// Removal overrides any insertion
fun <K, V> FunctionalMap<K, V>.remove(key: K): FunctionalMap<K, V> = { k ->
    if (k == key) null else this(k)
}

fun main() {
    var aMap: FunctionalMap<String, Int> = emptyMap()

    aMap = aMap.put("one", 1)
    aMap = aMap.put("two", 2)

    println(aMap["one"]) // Output: 1
    println(aMap["two"]) // Output: 2
    println(aMap["three"]) // Output: null

    aMap = aMap.remove("one")

    println(aMap["one"]) // Output: null
}