/**
 * Abstract class for a LoadingMap.
 * Overrides [get] return type as non-nullable type V.
 */
abstract class LoadingMap<K : Any, V : Any>
protected constructor(private val source: MutableMap<K, V> = hashMapOf())
: Map<K, V> by source {

  final override fun get(key: K): V = source[key] ?: load(key).apply { source[key] = this }
  
  protected abstract fun load(key: K): V

  companion object {

    /**
     * Pseudo-constructor for inlining the load function
     */
    inline operator fun <K : Any, V : Any> invoke(source: MutableMap<K, V> = hashMapOf(),
                                                  crossinline load: (K) -> V) =
        object : LoadingMap<K, V>(source) {
          override fun load(key: K): V = load(key)
        }

  }

}

/*
 * Sample LoadingMap<String, Int>
 */
val cache = LoadingMap { key: String ->
  key.length
}