A withDefault extension function

[andrew-at-rm]

Basically something the same as https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.collections/with-default.html

Not sure if it would be as simple as below

class CacheWithDefault<K : Any, V : Any>(
    val cache: Cache<K, V>,
    private val default: (K) -> V?
) : Cache<K, V> by cache {
    override fun get(key: K): V? = cache.get(key) ?: default(key)?.also { cache.put(key, it) }
}

fun <K : Any, V : Any> Cache<K, V>.withDefault(defaultValue: (K) -> V?): Cache<K, V> = when (this) {
    is CacheWithDefault -> this.cache.withDefault(defaultValue)
    is Cache -> CacheWithDefault(this, defaultValue)
}