Memoize.kt

class Memoize0<out R>(val f: () -> R) : () -> R {
    private val value = lazy(f)
    override fun invoke(): R {
        return value
    }
}

fun <R> memoize(block: () -> R): () -> R = Memoize0(block)

class Memoize1<in T, out R>(val f: (T) -> R) : (T) -> R {
    private val values = mutableMapOf<T, R>()
    override fun invoke(x: T): R {
        return values.getOrPut(x, { f(x) })
    }
}

fun <T, R> memoize(block: (T) -> R): (T) -> R = Memoize1(block)

// Usage
val memoizedFoo = memoize { x: Int -> foo(x) }
val memoizedBar = memoize(::bar)