Hello, I've a question about maps and potentially ...

# announcements

[roberto]

Hello, I've a question about maps and potentially lambdas, I've a map mapping int to objects creation, something like this:

val menuIdToItems: Map<Int, MyObject> by lazy {
mapOf(
menu1 to Item1.newInstance(),
menu2 to Item2.newInstance(),
menu3 to item3.newInstance(),
)

It's working fine, BUT I just realised that when I evaluate menuIdToItems[menu1], I actually need a NEW INSTANCE every time I evaluate menuIdToItems[menu1]. Right now, If I evaluate menuIdToItems[menu1] two times, second time will return the same instance that I got the first time.

Is there a way to do this using a map?

[gildor]

wrap Item1.newInstance() to lambda and then:
menuIdToItems[menu1]() or menuIdToItems[menu1].invoke()

[gildor]

And change map type to something like
Map<Int, () -> MyObject>

[roberto]

Note that I still want to keep returning the same instance for the case item2 and item3, If I do that, wouldn't have a type mismatch for other cases or I'll have to return new instance for all of them?

[gildor]

you need some abstraction with memoization

[gildor]

it means that create lambda with static value

[gildor]

not sure what is correct term for that, memoization or Identitiy (like in FP)

[gildor]

fun <T> memoized(instance: T): () -> T = { instance }

val menuIdToItems: Map<Int, () -> MyObject> by lazy {
        mapOf(
                menu1 to { Item1.newInstance() },
                menu2 to memoized(Item2.newInstance()),
                menu3 to memoized(item3.newInstance()),
        )
}

[roberto]

Awesome, I'm just trying what you suggest.

[gildor]

yeah, probably bad naming, also instance creation will not be lazy, probably better to use something like:

class Cached<T>(builder: () -> T) : () -> T {
    private val instance by lazy { builder() }
    override fun invoke(): T = instance
}

val menuIdToItems: Map<Int, () -> MyObject> by lazy {
        mapOf(
                menu1 to { Item1.newInstance() },
                menu2 to Cached { Item2.newInstance() },
                menu3 to Cached { item3.newInstance() },
        )
}

[gildor]

In this case instance of each item will be created only on first access and cached