Lens.kt

class Lens<Entity : Any, Value : Any?>(
    val get: (Entity) -> Value,
    val set: (Entity, Value) -> Entity
)

fun <Entity : Any, Value : Any?> Flow<Entity>.lens(lens: Lens<Entity, Value>): Flow<Value> =
    map { lens.get(it) }.distinctUntilChanged()

fun <Entity1 : Any, Entity2 : Any, Value : Any?> Lens<Entity1, Entity2>.compose(other: Lens<Entity2, Value>): Lens<Entity1, Value> =
    Lens(
        get = { entity -> other.get(get(entity)) },
        set = { entity1: Entity1, value -> 
            val entity2: Entity2 = get(entity1)
            set(entity1, other.set(entity2, value))
        }
    )