kotlin -- can I rewrite this map access to get rid of the ``!!``?

Question

Let's say I have a

enum class ConceptualPosition{
    A1,A2,A3,A4,A5,A6,A7,A8,
    B1,B2,B3,B4,B5,B6,B7,B8,
    C1,C2,C3,C4,C5,C6,C7,C8
}

And I now want to construct a Graph where every position has a Node.

import [...].domain.model.base.ConceptualPosition
import [...].domain.model.base.ConceptualPosition.*
import [...].domain.model.base.Node
import [...].domain.model.base.Node.MovableDirection.*

class MyGraph:Graph{
    private val internalPositions = mutableMapOf<ConceptualPosition, Node>()
    override val position by lazy {internalPositions.toMap().withDefault {
        throw IllegalArgumentException("position '$it' does not exist")
    }}

    init{
        //create nodes for all positions
        ConceptualPosition.values().forEach { internalPositions[it] = Node(it) }

        //now build graph
        position[A1]!!.connectBothWaysWith(position[A2]!!,RIGHT)
        position[A2]!!.connectBothWaysWith(position[A3]!!,RIGHT)
        [...]

    }
}

since I have a withDefault that immediately throws an exception and since Kotlin correctly infers the type of position to be Map<ConceptualPosition,Node>, I think I can be fairly sure that if a lookup does not fail, I get a non-null value out of it.

Kotlin apparently cannot safely infer that.

Can I refactor this (in a reasonable way) to get rid of the !!?

Answer 1

Use Map.getValue instead of get:

position.getValue(A1).connectBothWaysWith(position.getValue(A2),RIGHT)

Alternately, you could define your own version of withDefault with a more precise return type (the Kotlin standard library makes the equivalent private):

class MapWithDefault<K, V>(val map: Map<K, V>, val default: (K) -> V) : Map<K, V> by map {
    override fun get(key: K): V = map[key] ?: default(key)
}

fun <K, V> Map<K, V>.withDefault(default: (K) -> V) = MapWithDefault(this, default)