Nullable Comprehension

nc.kt

import kotlin.coroutines.*
import kotlin.coroutines.intrinsics.COROUTINE_SUSPENDED
import kotlin.coroutines.intrinsics.suspendCoroutineUninterceptedOrReturn

class NullableComprehension<R> : Continuation<R> {

    override val context: CoroutineContext = EmptyCoroutineContext

    private var internalResult: R? = null

    /**
     * Asserts that the value is not null or interrupts `coerceNull` computation completely and returns null.
     */
    suspend fun <T> T?.coerce(): T {
        return suspendCoroutineUninterceptedOrReturn {
            if (this != null) {
                it.resume(this)
            }
            COROUTINE_SUSPENDED
        }
    }

    override fun resumeWith(result: Result<R>) {
        internalResult = result.getOrThrow()
    }

    fun getResult(): R? = internalResult

}

/**
 * Creates computational context in which you can [NullableComprehension.coerce] nullable types
 */
fun <R> coerceNull(block: suspend NullableComprehension<R>.() -> R): R? {
    val context = NullableComprehension<R>()
    block.startCoroutine(context, context)
    return context.getResult()
}

fun sample() {
  val a: Int? = 2
  val b: Int? = 3
  val c: Int = 5
  val d: Int? = null
  
  val res: Int? = coerceNull {
    a.coerce() + b.coerce() + c + d.coerce()
  }
  println(res) // null
}