Constraint ADT for Flow Action and Reaction emit

adt_bound_constrain.kt

/** Dummy login model **/
@JvmInline
value class UserName(val value: String)

@JvmInline
value class Password(val value: String)

/**
 * A [Reaction] is always parametric to a type of [Action]
 */
sealed class Reaction<out A : Action>

/**
 * The [LoginReaction] can only be sent for [LoginAction]
 */
object LoginReaction : Reaction<LoginAction>()

/**
 * The [Reaction1] can only be sent for [OtherAction]
 */
object Reaction1 : Reaction<OtherAction>()


/**
 * The [WhateverReaction] is good for all [Action]
 */
object WhateverReaction : Reaction<Nothing>()

sealed class Action
data class LoginAction(val userName: UserName, val password: Password) : Action()
object OtherAction : Action()


suspend fun login(userName: UserName, password: Password) {
  val action = LoginAction(userName, password)
  val loginFlow = react(action) { LoginReaction } // this is fine because LoginReaction is valid for LoginAction
  return loginFlow.collect { reaction : LoginReaction ->
    //here reaction can only be of type LoginReaction
  }
}

/**
 * This is the motherload. This function ensures that a given [Action] [A]
 * creates a [Flow] of [Reaction] for [R], a Reaction that only accepts actions of type [A]
 */
inline fun <A : Action, R : Reaction<A>> react(
  action: A,
  crossinline f: (A) -> R
): Flow<R> = 
  flow { emit(f(action)) }

fun badProgram() {
  val action: LoginAction = LoginAction(UserName("test"), Password("test"))
  react(action) {
    Reaction1 //Type mismatch: inferred type is Reaction1 but Reaction<LoginAction> was expected
  }
}