coffee.kt

class Milk
class Beans
class Powder
class Water
class Coffee(water: Water, powder: Powder)

sealed interface BrewResult {
    class Success(val coffee: Coffee) : BrewResult
    sealed interface Failure : BrewResult {
        object InsufficientPowder : Failure
        object InsufficientWater : Failure
        object BrewerBroken : Failure
    }
}

sealed interface GetMilkResult {
    class Success(val milk: Milk) : GetMilkResult // don't expose success result
    sealed interface Failure : GetMilkResult, BrewResult {
        object Expired : Failure
        object Empty : Failure
    }
}

sealed interface GetBeansResult {
    class Success(val beans: Beans) : GetBeansResult // don't expose success result
    object Empty : GetBeansResult, BrewResult
}

sealed interface GrindBeansResult {
    class Success(val powder: Powder) : GrindBeansResult // don't expose success result
    sealed interface Failure : GrindBeansResult, BrewResult {
        object GrinderBroken : Failure
        object InsufficientBeans : Failure
    }
}

interface Service {
    fun getMilk(): GetMilkResult
    fun getBeans(): GetBeansResult
    fun grindBeans(beans: Beans): GrindBeansResult
    fun brew(powder: Powder, water: Water, milk: Milk): BrewResult
}

interface CoffeeMachine {
    fun brew(): BrewResult
}

class CoffeeMachineImpl(
    private val service: Service,
) : CoffeeMachine {
    override fun brew(): BrewResult {
        val milk = when (val getMilkResult = service.getMilk()) {
            is GetMilkResult.Success -> getMilkResult.milk
            is GetMilkResult.Failure -> return getMilkResult
        }
        val beans = when (val getBeansResult = service.getBeans()) {
            is GetBeansResult.Success -> getBeansResult.beans
            is GetBeansResult.Empty -> return getBeansResult
        }
        val powder = when (val grindBeansResult = service.grindBeans(beans)) {
            is GrindBeansResult.Success -> grindBeansResult.powder
            is GrindBeansResult.Failure -> return grindBeansResult
        }
        return service.brew(powder, Water(), milk)
    }
}

fun main(coffeeMachine: CoffeeMachine) {
    when (val brewResult = coffeeMachine.brew()) {
        is BrewResult.Success -> println("Enjoy your ${brewResult.coffee}")
        BrewResult.Failure.BrewerBroken -> println("Brewer is broken")
        BrewResult.Failure.InsufficientPowder -> println("You did not use enough powder")
        BrewResult.Failure.InsufficientWater -> println("You did not use enough water")
        GetBeansResult.Empty -> println("Please buy more beans first")
        GetMilkResult.Failure.Empty -> println("Sorry, we're out of mild")
        GetMilkResult.Failure.Expired -> println("Sorry, the milk is stale")
        GrindBeansResult.Failure.GrinderBroken -> println("Sorry, your grinder seems to be broken")
        GrindBeansResult.Failure.InsufficientBeans -> println("Sorry, you did not use enough beans to create powder")
    }
}