ArithmeticMutator.kt

data class State(
    val count: Int = 0
)

sealed class Action {
    abstract val value: Int

    data class Add(override val value: Int) : Action()
    data class Subtract(override val value: Int) : Action()
}

val mutator : Mutator<Action, StateFlow<State>> = stateFlowMutator<Action, State>(
    scope = scope,
    initialState = State(),
    started = SharingStarted.WhileSubscribed(),
    transform = { actions ->
        actions.toMutationStream {
            when (val action = type()) {
                is Action.Add -> action.flow
                    .map {
                        Mutation { copy(count = count + value) }
                    }
                is Action.Subtract -> action.flow
                    .map {
                        Mutation { copy(count = count - value) }
                    }
            }
        }
    }
)