reducer_fun_interface.kt

import kotlinx.coroutines.flow.*
import kotlinx.coroutines.*

data class State(val isLoading: Boolean = false, val items: List<Int> = emptyList())

fun interface Reducer {
    operator fun invoke(current: State): State
}

val loadingReducer = Reducer { it.copy(isLoading = true) }

val loadedReducer = Reducer{ it.copy(isLoading = false, items = listOf(1, 2, 3)) }

fun main() = runBlocking {
    flowOf<Reducer>(
    	loadingReducer,
        loadedReducer,
        loadingReducer
    )
    .scan(State()) { state, reducer -> reducer(state) }
    .collect(::println)
}

// State(isLoading=false, items=[])
// State(isLoading=true, items=[])
// State(isLoading=false, items=[1, 2, 3])
// State(isLoading=true, items=[1, 2, 3])