Redux reducer composed using new @functionBuilder api

ReducerBuidler.swift

protocol Action {}

struct Reducer<State> {
    let reduce: (State, Action) -> State
}

struct Increment: Action {}
struct Decrement: Action {}

func on<State, A>(_ actionType: A.Type, reduce: @escaping (State, A) -> State) -> Reducer<State> {
    Reducer<State> { state, action in
        guard let action = action as? A else {
            return state
        }
        
        return reduce(state, action)
    }
}

@_functionBuilder
struct ReduceBuilder<State> {
    static func buildBlock(_ reducers: Reducer<State>...) -> Reducer<State> {
        return Reducer<State> { state, action in
            var state = state
            for reducer in reducers {
                state = reducer.reduce(state, action)
            }
            return state
        }
    }
}

func reduce<State>(@ReduceBuilder<State> builder: @escaping () -> Reducer<State>) -> Reducer<State> {
    return builder()
}

let counter: Reducer<Int> = reduce {
    on(Increment.self) { state, action in state + 1 }
    on(Decrement.self) { state, action in state - 1 }
}

var state = 0
state = counter.reduce(state, Increment())
state = counter.reduce(state, Increment())
state = counter.reduce(state, Increment())
state = counter.reduce(state, Decrement())
state = counter.reduce(state, Decrement())