Inferred typealias defined in protocol by implementing a function

typealias.swift

protocol Action {
    typealias StateType
    func reduce(state: StateType) -> StateType
}

struct MyState { }

struct MyAction: Action {
    // Note: We don't have to define the StateType alias, it infers it.
    func reduce(state: MyState) -> MyState {
        return state
    }
}