undecidabot

  export const addMovie = (movie: string): Action => ({ tag: "AddMovie", movie });

  interface State {
    movies: string[]
  }

  // you may also use { tag: "Action", payload: string }
  // if you prefer something more structured
  type Action =
    | ["AddMovie", string]
    | ["RemoveMovie", string]
    | ["Reset"]

  const defaultState: State = { movies: [] }

  const reducer = (state: State, action: Action): State => {
    switch (action[0]) {
      case "AddMovie": return { movies: [action[1], ...state.movies] }
      case "RemoveMovie": return { movies: state.movies.filter(m => m !== action[1]) }
      case "Reset": return defaultState
    }
  }

  const someAction: Action = ["AddMovie", "The End of Evangelion"]