For example we have a feature reducer like this:

// selectCard :: String -> Action Stringexport const selectCard =  createAction(SELECT_CARD)// showFeedback :: String -> Action Stringexport const showFeedback =  createAction(SHOW_FEEDBACK)// reducer :: Action a -> (State AppState ()) | Nullconst reducer = ({ type, payload }) => {  switch (type) {    case SELECT_CARD:      return answer(payload)    case SHOW_FEEDBACK:      return feedback(payload)  }  return null}export default reducer

 

First of all, we can replace 'swtich' statement with normal Object:

const actionReducer = {  SELECT_CARD: answer,  SHOW_FEEDBACK: feedback}  // reducer :: Action a -> (State AppState ()) | Nullconst reducer = ({ type, payload }) => (actionReducer[type] || Function.prototype)(payload)

In case of ´actionReducer[type]´ return undefined, we default a function by `Function.prototype`, ready to take a payload.

 

const reducer = ({ type, payload }) => (actionReducer[type] || Function.prototype)(payload)

This type of code is suitable for using 'Maybe'.

// createReducer :: ActionReducer -> Reducerexport const createReducer = actionReducer =>  ({ type, payload }) =>    prop(type, actionReducer)      .map(applyTo(payload))

Refactor:

// showFeedback :: String -> Action String export const showFeedback =  createAction(SHOW_FEEDBACK)const reducer = createReducer({  SELECT_CARD: answer,  SHOW_FEEDBACK: feedback})// reducer :: Action a -> (State AppState ()) | Null// const reducer = ({ type, payload }) => //   (actionReducer[type] || Function.prototype)(payload)export default reducer

 

For this workflow, the following code should also be chagned to handle Maybe type:

// Fromimport turn from './turn'//reducer :: (AppState, Action a) -> AppStateconst reducer = (prev, action) =>  const result = turn(action)  return isSameType(State, result)    ? result.execWith(prev)    : prev}export default reducer // Toimport turn from './turn'//reducer :: (AppState, Action a) -> AppStateconst reducer = (prev, action) =>  turn(action)    .chain(safe(isSameType(State)))//   ? result.execWith(prev) //   : prevexport default reducer

 

Here ´turn(action)´ return a Maybe type, we still need to check whether inside Maybe, it is `State` type, 

.chain(safe(isSameType(State)))

 

If it is, then we call `execWith` otherwise we return previous state:

const reducer = (prev, action) => turn(action)   .chain(safe(isSameType(State)))   .map(execWith(prev))   .option(prev)