[Functional Programming ADT] Combine Multiple State ADT Based Redux Reducers

Posted 2021-02-08 answer1215

Redux provides a convenient helper for combining many reducers called combineReducer, but it focuses in on specific attributes on our state, making it incompatible with using the State ADT. We would like a way to avoid keeping all of our reducers in a single file and want the ability to still combine them in a manner that works with the State ADT.

So we will put together our own helper that we also call combineReducers, but explore how we can use the First Monoid and mreduceMap to get us all the power that the Redux helper provides, but setup for our unique needs. As a bonus we will get a sneak peak of the power of using the flipcombinator to create easy to read compositions without pesky argument juggling

 

// combineReducers :: [ Reducer ] -> Reducer
/*
export const combineReducers = reducers => action =>
  mreduceMap(First, applyTo(action), reducers);
*/
// We take reducers first and action second, but we use action first, reducers second.
// It is good case to use flip
/*
export const combineReducers = flip(action =>
  mreduceMap(First, applyTo(action))
);*/

// We can use compose to remove action param, applyTo will get action
// Then the return result will be passed into mreduceMap(First)
export const combineReducers = flip(
  compose(
    mreduceMap(First),
    applyTo
  )
);