reducerWithImmutable.js

export default function reducer(state=initialState, action) {
  switch(action.type) {
    case defs.MAKE_PAYMENT_SUCCESS:
      return state.withMutations(s =>
        s.set('modalOpen', false)
         .set('loading', false)
         .setIn(['loans', action.loan.id], Immutable.fromJS(action.loan))
      )
    case defs.MAKE_PAYMENT_FAILED:
      return state.withMutations(s =>
        s.set('modalOpen', false)
         .set('loading', false)
         .set('paymentError', action.error)
      )
    // ...
  }
}