actions.js

// mutators.js
export const closeModal = s => s.set('modalOpen', false)
export const stopLoading = s => s.set('loading', false)
export const updateLoan = loan => s => s.setIn(['loans', loan.id], Immutable.fromJS(loan))
export const setPaymentError = error => s => s.set('paymentError', error)
// utils.js
const applyFn = (state, fn) => fn(state)
export const pipe = (fns, state) =>
  state.withMutations(s => fns.reduce(applyFn, s))
// reducer.js
import * as mutate from './mutators'
export default function reducer(state=initialState, action) {
  switch(action.type) {
    case defs.MAKE_PAYMENT_SUCCESS:
      return pipe([
        mutate.closeModal,
        mutate.stopLoading,
        mutate.updateLoan(action.loan),
      ], state)
    case defs.MAKE_PAYMENT_FAILED:
      return pipe([
        mutate.closeModal,
        mutate.stopLoading,
        mutate.setPaymentError(action.error),
      ], state)
    // ...
  }
}