function counter(state = 0, action) {
  switch (action.type) {
    case 'INCREMENT':
      return state + 1
    case 'DECREMENT':
      return state - 1
    default:
      return state
  }
}

//change to

const counter = (state = 0, action) =>
  action.type === 'INCREMENT'   ? state + 1
  : action.type === 'DECREMENT' ? state - 1
                                : state

// change to

const switchcase = cases => defaultCase => key =>
  cases.hasOwnProperty(key) ? cases[key] : defaultCase

const counter = (state = 0, action) =>
  switchcase({
    'INCREMENT': state + 1,
    'DECREMENT': state -1
  })(state)(action.type)

// change to

const switchcaseF = cases => defaultCase => key =>
  switchcase(cases)(defaultCase)(key)()

const counter = (state = 0, action) =>
  switchcaseF({
    'INCREMENT': () => state + 1,
    'DECREMENT': () => state -1
  })(() => state)(action.type)

// change to
const executeIfFunction = f =>
  f instanceof Function ? f() : f

const switchcaseF = cases => defaultCase => key =>
  executeIfFunction(switchcase(cases)(defaultCase)(key))

const counter = (state = 0, action) =>
  switchcaseF({
    'INCREMENT': () => state + 1,
    'DECREMENT': () => state -1
  })(state)(action.type)

const counter = (state = 0, action) =>
  switchcaseF({
    'RESET': 0,
    'INCREMENT': () => state + 1,
    'DECREMENT': () => state -1
  })(state)(action.type)