Function reducer

test/typescript/store.ts:30–42

(
  state: State | undefined = {
    a: 'a',
    b: {
      c: 'c',
      d: 'd'
    },
    e: brandedString
  },
  action: Action
)

}

const reducer: Reducer<State> = (
  state: State | undefined = {
    a: 'a',
    b: {
      c: 'c',
      d: 'd'
    },
    e: brandedString
  },
  action: Action
): State => {
  return state
}

const reducerWithAction: Reducer<State, DerivedAction> = (
  state: State | undefined = {