Type the redux middleware

[caasi]

Flow keep picking the wrong branch in a union type of two functions(Dispatch).

Type the function will help me to describe my async acitons like const setDate: RawAction<[string], string>.

type InjectStore<A, B> = RawAction<A, B> => Action<A, B>
export type Dispatch
  = DispatchAPI<PlainAction>
  | InjectStore<*, *>

const app: ? = store => next => action =>
  typeof action === 'function'
    ? action(store)
    : next(action)

see: facebook/flow#1948 for more information about union type and functions.