// replacing constants
import {fetchTodos} from './actions'

export default function(state = ..., action){
  switch(action.type){

    case fetchTodos.TYPE: // 'FETCH_TODOS@ASYNC_REQUEST'
       return ...
    case fetchTodos.success.TYPE: // 'FETCH_TODOS@ASYNC_SUCCESS'
       return ...
    casse fetchTodos.failure.TYPE:  // 'FETCH_TODOS@ASYNC_FAILURE'
       return ...

    // used less, but are available...
    case fetchTodos.progress.TYPE:
       return ...
    case fetchTodos.cancel.TYPE:
       return ...
  }
}

// some where in your code
import {fetchTodos} from './actions'

fetchTodos()
// ==> { type: 'FETCH_TODOS@ASYNC_REQUEST' }

fetchTodos.success()
// ==> { type: 'FETCH_TODOS@ASYNC_SUCCESS' }

fetchTodos.failure()
// ==> { type: 'FETCH_TODOS@ASYNC_FAILURE' }

fetchTodos.progress()
// ==> { type: 'FETCH_TODOS@ASYNC_PROGRESS' }

fetchTodos.cancel()
// ==> { type: 'FETCH_TODOS@ASYNC_CANCEL' }