Is there a better way to write nested cases in Idris?

Here is a piece of Idris code from Edwin's book:

data DataStore : Type where
  MkData : (size : Nat) ->
           (items : Vect size String) ->
           DataStore

proccessInput : DataStore -> String -> Maybe (String, DataStore)
proccessInput store@(MkData size items) input =
  case map trim $ span (/= ' ') input of
    ("add", item) => Just ("ID " ++ show size ++ "\n", MkData _ (item :: items))
    ("get", val) => case all isDigit (unpack val) of
      False => Just ("Invalid command\n", store)
      True => case integerToFin (cast val) size of
        Nothing => Just ("Out of range\n", store)
        (Just id) => Just (index id items ++ " \n", store)
    ("quit", "") => Nothing
    (_, _) => Just ("Invalid command\n", store)

I've already eliminate out some pieces of codes that I think is redundant( some helper functions), But I still found that the code is somewhat redundant. The case split is really ugly I think.