-- isString.hs: functions implementing the fsm in string-fsm.jpeg (this folder)

-- The 'isString :: String -> Bool' line declares isString to be a function which takes
--   a Haskell string and returns true if that string matches the FSM.

isString :: String -> Bool
isString chars = isStringStart chars

isStringStart text = if null text
                     then False
                     else if (head text) == 'Q'
                          then isStringMiddle (tail text)
                          else False

isStringMiddle text = if null text
                      then False
                      else 
                        let next:rest = text in
                          if next == 'a' 
                          then isStringMiddle rest
                          else if next == 'Q'
                               then isDone rest
                               else if next == 'B'
                                    then isEscape rest
                                    else False

isEscape text = if null text
                then False
                else if (head text) == 'Q'
                then isStringMiddle (tail text)
                else False

isDone text = null text