-- isStringWithPatterns.hs: functions implementing the fsm in string-fsm.jpeg (this folder)
--   This version uses patterns for function arguments

-- 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.
-- Note: this code will fail if a character other than Q, a, or B appears in the string.
--       Handling such input requires using "guards" in function definitions.

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

isStringStart [] = False
isStringStart ('Q':rest) = isStringMiddle rest
isStringStart ('a':rest) = False
isStringStart ('B':rest) = False

isStringMiddle [] = False
isStringMiddle ('a':rest) = isStringMiddle rest
isStringMiddle ('Q':rest) = isDone rest
isStringMiddle ('B':rest) = isEscape rest

isEscape [] = False
isEscape ('Q':rest) = isStringMiddle rest
isEscape ('a':_)    = False     -- _ means "don't care"
isEscape ('B':_)    = False

isDone text = null text