code snippet from the video

jpg-automaton.hs

type State = Int

checkInput :: State -> String -> State
checkInput s [] = s
checkInput 0 (x:xs)
  | x == 'j'  = checkInput 1 xs
  | x == 'p'  = checkInput 0 xs
  | x == 'g'  = checkInput 0 xs
  | otherwise = checkInput 0 xs
checkInput 1 (x:xs)
  | x == 'j'  = checkInput 1 xs
  | x == 'p'  = checkInput 2 xs
  | x == 'g'  = checkInput 0 xs
  | otherwise = checkInput 0 xs
checkInput 2 (x:xs)
  | x == 'j'  = checkInput 1 xs
  | x == 'p'  = checkInput 0 xs
  | x == 'g'  = checkInput 3 xs
  | otherwise = checkInput 0 xs
checkInput 3 (x:xs)
  | x == 'j'  = checkInput 1 xs
  | x == 'p'  = checkInput 0 xs
  | x == 'g'  = checkInput 0 xs
  | otherwise = checkInput 0 xs
  
automatonAccepts :: String -> Bool
automatonAccepts str = 3 == checkInput 0 str

main = do
  putStrLn . show $ automatonAccepts "jpg"