haskellのStateモナドでAutomaton

automaton.hs

import Control.Monad.Trans.State

data Auto = Auto {
  getNode :: Int,
  getValue :: [Int]
} deriving Show

type AutoOp = State Auto ()

f :: Char -> AutoOp
f c = do
  Auto n v <- get
  put $ Auto (next n c) (n:v)
  return ()

next n c = case (n, c) of
    (0, 'a') -> 1
    (0, _)   -> 2
    (1, 'b') -> 3
    (1, _)   -> 4
    (2, 'c') -> 3
    (2, _)   -> 4
    (3, _)   -> 5
    (4, _)   -> 5
    _        -> -1

main = do
  let s0 = Auto 0 []
  print $ (`runState` s0) $ foldr1 (>>) (f <$> "abc")