data FSM state input output = FSM -- Input and output alphabet are constricted at compile time by datatype
  { start :: state, -- Start state
    trans :: state -> input -> state, -- Transition function
    accept :: [state], -- Set of accept states
    out :: OutFunc state input output -- Output function, either Moore or Mealy
  }

data OutFunc state input output
  = Moore (state -> output)
  | Mealy (state -> input -> output)

runFSM :: Eq state => FSM state input output -> [input] -> (state, Bool)
runFSM (FSM start trans accept out) inputs = (finalState, elem finalState accept)
  where
    -- finalState = foldl' trans start input
    finalState = go inputs start

    -- Output only changes when state changes occur
    --go :: [input] -> state -> state
    go [] acc = acc
    go (x : xs) currentState = go xs (trans currentState x)




data Binary = Zero | One deriving (Enum, Eq, Show)

data EvenState = E0 | E1 deriving (Eq, Show)

isEven :: FSM EvenState Binary ()
isEven = FSM
  { start  = E0
  , trans  = trans
  , accept = [E0]
  , out    = Moore $ const ()
  }
  where
    trans :: EvenState -> Binary -> EvenState
    trans E0 _ = E1
    trans E1 _ = E0