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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
 

prog.c:1:1: error: unknown type name ‘data’
 data FSM state input output = FSM -- Input and output alphabet are constricted at compile time by datatype
 ^~~~
prog.c:1:10: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘state’
 data FSM state input output = FSM -- Input and output alphabet are constricted at compile time by datatype
          ^~~~~
prog.c:1:10: error: unknown type name ‘state’
prog.c:8:1: error: unknown type name ‘data’
 data OutFunc state input output
 ^~~~
prog.c:8:14: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘state’
 data OutFunc state input output
              ^~~~~
prog.c:8:14: error: unknown type name ‘state’
prog.c:15:26: warning: missing terminating ' character
     -- finalState = foldl' trans start input
                          ^
prog.c:15:26: error: missing terminating ' character
     -- finalState = foldl' trans start input
                          ^~~~~~~~~~~~~~~~~~~
prog.c:37:3: error: unknown type name ‘where’
   where
   ^~~~~
prog.c:38:11: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘:’ token
     trans :: EvenState -> Binary -> EvenState
           ^

Standard output is empty