gistfile1.hs

import Control.Monad.RWS
import Data.Map
import qualified Data.Map as Map

type Knot k s a = RWS k k s a

tie :: Knot k s a -> s -> (a, s, k)
tie knot s0 =
  let (a, s1, k) = runRWS knot k s0 -- this is fucking interesting
  in (a, s1, k)

data Node = Node {
  value :: Int,
  next  :: Node
} 

data ParserState = ParserState {
  _map :: Map Int Node, 
  remaining :: [(Int, Int)]
}

type NodeParser a = Knot (Map Int Node) ParserState a

parse :: NodeParser ()
parse = do
  s <- get
  case remaining s of
    [] -> return ()
    ((val, ref) : rest) -> do
      let mm = _map s
      put  $ ParserState (Map.insert val (Node val $ mm ! ref) mm) rest
      parse

-- example :: Node
example =
  let (_, s, _) = tie parse $ ParserState Map.empty [(10, 2), (2, 3), (3, 10)]
  in _map s Map.! 10

main = putStrLn $ show $ value $ next example