pipesGameState.hs

import Control.Monad
import Control.Applicative
import Control.Proxy
import qualified Control.Proxy.Trans.State as S
import System.IO
import Prelude hiding (Left, Right)

type Pos = (Int, Int)
data Input = Up | Down | Left | Right | Invalid deriving Show

toInput :: Char -> Input
toInput c = case c of
    'u' -> Up
    'd' -> Down
    'l' -> Left
    'r' -> Right
    _   -> Invalid

move :: Input -> Pos -> Pos
move i p@(x,y) = case i of
  Up      -> (x,y-1)
  Down    -> (x,y+1)
  Left    -> (x-1,y)
  Right   -> (x+1,y)
  Invalid -> p

inputs :: Proxy p => () -> Producer p Input IO ()
inputs () = runIdentityP . forever $ lift (toInput <$> getChar) >>= respond

-- embed some local state into this proxy that will hold the current Pos
-- and accept an initial position as an arg
moves :: (Monad m, Proxy p) => Pos -> () -> Pipe p Input Pos m ()
moves p () = S.evalStateP p $ forever $ do
    s <- move <$> request () <*> S.get
    S.put s
    respond s

main = do
  hSetEcho stdin False
  runProxy $ inputs >-> moves (0, 0) >-> printD