monad-skeleton

This package provides Skeleton, an operational monad. The internal encoding gives O(1) bind and monadic reflection.

Skeleton promotes unit instructions to a monad. It is isomorphic to MonadView (Skeleton t):

data MonadView t m x where
  Return :: a -> MonadView t m a
  (:>>=) :: !(t a) -> (a -> m b) -> MonadView t m b

boned :: MonadView t (Skeleton t) a -> Skeleton t a
debone :: Skeleton t a -> MonadView t (Skeleton t) a

GADTs are handy to define instructions:

data Interaction x where
  Get :: Interaction String
  Put :: String -> Interaction ()

echo :: Skeleton Interaction ()
echo = bone Get >>= bone . Put

Use debone to interpret a computation.

interpret :: Skeleton Interaction a -> IO a
interpret m = case debone m of
  Return a -> return a
  Get :>>= k -> getLine >>= interpret . k
  Put s :>>= k -> putStrLn s >>= interpret . k