module Language.Haskell.FreeTheorems.Variations.PolySeq.M (M,runM,newLab,abort,track) where

type State = (Int,[String])

data M a = M (State -> [(a,State)])

instance Monad M where
    return a = M (\s -> [(a,s)])
    M m >>= k = M (\s -> concatMap (\(a,s') -> case k a of M l -> l s') (m s))

runM :: M a -> Maybe (a, [String])
runM (M m) = case m (1,[]) of
	       []        -> Nothing
	       ((a,(_,ls)):_) -> Just (a,reverse ls)

newLab :: M Int
newLab = M(\(i,ls) -> [(i,(i+1,ls))])

abort :: M a
abort = M (\s -> [])

track :: String -> M ()
track l = M (\(i,ls) -> [((),(i,l:ls))])