module NonDet where

data L e a = Failure (Maybe e) | Success a (L e a)
  deriving Show

unitL :: a -> L e a
unitL a = Success a (Failure Nothing)

(Failure _)  `appendL` xs = xs
Success x xs `appendL` ys = Success x (xs `appendL` ys)

bindL :: L e a -> (a -> L e b) -> L e b
(Success a es) `bindL` k = k a  `appendL` (es `bindL` k)
(Failure e)    `bindL` k = Failure e

failL :: e -> L e a
failL e = Failure (Just e)

tryL :: L e a -> (Maybe e -> L e a) -> L e a
tryL (Success v vs) h = Success v (tryL vs h)
tryL (Failure e) h    = h e

toList :: L e a -> [a]
toList (Success x xs) = x : toList xs
toList _              = []