module DoubleStack
    where

newtype DS a = DS (a,[a],[a])
    deriving (Eq, Ord, Read, Show)
{- |
Unsafe. -}
fromListL :: [a] -> DS a
fromListL (x:xs) = DS (x,xs,[])
fromListL _ = undefined

singleton :: a -> DS a
singleton x = DS (x,[],[])

head :: DS a -> a
head (DS (x,_,_)) = x
left, right :: DS a -> [a]
left (DS (_,l,_)) = l
right (DS (_,_,r)) = r

pushL :: DS a -> DS a
pushL (DS (x,xs, y:ys )) = DS (y, x:xs ,ys)
pushL ds = ds

pushR :: DS a -> DS a
pushR (DS (y, x:xs ,ys)) = DS (x,xs, y:ys )
pushR ds = ds

cycleL :: DS a -> Maybe (DS a)
cycleL (DS (x,xs, y:ys )) = Just $ DS (y, x:xs ,ys)
cycleL _ = Nothing

cycleR :: DS a -> Maybe (DS a)
cycleR (DS (y, x:xs ,ys)) = Just $ DS (x,xs, y:ys )
cycleR _ = Nothing