Add airlocks

src/DoubleStack.hs

@@ -0,0 +1,33 @@
+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,[])
+
+singleton :: a -> DS a
+singleton x = DS (x,[],[])
+
+head (DS (x,_,_)) = x
+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