Implement stable merging of Streams

src/Stream.hs

@@ -18,6 +18,18 @@ fromList (pair:pairs) = Stream [(pair, fromList pairs)]
 infinite :: a -> Stream a
 infinite a = Stream [(a, infinite a)]
 
+merge :: Eq a => Stream a -> Stream a -> Stream a
+merge (Stream aPairs) (Stream bPairs) = Stream $ foldl mergeOrAdd aPairs bPairs
+  where
+    openAt _ l [] = (l, [])
+    openAt p up l@(x:xs) = if p x then (up, l) else openAt p (x:up) xs
+    zipUp ([], l) = l
+    zipUp (x:xs, l) = zipUp (xs, x:l)
+    mergeOrAdd accumulator (b, bStream) = zipUp $
+      case openAt ((== b) . fst) [] accumulator of
+        (up, []) -> (up, [(b, bStream)])
+        (up, (x, xStream):down) -> (up, (x, merge xStream bStream):down)
+
 showTree :: Show a => Stream a -> String
 showTree = intercalate "\n" . getLines
   where