Add more sounds

src/ListHelp.hs

@@ -7,9 +7,24 @@ module ListHelp
     , safeHead
     , safeUncons
     , errorHead
-    )where
+    , foldPairs
+    , loopPairs
+    ) where
 import Data.List
 
+loopPairs :: [a] -> [(a,a)]
+loopPairs (a:as) = snd $ foldr f (Just a,[]) (a:as)
+  where
+    f x (Nothing,ps) = (Just x,ps)
+    f x (Just y,ps)  = (Just x,(x,y):ps)
+loopPairs [] = error "tried to make loop with empty list of elements"
+
+foldPairs :: [a] -> [(a,a)]
+foldPairs = snd . foldr f (Nothing, [])
+  where
+    f x (Nothing,ps) = (Just x,ps)
+    f x (Just y,ps)  = (Just x,(x,y):ps)
+
 initOrNull :: [a] -> [a]
 initOrNull [] = []
 initOrNull xs = init xs