Add tree structure generation

src/Dodge/Layout/Tree/Either.hs

@@ -0,0 +1,22 @@
+{-|
+Combining and composing trees with 'Either' nodes.
+-}
+module Dodge.Layout.Tree.Either
+    ( expandTreeBy
+    ) where
+import Data.Tree
+
+-- | 'Left' elements get new children as given by the node function, 
+-- 'Right' elements inherit the children from the base tree
+expandTreeBy 
+    :: (a -> Tree (Either b b)) -- ^ Node function
+    -> Tree a                   -- ^ Base tree
+    -> Tree b
+expandTreeBy f (Node x []) = fmap removeEither (f x)
+expandTreeBy f (Node x xs) = appendAndRemove $ f x
+  where
+    appendAndRemove (Node (Left y)  ys) = Node y (map appendAndRemove ys)
+    appendAndRemove (Node (Right y) _ ) = Node y (map (expandTreeBy f) xs)
+
+removeEither (Left y)  = y
+removeEither (Right y) = y