1 files changed, 40 insertions, 0 deletions
diff --git a/TreeZipperUtils.hs b/TreeZipperUtils.hs
index b6410a3..0c6dc00 100644
--- a/TreeZipperUtils.hs
+++ b/TreeZipperUtils.hs
@@ -1,5 +1,6 @@
 module TreeZipperUtils where
 
+import Data.Maybe
 import Data.Tree
 import Data.Tree.Zipper
 
@@ -10,3 +11,42 @@ path loc = toParent loc : parents loc
 -- Return parent stack compatible form of loc.
 toParent :: TreePos Full a -> (Forest a, a, Forest a)
 toParent loc = (before loc, label loc, after loc)
+
+
+modifyFirstParentLabelWhere
+    :: (a -> Bool)
+    -> (a -> a)
+    -> TreePos Full a
+    -> TreePos Full a
+modifyFirstParentLabelWhere p f loc0 =
+    case parent loc0 of
+        Nothing -> loc0
+        Just loc0' -> go (byChildIndex loc0) loc0'
+  where
+
+    go rewind loc =
+        if p (label loc)
+            then
+                rewind (modifyLabel f loc)
+            else
+                case parent loc of
+                    Nothing -> rewind loc
+                    Just loc' ->
+                        go (rewind . byChildIndex loc) loc'
+
+    -- generator for a rewind step
+    byChildIndex :: TreePos Full a -> (TreePos Full a -> TreePos Full a)
+    byChildIndex loc =
+        -- The use of fromJust is safe here because we're only modifying
+        -- labels and not the tree structure and thus the index is valid.
+        fromJust . childAt (childIndex loc)
+
+
+-- XXX This could be named more general, like countPrevSiblings?
+-- XXX Can we kill the recursion?
+childIndex :: TreePos Full a -> Int
+childIndex =
+    go 0
+  where
+    go index =
+        maybe index (go $ index + 1) . prev