funclang series4: Added some comments and fixed existing key issue.

funclang-taddeus/series4/ass9.ml

@@ -15,17 +15,17 @@ let rec insert trie key value = match trie with
     Empty -> Node (key, value, []) (* Root *)
     | Node (k, v, c) ->
         if k = key then
-            raise (Failure "Inserted key already exists in trie")
-        else if not (starts_with key k) then
-            raise (Failure "Inserted key does not start with node key")
-        else
+            (* Key already exists, replace its value *)
+            Node (k, value, c)
+        else if starts_with key k then
+            (* Key prefix matches, trye to insert in children *)
             let match_node node = match node with
                 Empty -> false
                 | Node (k, v, c) -> starts_with key k
             in
             let matches = map match_node c in
             if fold_left (||) false matches then
-                (* Match in child node, do not add new child *)
+                (* Match in child node, insert in matching child *)
                 let apply_match (matched, node) =
                     if matched then insert node key value else node
                 in
@@ -33,6 +33,9 @@ let rec insert trie key value = match trie with
             else
                 (* No match in any child node, add new child *)
                 Node (k, v, c @ [Node (key, value, [])])
+        else
+            (* Key prefix does not match, key cannot be inserted *)
+            raise (Failure "Inserted key does not start with node key")
 
 (* Helper function, checks if a trie is not empty *)
 let non_empty = function | Empty -> false | _ -> true