diff --git a/editor/Unison/Parser.elm b/editor/Unison/Parser.elm
index c4bf7d7ae..64bc405b6 100644
--- a/editor/Unison/Parser.elm
+++ b/editor/Unison/Parser.elm
@@ -6,46 +6,92 @@ import Dict as M
 
 type Msg = [String]
 
--- type P a 
--- type P a = J.Value -> Either Msg a
--- type Parser a = { focus : Path, parser : P a } 
--- combinators like lift2 can merge common paths
+type P a = J.Value -> Either Msg a
 
-type Parser a = J.Value -> Either Msg a
+data E = Key String | Index Int
+type Path = [E]
+  
+type Parser a = { focus : Path, parse : P a }
 
-run : Parser a -> J.Value -> Either String a
-run p v = case p v of
-  Left stack -> Left (join "\n" stack)
-  Right a -> Right a
+safeIndex : Int -> [a] -> Maybe a
+safeIndex i xs = case drop i xs of
+  [] -> Nothing
+  h :: t -> Just h
+
+getPath : Path -> J.Value -> Either Msg J.Value
+getPath p v =
+  let 
+    failure loc v = Left [
+      "invalid path: " ++ show p,
+      "remainder: " ++ show loc,
+      "focus: " ++ J.toString "" v
+    ]
+    go loc v = case loc of
+      [] -> Right v
+      Key k :: t -> case v of
+        J.Object obj -> case M.get k obj of
+          Nothing -> failure loc v 
+          Just v -> go t v 
+        _ -> failure loc v 
+      Index i :: t -> case v of
+        J.Array vs -> case safeIndex i vs of
+          Nothing -> failure loc v
+          Just v -> go t v
+        _ -> failure loc v
+  in go p v 
+
+run : Parser a -> J.Value -> Either Msg a
+run p v = either Left p.parse (getPath p.focus v)
 
 unit : a -> Parser a
-unit a _ = Right a
+unit a = { focus = [], parse = \v -> Right a }
+
+value : Parser J.Value
+value = { focus = [], parse = Right } 
 
 fail : String -> Parser a
-fail msg _ = Left [msg]
+fail msg = { focus = [], parse = \v -> Left [msg] }
 
 scope : String -> Parser a -> Parser a
-scope msg p v = case p v of 
-  Left stack -> Left (msg :: stack)
-  a          -> a
+scope msg p = 
+  let focus = p.focus
+      parse v = case p.parse v of 
+        Left stack -> Left (msg :: stack)
+        a          -> a
+  in { focus = focus, parse = parse }
 
 bind : (a -> Parser b) -> Parser a -> Parser b
-bind f p v = case p v of
-  Right a -> f a v
-  Left e -> Left e 
+bind f p = 
+  let focus = p.focus
+      parse v = case p.parse v of
+        Right a -> run (f a) v
+        Left e -> Left e
+  in { focus = focus, parse = parse }
 
 infixl 3 >>= 
 (>>=) : Parser a -> (a -> Parser b) -> Parser b
 a >>= f = bind f a
 
 map : (a -> b) -> Parser a -> Parser b
-map f = bind (unit . f) 
+map f p = 
+  let parse v = case p.parse v of 
+    Left e -> Left e
+    Right a -> Right (f a)
+  in { p | parse <- parse }
 
 lift2 : (a -> b -> c) -> Parser a -> Parser b -> Parser c
-lift2 f a b v = case (a v, b v) of
-  (Left e, _) -> Left e
-  (_, Left e) -> Left e
-  (Right a, Right b) -> Right (f a b) 
+lift2 f a b = 
+  let common = a.focus `zip` b.focus
+            |> concatMap (\p -> if fst p == snd p then [fst p] else []) 
+      restL = drop (length common) a.focus 
+      restR = drop (length common) b.focus
+      parseL v = either Left a.parse (getPath restL v)
+      parseR v = either Left b.parse (getPath restR v)
+      parse v = case (parseL v, parseR v) of
+        (Left e, _) -> Left e
+        (_, Left e) -> Left e
+        (Right a, Right b) -> Right (f a b) 
+  in { focus = common, parse = parse }
 
 apply : Parser (a -> b) -> Parser a -> Parser b
 apply f a = lift2 (<|) f a 
@@ -58,54 +104,30 @@ infixl 4 #|
 (#|) : (a -> b) -> Parser a -> Parser b
 f #| a = map f a
 
-key : String -> Parser J.Value
-key k v = case v of 
-  J.Object obj -> M.get k obj 
-               |> maybe (Left ["not found"]) Right
-  e -> Left ["not found: " ++ J.toString "" v]
-
 string : Parser String
-string v = case v of 
-  J.String s -> Right s
-  _ -> Left ["not a string: " ++ J.toString "" v]
+string = value >>= \v -> case v of 
+  J.String s -> unit s
+  _ -> fail ("not a string: " ++ J.toString "" v)
 
 int : Parser Int
-int v = case v of
-  J.Number v -> Right (floor v)
-  _ -> Left ["not a number: " ++ J.toString "" v]
-  
-number : Parser Float
-number v = case v of 
-  J.Number v -> Right v
-  _ -> Left ["not a number: " ++ J.toString "" v]
-  
-asString : Parser J.Value -> Parser String
-asString p v = case v of 
-  J.String s -> Right s
-  _ -> Left ["not a string: " ++ J.toString "" v]
+int = value >>= \v -> case v of
+  J.Number v -> unit (floor v)
+  _ -> fail ("not a number: " ++ J.toString "" v)
 
-atKey : String -> Parser a -> Parser a  
-atKey fld p = 
-  let p2 v = case v of
-    J.Object obj -> M.get fld obj 
-                 |> maybe (Left ["not found"]) p   
-    e -> Left ["not found: " ++ J.toString "" v]
-  in scope fld p2
+number : Parser Float
+number = value >>= \v -> case v of 
+  J.Number v -> unit v
+  _ -> fail ("not a number: " ++ J.toString "" v)
+  
+atKey : String -> Parser a -> Parser a
+atKey k p = { p | focus <- Key k :: p.focus }
 
 atIndex : Int -> Parser a -> Parser a  
-atIndex ind p = 
-  let p2 v = case v of
-    J.Array vs -> 
-      let sub = drop ind vs 
-      in if isEmpty sub 
-         then Left ["out of bounds: " ++ show ind ++ " " ++ J.toString "" v] 
-         else p (head sub)
-    e -> Left ["not found: " ++ J.toString "" v]
-  in scope ("index: " ++ show ind) p2
+atIndex ind p = { p | focus <- Index ind :: p.focus }
 
 union : String -> String -> (String -> Parser a) -> Parser a
 union tag contents f = 
-  asString (key tag) >>= \t -> atKey contents (f t)
+  atKey tag string >>= \t -> atKey contents (f t)
 
 union' : (String -> Parser a) -> Parser a
 union' = union "tag" "contents"