diff --git a/src/Elm/Kernel/Debug.js b/src/Elm/Kernel/Debug.js
index d2b04c98..f68ef007 100644
--- a/src/Elm/Kernel/Debug.js
+++ b/src/Elm/Kernel/Debug.js
@@ -148,7 +148,7 @@ function _Debug_toAnsiString(ansi, value)
 		return _Debug_stringColor(ansi, '<' + value.byteLength + ' bytes>');
 	}
 
-	if (typeof File === 'function' && value instanceof File)
+	if (typeof File !== 'undefined' && value instanceof File)
 	{
 		return _Debug_internalColor(ansi, '<' + value.name + '>');
 	}
diff --git a/src/Elm/Kernel/List.js b/src/Elm/Kernel/List.js
index 2441718b..40f31af7 100644
--- a/src/Elm/Kernel/List.js
+++ b/src/Elm/Kernel/List.js
@@ -70,6 +70,23 @@ var _List_map5 = F6(function(f, vs, ws, xs, ys, zs)
 	return _List_fromArray(arr);
 });
 
+var _List_append = F2(_List_ap);
+
+function _List_ap(xs, ys)
+{
+	if (!xs.b)
+	{
+		return ys;
+	}
+	var root = _List_Cons(xs.a, ys);
+	xs = xs.b
+	for (var curr = root; xs.b; xs = xs.b) // WHILE_CONS
+	{
+		curr = curr.b = _List_Cons(xs.a, ys);
+	}
+	return root;
+}
+
 var _List_sortBy = F2(function(f, xs)
 {
 	return _List_fromArray(_List_toArray(xs).sort(function(a, b) {
diff --git a/src/Elm/Kernel/Utils.js b/src/Elm/Kernel/Utils.js
index 66406990..74fc2613 100644
--- a/src/Elm/Kernel/Utils.js
+++ b/src/Elm/Kernel/Utils.js
@@ -178,16 +178,6 @@ function _Utils_ap(xs, ys)
 		return xs + ys;
 	}
 
-	// append Lists
-	if (!xs.b)
-	{
-		return ys;
-	}
-	var root = __List_Cons(xs.a, ys);
-	xs = xs.b
-	for (var curr = root; xs.b; xs = xs.b) // WHILE_CONS
-	{
-		curr = curr.b = __List_Cons(xs.a, ys);
-	}
-	return root;
+        // append Lists
+        return __List_ap(xs, ys);
 }
diff --git a/src/List.elm b/src/List.elm
index 9ef2b176..1fdb015f 100644
--- a/src/List.elm
+++ b/src/List.elm
@@ -132,7 +132,17 @@ element (starting at zero).
 -}
 indexedMap : (Int -> a -> b) -> List a -> List b
 indexedMap f xs =
-  map2 f (range 0 (length xs - 1)) xs
+  reverse (indexedMapHelper 0 f xs [])
+
+
+indexedMapHelper :  (Int -> a -> b)->Int -> List a -> List b -> List b
+indexedMapHelper fn index list result =
+  case list of
+    [] ->
+      result
+
+    x :: xs ->
+      indexedMapHelper fn (index + 1) xs (cons (fn index x) result)
 
 
 {-| Reduce a list from the left.
@@ -198,7 +208,7 @@ foldrHelper fn acc ctr ls =
                                 d :: r4 ->
                                     let
                                         res =
-                                            if ctr > 500 then
+                                            if ctr > 1000 then
                                                 foldl fn acc (reverse r4)
                                             else
                                                 foldrHelper fn acc (ctr + 1) r4
@@ -227,17 +237,16 @@ from an untrusted source and you want to turn them into numbers:
 -}
 filterMap : (a -> Maybe b) -> List a -> List b
 filterMap f xs =
-  foldr (maybeCons f) [] xs
-
-
-maybeCons : (a -> Maybe b) -> a -> List b -> List b
-maybeCons f mx xs =
-  case f mx of
-    Just x ->
-      cons x xs
+  let
+    helper mx acc =
+      case f mx of
+        Just x ->
+          cons x acc
 
-    Nothing ->
-      xs
+        Nothing ->
+          acc
+  in
+    foldr helper [] xs
 
 
 -- UTILITIES
@@ -363,13 +372,8 @@ product numbers =
 You can also use [the `(++)` operator](Basics#++) to append lists.
 -}
 append : List a -> List a -> List a
-append xs ys =
-  case ys of
-    [] ->
-      xs
-
-    _ ->
-      foldr cons ys xs
+append =
+  Elm.Kernel.List.append
 
 
 {-| Concatenate a bunch of lists into a single list:
@@ -387,7 +391,12 @@ concat lists =
 -}
 concatMap : (a -> List b) -> List a -> List b
 concatMap f list =
-  concat (map f list)
+  let
+    helper val acc =
+      append (f val) acc
+  in
+    foldr helper [] list
+
 
 
 {-| Places the given value between all members of the given list.