a bit of refactoring, moving code to update 'squares' vector in app-with.sml to its own function (to make it reuseable)

fcore/app-with.sml

@@ -2,6 +2,14 @@ structure AppWith =
 struct
   open AppType
 
+  fun updateSquares (squares, newX, newY, item) =
+    Vector.mapi
+      (fn (idx, el) =>
+         if idx = newX then
+           Vector.mapi (fn (iidx, iel) => if iidx = newY then item else iel) el
+         else
+           el) squares
+
   fun addSquare (app, newX, newY, arrowX, arrowY) : app_type =
     let
       val
@@ -31,16 +39,13 @@ struct
         , redo
         } = app
 
-      val squares =
-        Vector.mapi
-          (fn (idx, el) =>
-             if idx = newX then
-               Vector.mapi
-                 (fn (iidx, iel) =>
-                    if iidx = newY then {r = r, g = g, b = b, a = a} else iel)
-                 el
-             else
-               el) squares
+      val yAxis = Vector.sub (squares, newX)
+      val undoData = Vector.sub (yAxis, newY)
+      val undo = undoData :: undo
+      val redo = []
+
+      val item = {r = r, g = g, b = b, a = a}
+      val squares = updateSquares (squares, newX, newY, item)
     in
       { mode = mode
       , squares = squares