save grid to custom format

fcore/common-update.sml

@@ -1,8 +1,22 @@
 structure CommonUpdate =
 struct
-  (* unimplemented *)
-  fun getSaveSquaresMsg model = (model, [])
+  open AppType
 
+  open DrawMessage
+  open FileMessage
+  open InputMessage
+  open UpdateMessage
+
+  fun getSaveSquaresMsg (model: app_type) =
+    let
+      val {canvasWidth, canvasHeight, squares, ...} = model
+      val str = CollisionTree.toString (squares, canvasWidth, canvasHeight)
+      val msg = SAVE_SQUARES str
+    in
+      (model, [FILE msg])
+    end
+
+  (* unimplemented *)
   fun getLoadSquaresMsg model = (model, [])
 
   fun getExportSquaresMsg model = (model, [])