changed update to use "explicit model" style

Author: Tyson Williams

src/Samples/SubModelSeq/Program.fs

@@ -169,13 +169,14 @@ module App =
     |> Func.dedup
     |> RoseTree.mapChildren
 
-  let update = function
-    | ToggleGlobalState -> not |> SomeGlobalState.map
-    | CounterMsg (nId, msg) -> msg |> Counter.update |> Identifiable.mapIfId nId |> RoseTree.mapData |> DummyRoot.map
-    | AddCounter pId -> () |> Guid.NewGuid |> addChildCounter |> mapIf (hasId pId) |> RoseTree.map |> DummyRoot.map
-    | Remove nId -> nId |> hasId >> not |> List.filter |> RoseTree.mapChildren |> RoseTree.map |> DummyRoot.map
-    | MoveUp nId -> nId |> swapCounters List.swapWithPrev |> RoseTree.map |> DummyRoot.map
-    | MoveDown nId -> nId |> swapCounters List.swapWithNext |> RoseTree.map |> DummyRoot.map
+  let update msg m =
+    match msg with
+    | ToggleGlobalState -> { m with SomeGlobalState = not m.SomeGlobalState }
+    | CounterMsg (nId, msg) -> { m with DummyRoot = (msg |> Counter.update |> Identifiable.mapIfId nId |> RoseTree.mapData) m.DummyRoot }
+    | AddCounter pId -> { m with DummyRoot = (Guid.NewGuid () |> addChildCounter |> mapIf (hasId pId) |> RoseTree.map) m.DummyRoot }
+    | Remove nId -> { m with DummyRoot = (nId |> hasId >> not |> List.filter |> RoseTree.mapChildren |> RoseTree.map) m.DummyRoot }
+    | MoveUp nId -> { m with DummyRoot = (nId |> swapCounters List.swapWithPrev |> RoseTree.map) m.DummyRoot }
+    | MoveDown nId -> { m with DummyRoot = (nId |> swapCounters List.swapWithNext |> RoseTree.map) m.DummyRoot }
 
   /// Returns all top-level counters.
   let topLevelCounters m =