Encapsulate State to make it impossible to read

This was NightLightStateMachine's purpose, but it fits equally well
in the actual production code.

NightLight.Core.Tests/NightLightStateMachine.fs

@@ -7,7 +7,7 @@ open NightLight.Core.Models
 open NightLight.Core.Core
 
 type NightLightStateMachine(now: DateTime) =
-    let mutable state = { Time = now }
+    let mutable state = State now
 
     let transmittedCommands = new List<Message>()
 
@@ -18,7 +18,7 @@ type NightLightStateMachine(now: DateTime) =
 
     let sendEvent event =
         result {
-            let! newState, commands = onEventReceived state event
+            let! newState, commands = state.OnEventReceived event
             state <- newState
             transmittedCommands.AddRange commands
         }

NightLight.Core/Core.fs

@@ -1,5 +1,6 @@
 module NightLight.Core.Core
 
+open System
 open NightLight.Core.Models
 open NightLight.Core.PartsOfDay
 open NightLight.Core.ZigbeeEvents
@@ -16,16 +17,17 @@ let internal generateZigbeeCommandToFixLight partOfDay light =
 
     generateZigbeeCommand light.FriendlyName color brightness
 
-let onEventReceived (state: State) (event: Event) : Result<State * Message seq, ParseEventError> =
+type State(time: DateTime) =
+    member this.OnEventReceived(event: Event) : Result<State * Message seq, ParseEventError> =
         result {
-        let partOfDay = getPartOfDay state.Time
+            let partOfDay = getPartOfDay time
 
             match event with
             | ReceivedZigbeeEvent payload ->
                 let! zigbeeEvent = parseZigbeeEvent payload |> Result.mapError ParseZigbeeEventError
 
                 return
-                state,
+                    this,
                     match zigbeeEvent with
                     | DeviceAnnounce friendlyName ->
                         let maybeLight = tryFindLight friendlyName
@@ -33,9 +35,9 @@ let onEventReceived (state: State) (event: Event) : Result<State * Message seq,
                         match maybeLight with
                         | Some light -> generateZigbeeCommandToFixLight partOfDay light |> Seq.singleton
                         | None -> Seq.empty
-        | TimeChanged time ->
+            | TimeChanged newTime ->
-            let newState = { Time = time }
+                let newState = State newTime
-            let newPartOfDay = getPartOfDay time
+                let newPartOfDay = getPartOfDay newTime
 
                 return
                     newState,

NightLight.Core/Models.fs

@@ -2,8 +2,6 @@ module NightLight.Core.Models
 
 open System
 
-type State = { Time: DateTime }
-
 type Message = { Topic: string; Payload: string }
 
 type Event =

NightLight/Program.fs

@@ -77,7 +77,7 @@ let mainAsync _ =
         let mqttClientOptions = MqttClientOptionsBuilder().WithTcpServer(server).Build()
 
         let stateLock = new SemaphoreSlim(1, 1)
-        let mutable state = { Time = DateTime.Now }
+        let mutable state = State DateTime.Now
 
         mqttClient.add_ApplicationMessageReceivedAsync (fun e ->
             async {