Fail on second router

src/Saturn/Application.fs

@@ -224,19 +224,28 @@ module Application =
         )
 
     ///Defines top-level router used for the application
+    ///
+    ///This can only be called once, and not after `no_router`!
     [<CustomOperation("use_router")>]
     member __.Router(state, handler) =
-      {state with Router = Some handler}
+      match state.NoRouter, state.Router with
+      | false, None -> {state with Router = Some handler}
+      | true, _ -> failwith "Cannot add a router, after `no_router` was set!"
+      | _, Some _ -> failwith "Cannot add a second router!"
 
     ///Defines top-level endpoint router used for the application
     [<CustomOperation("use_endpoint_router")>]
     member __.EndpointRouter(state, routes) =
       {state with EndpointRouter = Some routes}
 
     ///Disable warning message about lack of `router` definition. Should be used for channels-only or gRPC applications.
+    ///
+    ///This cannot be called after `use_router`!
     [<CustomOperation("no_router")>]
     member __.NoRouter(state) =
-      {state with NoRouter = true}
+      match state.Router with
+      | Some _ -> failwith "Cannot set `no_router` after a router with `use_router` has been set!"
+      | _ -> {state with NoRouter = true}
 
     ///Disables any configuration of webhost. Could be used for generic `IHostBuilder` applications not using Kestrel/IIS
     [<CustomOperation("no_webhost")>]

tests/Saturn.UnitTests/SimpleTests.fs

@@ -41,3 +41,37 @@ let tests =
         Expect.equal (getBody ctx) """{"id":"myId","links":["myLink1","myLink2"]}""" "Result should be equal"
 
   ]
+
+//---------------------------`Application only takes one router` tests----------------------------------------
+
+[<Tests>]
+let routerTests =
+  testList "Application only takes one router" [
+    testCase "Second router throws" (fun _ ->
+      let app () =
+        application {
+          use_router (text "")
+          use_router (text "")
+        }
+
+      Expect.throws (app >> ignore) "Application did not fail on second router!"
+    )
+    testCase "Adding a router after `no_router` throws" (fun _ ->
+      let app () =
+        application {
+          no_router
+          use_router (text "")
+        }
+
+      Expect.throws (app >> ignore) "Application did not fail on router after no_router!"
+    )
+    testCase "Adding a `no_router after `use_router` throws" (fun _ ->
+      let app () =
+        application {
+          use_router (text "")
+          no_router
+        }
+
+      Expect.throws (app >> ignore) "Application did not fail on no_router after use_router!"
+    )
+  ]