WIP: add forward

src/ProgramTest.elm

@@ -235,6 +235,7 @@ type ProgramTest model msg effect
             Maybe
                 { currentLocation : Url
                 , browserHistory : List Url
+                , browserForward : List Url
                 }
         , effectSimulation : Maybe (EffectSimulation msg effect)
         }
@@ -320,6 +321,7 @@ createHelper program options =
                     Just
                         { currentLocation = baseUrl
                         , browserHistory = []
+                        , browserForward = []
                         }
         , effectSimulation = Maybe.map EffectSimulation.init options.deconstructEffect
         }
@@ -1467,6 +1469,21 @@ queueSimulatedEffect effect programTest =
                                                 programTest
                                                     |> routeChangeHelper ("simulating effect: SimulatedEffect.Navigation.Back " ++ String.fromInt n) 2 (Url.toString first)
 
+                        SimulatedEffect.Forward n ->
+                            case state.navigation of
+                                Nothing ->
+                                    programTest
+
+                                Just navigation ->
+                                    case List.head navigation.browserForward of
+                                        Nothing ->
+                                            -- there's nothing to go forward to
+                                            programTest
+
+                                        Just first ->
+                                            programTest
+                                                |> routeChangeHelper ("simulating effect: SimulatedEffect.Navigation.Forward " ++ String.fromInt n) 0 (Url.toString first)
+
 
 drain : ProgramTest model msg effect -> ProgramTest model msg effect
 drain =
@@ -2045,10 +2062,10 @@ routeChangeHelper functionName removeFromBackStack url programTest =
                 Nothing ->
                     Finished (ProgramDoesNotSupportNavigation functionName)
 
-                Just { currentLocation, browserHistory } ->
+                Just navigation ->
                     let
                         newLocation =
-                            Url.Extra.resolve currentLocation url
+                            Url.Extra.resolve navigation.currentLocation url
 
                         processRouteChange =
                             case state.program.onRouteChange newLocation of
@@ -2063,10 +2080,14 @@ routeChangeHelper functionName removeFromBackStack url programTest =
                         { state
                             | navigation =
                                 Just
-                                    { currentLocation = newLocation
-                                    , browserHistory =
-                                        (currentLocation :: browserHistory)
-                                            |> List.drop removeFromBackStack
+                                    { navigation
+                                        | currentLocation = newLocation
+                                        , browserHistory =
+                                            (navigation.currentLocation :: navigation.browserHistory)
+                                                |> List.drop removeFromBackStack
+                                        , browserForward =
+                                            (navigation.currentLocation :: navigation.browserHistory)
+                                                |> List.take 1
                                     }
                         }
                         |> processRouteChange

src/SimulatedEffect.elm

@@ -14,6 +14,7 @@ type SimulatedEffect msg
     | PushUrl String
     | ReplaceUrl String
     | Back Int
+    | Forward Int
 
 
 type SimulatedTask x a

src/SimulatedEffect/Cmd.elm

@@ -60,3 +60,6 @@ map f effect =
 
         SimulatedEffect.Back n ->
             SimulatedEffect.Back n
+
+        SimulatedEffect.Forward n ->
+            SimulatedEffect.Forward n

src/SimulatedEffect/Navigation.elm

@@ -1,4 +1,4 @@
-module SimulatedEffect.Navigation exposing (pushUrl, replaceUrl, back)
+module SimulatedEffect.Navigation exposing (pushUrl, replaceUrl, back, forward)
 
 {-| This module parallels [elm/browsers's `Browser.Navigation` module](https://package.elm-lang.org/packages/elm/browser/1.0.1/Browser-Navigation).
 _Pull requests are welcome to add any functions that are missing._
@@ -9,7 +9,7 @@ to help you implement the function to provide when using [`ProgramTest.withSimul
 
 # Navigate within Page
 
-@docs pushUrl, replaceUrl, back
+@docs pushUrl, replaceUrl, back, forward
 
 -}
 
@@ -37,3 +37,10 @@ replaceUrl =
 back : Int -> SimulatedEffect msg
 back =
     SimulatedEffect.Back
+
+
+{-| Go forward some number of pages.
+-}
+forward : Int -> SimulatedEffect msg
+forward =
+    SimulatedEffect.Forward

tests/ProgramTestTests/SimulatedEffects/NavigationTest.elm

@@ -98,4 +98,27 @@ all =
                 TestingProgram.application (SimulatedEffect.Navigation.pushUrl "https://example.com/new")
                     |> ProgramTest.update (ProduceEffects (SimulatedEffect.Navigation.back 0))
                     |> ProgramTest.expectBrowserUrl (Expect.equal "https://example.com/new")
+        , test "simulating forward undoes a back" <|
+            \() ->
+                TestingProgram.application (SimulatedEffect.Navigation.pushUrl "https://example.com/new")
+                    |> ProgramTest.update (ProduceEffects (SimulatedEffect.Navigation.back 1))
+                    |> ProgramTest.update Clear
+                    |> ProgramTest.update (ProduceEffects (SimulatedEffect.Navigation.forward 1))
+                    |> ProgramTest.expectModel (Expect.equal [ "OnUrlChange: https://example.com/new" ])
+        , test "simulating forward 2 only does one URL change" <|
+            \() ->
+                TestingProgram.application (SimulatedEffect.Navigation.pushUrl "https://example.com/new")
+                    |> ProgramTest.update (ProduceEffects (SimulatedEffect.Navigation.pushUrl "https://example.com/new2"))
+                    -- back: [new, path], current: new2, forward: []
+                    |> Debug.log "A"
+                    |> ProgramTest.update (ProduceEffects (SimulatedEffect.Navigation.back 2))
+                    -- back: [], current: path, forward: [new, new2]
+                    |> Debug.log "B"
+                    |> ProgramTest.update Clear
+                    |> ProgramTest.update (ProduceEffects (SimulatedEffect.Navigation.forward 2))
+                    -- back: [new, path], current: new2, forward: []
+                    |> Debug.log "C"
+                    |> ProgramTest.expectModel (Expect.equal [ "OnUrlChange: https://example.com/new2" ])
+        , todo "forward 1 after back 2?"
+        , todo "what happens for real with invalid n?"
         ]