Put new levels at the right position in the list

New levels need to go at the *end* of the list of levels, not the
beginning - otherwise we jump to the proper position on the new level
but the current level stays the same (oops).

src/Xanthous/App.hs

@@ -300,8 +300,7 @@ handleCommand GoDown = do
     let newLevelNum = Levels.pos levs + 1
     levs' <- nextLevel (levelToEntityMap <$> genLevel newLevelNum) levs
     cEID <- use characterEntityID
-    pCharacter <- use $ entities . at cEID
-    entities . at cEID .= Nothing
+    pCharacter <- entities . at cEID <<.= Nothing
     levels .= levs'
     entities . at cEID .= pCharacter
   else say_ ["cant", "goDown"]

src/Xanthous/Data/Levels.hs

@@ -102,7 +102,7 @@ nextLevel genLevel levs
   = pure $ seeks succ levs
   | otherwise
   = genLevel <&> \level ->
-      seek (pos levs + 1) . partialMkLevels $ level <| allLevels levs
+      seek (pos levs + 1) . partialMkLevels $ allLevels levs |> level
 
 -- | Go to the previous level. Returns Nothing if 'pos' is 0
 prevLevel :: Levels level -> Maybe (Levels level)

test/Xanthous/Data/LevelsSpec.hs

@@ -37,6 +37,12 @@ test = testGroup "Xanthous.Data.Levels"
       , testProperty "extract is total" $ \(levels :: Levels Int) genned ->
           let levels' = runIdentity . nextLevel (Identity genned) $ levels
           in total $ extract levels'
+      , testProperty "uses the generated level as the next level"
+        $ \(levels :: Levels Int) genned ->
+          let levels' = seek (length levels - 1) levels
+              levels'' = runIdentity . nextLevel (Identity genned) $ levels'
+          in counterexample (show levels'')
+             $ extract levels'' === genned
       ]
     , testGroup "prevLevel"
       [ testProperty "seeks backwards" $ \(levels :: Levels Int) ->