Make enumerateUnits a Traversal instad of just a Fold

src/GameModel.hs

@@ -938,16 +938,26 @@ cidsInRange board range locus = board ^.. inner
 ofPlayer :: Player -> Traversal' CharacterIdentifier CharacterIdentifier
 ofPlayer player = filtered (owner >>> (== player))
 
-lookupCIDs :: Monoid r => [CharacterIdentifier] -> Getting r BoardState (CharacterIdentifier, Character)
-lookupCIDs chars = to lookupCIDs' . each
+-- | A traversal over pairs in the form (`CharacterIdentifier`, `Character`)
+--
+-- On modifying these pairs:  Modifications to the `Character` will be applied to the
+-- original character, but __changes to the `CharacterIdentifier` will be completely
+-- ignored__.  This means you /cannot/ use this traversal to change the order of
+-- characters, switch characters between players, or anything like that.
+enumerateUnits :: Traversal' BoardState (CharacterIdentifier, Character)
+enumerateUnits = enumerateUnits'
   where
-    ixCharacter' :: CharacterIdentifier -> Getting (First Character) BoardState Character
-    ixCharacter' cid = ixCharacter cid
-    lookupCIDs' :: BoardState -> [(CharacterIdentifier, Character)]
-    lookupCIDs' boardstate = mapMaybe (liftA2 (fmap . (,)) id ((boardstate ^?) . ixCharacter')) chars
-
-enumerateUnits :: SimpleFold BoardState (CharacterIdentifier, Character)
-enumerateUnits f board = lookupCIDs (board ^.. eachCID) f board
+    enumerateUnits' :: forall f. Applicative f
+                   => ((CharacterIdentifier, Character) -> f (CharacterIdentifier, Character))
+                   -> BoardState -> f BoardState
+    enumerateUnits' f = characters fCharacters
+      where
+        fUnit :: Player -> Int -> Character -> f Character
+        fUnit player indx c = snd <$> f ((player, indx), c)
+        fRoster :: Player -> [Character] -> f [Character]
+        fRoster player roster = zipWithM (fUnit player) [0..] roster
+        fCharacters :: PPair [Character] -> f (PPair [Character])
+        fCharacters (PPair p1 p2) = PPair <$> fRoster Min p1 <*> fRoster Max p2
 
 untappedUnits :: SimpleFold BoardState CharacterIdentifier
 untappedUnits = enumerateUnits . filtered (untapped . snd) . _1