@@ -2,150 +2,98 @@
22{-# LANGUAGE DeriveTraversable #-}
33{-# LANGUAGE FlexibleContexts #-}
44{-# LANGUAGE FlexibleInstances #-}
5−{-# LANGUAGE GeneralizedNewtypeDeriving #-}
65{-# LANGUAGE LambdaCase #-}
76{-# LANGUAGE MultiParamTypeClasses #-}
87{-# LANGUAGE PatternGuards #-}
9−{-# LANGUAGE PatternSynonyms #-}
108{-# LANGUAGE ScopedTypeVariables #-}
11−{-# LANGUAGE StandaloneDeriving #-}
129{-# LANGUAGE TemplateHaskell #-}
1310{-# LANGUAGE TypeApplications #-}
1411{-# LANGUAGE TypeFamilies #-}
15−{-# LANGUAGE ViewPatterns #-}
1612
1713-- | 'Patch'es on 'Map' that can insert, delete, and move values from one key to
1814-- another
19−module Data.Patch.MapWithMove
20− ( PatchMapWithMove
21− ( PatchMapWithMove
22− , unPatchMapWithMove
23− , ..
24− )
25− , patchMapWithMove
26− , patchMapWithMoveInsertAll
27− , insertMapKey
28− , moveMapKey
29− , swapMapKey
30− , deleteMapKey
31− , unsafePatchMapWithMove
32− , patchMapWithMoveNewElements
33− , patchMapWithMoveNewElementsMap
34− , patchThatSortsMapWith
35− , patchThatChangesAndSortsMapWith
36− , patchThatChangesMap
15+module Data.Patch.MapWithMove where
3716
38− -- * Node Info
39− , NodeInfo
40− ( NodeInfo
41− , _nodeInfo_to
42− , _nodeInfo_from
43− , ..
44− )
45− , bitraverseNodeInfo
46− , nodeInfoMapFrom
47− , nodeInfoMapMFrom
48− , nodeInfoSetTo
49−
50− -- * From
51− , From
52− ( From_Insert
53− , From_Delete
54− , From_Move
55− , ..
56− )
57− , bitraverseFrom
58−
59− -- * To
60− , To
61− ) where
62−
63−import Data.Coerce
64−import Data.Kind (Type)
6517import Data.Patch.Class
66−import Data.Patch.MapWithPatchingMove (PatchMapWithPatchingMove(..), To)
67−import qualified Data.Patch.MapWithPatchingMove as PM -- already a transparent synonym
6818
69−import Control.Lens
19+import Control.Arrow
20+import Control.Lens hiding (from, to)
21+import Control.Monad.Trans.State
22+import Data.Foldable
23+import Data.Function
7024import Data.List
7125import Data.Map (Map)
7226import qualified Data.Map as Map
73−import Data.Proxy
74−#if !MIN_VERSION_base(4,11,0)
75−import Data.Semigroup (Semigroup (..))
27+import Data.Maybe
28+#if !MIN_VERSION_base(4,10,0)
29+import Data.Semigroup (Semigroup (..), (<>))
7630#endif
77−import Data.Traversable (foldMapDefault)
31+import qualified Data.Set as Set
32+import Data.These (These(..))
33+import Data.Tuple
7834
7935-- | Patch a Map with additions, deletions, and moves. Invariant: If key @k1@
8036-- is coming from @From_Move k2@, then key @k2@ should be going to @Just k1@,
8137-- and vice versa. There should never be any unpaired From/To keys.
82−newtype PatchMapWithMove k (v :: Type) = PatchMapWithMove'
83− { -- | Extract the underlying 'PatchMapWithPatchingMove k (Proxy v)'
84− unPatchMapWithMove' :: PatchMapWithPatchingMove k (Proxy v)
38+newtype PatchMapWithMove k v = PatchMapWithMove
39+ { -- | Extract the internal representation of the 'PatchMapWithMove'
40+ unPatchMapWithMove :: Map k (NodeInfo k v)
8541 }
8642 deriving ( Show, Read, Eq, Ord
87−-- Haddock cannot handle documentation here before GHC 8.6
88− ,
89−#if __GLASGOW_HASKELL__ >= 806
90− -- | Compose patches having the same effect as applying the
91− -- patches in turn: @'applyAlways' (p <> q) == 'applyAlways' p .
92− -- 'applyAlways' q@
93−#endif
94− Semigroup
95− , Monoid
43+ , Functor, Foldable, Traversable
9644 )
9745
98−pattern Coerce :: Coercible a b => a -> b
99−pattern Coerce x <- (coerce -> x)
100− where Coerce x = coerce x
101−
102−{-# COMPLETE PatchMapWithMove #-}
103−pattern PatchMapWithMove :: Map k (NodeInfo k v) -> PatchMapWithMove k v
104−-- | Extract the representation of the 'PatchMapWithMove' as a map of
105−-- 'NodeInfo'.
106−unPatchMapWithMove :: PatchMapWithMove k v -> Map k (NodeInfo k v)
107−pattern PatchMapWithMove { unPatchMapWithMove } = PatchMapWithMove' (PatchMapWithPatchingMove (Coerce unPatchMapWithMove))
108−
109−_PatchMapWithMove
110− :: Iso
111− (PatchMapWithMove k0 v0)
112− (PatchMapWithMove k1 v1)
113− (Map k0 (NodeInfo k0 v0))
114− (Map k1 (NodeInfo k1 v1))
115−_PatchMapWithMove = iso unPatchMapWithMove PatchMapWithMove
46+-- | Holds the information about each key: where its new value should come from,
47+-- and where its old value should go to
48+data NodeInfo k v = NodeInfo
49+ { _nodeInfo_from :: !(From k v)
50+ -- ^ Where do we get the new value for this key?
51+ , _nodeInfo_to :: !(To k)
52+ -- ^ If the old value is being kept (i.e. moved rather than deleted or
53+ -- replaced), where is it going?
54+ }
55+ deriving (Show, Read, Eq, Ord, Functor, Foldable, Traversable)
11656
117−instance Functor (PatchMapWithMove k) where
118− fmap f = runIdentity . traverse (Identity . f)
57+-- | Describe how a key's new value should be produced
58+data From k v
59+ = From_Insert v -- ^ Insert the given value here
60+ | From_Delete -- ^ Delete the existing value, if any, from here
61+ | From_Move !k -- ^ Move the value here from the given key
62+ deriving (Show, Read, Eq, Ord, Functor, Foldable, Traversable)
11963
120−instance Foldable (PatchMapWithMove k) where
121− foldMap = foldMapDefault
64+-- | Describe where a key's old value will go. If this is 'Just', that means
65+-- the key's old value will be moved to the given other key; if it is 'Nothing',
66+-- that means it will be deleted.
67+type To = Maybe
12268
123−instance Traversable (PatchMapWithMove k) where
124− traverse =
125− _PatchMapWithMove .
126− traverse .
127− traverse
69+makeWrapped ''PatchMapWithMove
12870
12971instance FunctorWithIndex k (PatchMapWithMove k)
13072instance FoldableWithIndex k (PatchMapWithMove k)
13173instance TraversableWithIndex k (PatchMapWithMove k) where
13274 itraverse = itraversed . Indexed
133− itraversed =
134− _PatchMapWithMove .>
135− itraversed <.
136− traverse
75+ itraversed = _Wrapped .> itraversed <. traversed
13776
13877-- | Create a 'PatchMapWithMove', validating it
13978patchMapWithMove :: Ord k => Map k (NodeInfo k v) -> Maybe (PatchMapWithMove k v)
140−patchMapWithMove = fmap PatchMapWithMove' . PM.patchMapWithPatchingMove . coerce
79+patchMapWithMove m = if valid then Just $ PatchMapWithMove m else Nothing
80+ where valid = forwardLinks == backwardLinks
81+ forwardLinks = Map.mapMaybe _nodeInfo_to m
82+ backwardLinks = Map.fromList $ catMaybes $ flip fmap (Map.toList m) $ \(to, v) ->
83+ case _nodeInfo_from v of
84+ From_Move from -> Just (from, to)
85+ _ -> Nothing
14186
14287-- | Create a 'PatchMapWithMove' that inserts everything in the given 'Map'
14388patchMapWithMoveInsertAll :: Map k v -> PatchMapWithMove k v
144−patchMapWithMoveInsertAll = PatchMapWithMove' . PM.patchMapWithPatchingMoveInsertAll
89+patchMapWithMoveInsertAll m = PatchMapWithMove $ flip fmap m $ \v -> NodeInfo
90+ { _nodeInfo_from = From_Insert v
91+ , _nodeInfo_to = Nothing
92+ }
14593
14694-- | Make a @'PatchMapWithMove' k v@ which has the effect of inserting or updating a value @v@ to the given key @k@, like 'Map.insert'.
14795insertMapKey :: k -> v -> PatchMapWithMove k v
148−insertMapKey k v = PatchMapWithMove' $ PM.insertMapKey k v
96+insertMapKey k v = PatchMapWithMove . Map.singleton k $ NodeInfo (From_Insert v) Nothing
14997
15098-- |Make a @'PatchMapWithMove' k v@ which has the effect of moving the value from the first key @k@ to the second key @k@, equivalent to:
15199--
@@ -153,53 +101,84 @@ insertMapKey k v = PatchMapWithMove' $ PM.insertMapKey k v
153101-- 'Map.delete' src (maybe map ('Map.insert' dst) (Map.lookup src map))
154102-- @
155103moveMapKey :: Ord k => k -> k -> PatchMapWithMove k v
156−moveMapKey src dst = PatchMapWithMove' $ PM.moveMapKey src dst
104+moveMapKey src dst
105+ | src == dst = mempty
106+ | otherwise =
107+ PatchMapWithMove $ Map.fromList
108+ [ (dst, NodeInfo (From_Move src) Nothing)
109+ , (src, NodeInfo From_Delete (Just dst))
110+ ]
157111
158112-- |Make a @'PatchMapWithMove' k v@ which has the effect of swapping two keys in the mapping, equivalent to:
159113--
160114-- @
161115-- let aMay = Map.lookup a map
162116-- bMay = Map.lookup b map
163−-- in maybe id (Map.insert a) (bMay <> aMay)
164−-- . maybe id (Map.insert b) (aMay <> bMay)
117+-- in maybe id (Map.insert a) (bMay `mplus` aMay)
118+-- . maybe id (Map.insert b) (aMay `mplus` bMay)
165119-- . Map.delete a . Map.delete b $ map
166120-- @
167121swapMapKey :: Ord k => k -> k -> PatchMapWithMove k v
168−swapMapKey src dst = PatchMapWithMove' $ PM.swapMapKey src dst
122+swapMapKey src dst
123+ | src == dst = mempty
124+ | otherwise =
125+ PatchMapWithMove $ Map.fromList
126+ [ (dst, NodeInfo (From_Move src) (Just src))
127+ , (src, NodeInfo (From_Move dst) (Just dst))
128+ ]
169129
170130-- |Make a @'PatchMapWithMove' k v@ which has the effect of deleting a key in the mapping, equivalent to 'Map.delete'.
171131deleteMapKey :: k -> PatchMapWithMove k v
172−deleteMapKey = PatchMapWithMove' . PM.deleteMapKey
132+deleteMapKey k = PatchMapWithMove . Map.singleton k $ NodeInfo From_Delete Nothing
173133
174134-- | Wrap a @'Map' k (NodeInfo k v)@ representing patch changes into a @'PatchMapWithMove' k v@, without checking any invariants.
175135--
176136-- __Warning:__ when using this function, you must ensure that the invariants of 'PatchMapWithMove' are preserved; they will not be checked.
177137unsafePatchMapWithMove :: Map k (NodeInfo k v) -> PatchMapWithMove k v
178−unsafePatchMapWithMove = coerce PM.unsafePatchMapWithPatchingMove
138+unsafePatchMapWithMove = PatchMapWithMove
179139
180140-- | Apply the insertions, deletions, and moves to a given 'Map'
181141instance Ord k => Patch (PatchMapWithMove k v) where
182142 type PatchTarget (PatchMapWithMove k v) = Map k v
183− apply (PatchMapWithMove' p) = apply p
143+ apply (PatchMapWithMove p) old = Just $! insertions `Map.union` (old `Map.difference` deletions) --TODO: return Nothing sometimes --Note: the strict application here is critical to ensuring that incremental merges don't hold onto all their prerequisite events forever; can we make this more robust?
144+ where insertions = flip Map.mapMaybeWithKey p $ \_ ni -> case _nodeInfo_from ni of
145+ From_Insert v -> Just v
146+ From_Move k -> Map.lookup k old
147+ From_Delete -> Nothing
148+ deletions = flip Map.mapMaybeWithKey p $ \_ ni -> case _nodeInfo_from ni of
149+ From_Delete -> Just ()
150+ _ -> Nothing
184151
185152-- | Returns all the new elements that will be added to the 'Map'.
186153patchMapWithMoveNewElements :: PatchMapWithMove k v -> [v]
187−patchMapWithMoveNewElements = PM.patchMapWithPatchingMoveNewElements . unPatchMapWithMove'
154+patchMapWithMoveNewElements = Map.elems . patchMapWithMoveNewElementsMap
188155
189156-- | Return a @'Map' k v@ with all the inserts/updates from the given @'PatchMapWithMove' k v@.
190157patchMapWithMoveNewElementsMap :: PatchMapWithMove k v -> Map k v
191−patchMapWithMoveNewElementsMap = PM.patchMapWithPatchingMoveNewElementsMap . unPatchMapWithMove'
158+patchMapWithMoveNewElementsMap (PatchMapWithMove p) = Map.mapMaybe f p
159+ where f ni = case _nodeInfo_from ni of
160+ From_Insert v -> Just v
161+ From_Move _ -> Nothing
162+ From_Delete -> Nothing
192163
193164-- | Create a 'PatchMapWithMove' that, if applied to the given 'Map', will sort
194165-- its values using the given ordering function. The set keys of the 'Map' is
195166-- not changed.
196167patchThatSortsMapWith :: Ord k => (v -> v -> Ordering) -> Map k v -> PatchMapWithMove k v
197−patchThatSortsMapWith cmp = PatchMapWithMove' . PM.patchThatSortsMapWith cmp
168+patchThatSortsMapWith cmp m = PatchMapWithMove $ Map.fromList $ catMaybes $ zipWith g unsorted sorted
169+ where unsorted = Map.toList m
170+ sorted = sortBy (cmp `on` snd) unsorted
171+ f (to, _) (from, _) = if to == from then Nothing else
172+ Just (from, to)
173+ reverseMapping = Map.fromList $ catMaybes $ zipWith f unsorted sorted
174+ g (to, _) (from, _) = if to == from then Nothing else
175+ let Just movingTo = Map.lookup to reverseMapping
176+ in Just (to, NodeInfo (From_Move from) $ Just movingTo)
198177
199178-- | Create a 'PatchMapWithMove' that, if applied to the first 'Map' provided,
200179-- will produce a 'Map' with the same values as the second 'Map' but with the
201180-- values sorted with the given ordering function.
202−patchThatChangesAndSortsMapWith :: (Ord k, Ord v) => (v -> v -> Ordering) -> Map k v -> Map k v -> PatchMapWithMove k v
181+patchThatChangesAndSortsMapWith :: forall k v. (Ord k, Ord v) => (v -> v -> Ordering) -> Map k v -> Map k v -> PatchMapWithMove k v
203182patchThatChangesAndSortsMapWith cmp oldByIndex newByIndexUnsorted = patchThatChangesMap oldByIndex newByIndex
204183 where newList = Map.toList newByIndexUnsorted
205184 newByIndex = Map.fromList $ zip (fst <$> newList) $ sortBy cmp $ snd <$> newList
@@ -207,106 +186,102 @@ patchThatChangesAndSortsMapWith cmp oldByIndex newByIndexUnsorted = patchThatCha
207186-- | Create a 'PatchMapWithMove' that, if applied to the first 'Map' provided,
208187-- will produce the second 'Map'.
209188patchThatChangesMap :: (Ord k, Ord v) => Map k v -> Map k v -> PatchMapWithMove k v
210−patchThatChangesMap oldByIndex newByIndex = PatchMapWithMove' $
211− PM.patchThatChangesMap oldByIndex newByIndex
212−
213−--
214−-- NodeInfo
215−--
216−
217−-- | Holds the information about each key: where its new value should come from,
218−-- and where its old value should go to
219−newtype NodeInfo k (v :: Type) = NodeInfo' { unNodeInfo' :: PM.NodeInfo k (Proxy v) }
220−
221−deriving instance (Show k, Show p) => Show (NodeInfo k p)
222−deriving instance (Read k, Read p) => Read (NodeInfo k p)
223−deriving instance (Eq k, Eq p) => Eq (NodeInfo k p)
224−deriving instance (Ord k, Ord p) => Ord (NodeInfo k p)
225−
226−{-# COMPLETE NodeInfo #-}
227−pattern NodeInfo :: To k -> From k v -> NodeInfo k v
228−_nodeInfo_to :: NodeInfo k v -> To k
229−_nodeInfo_from :: NodeInfo k v -> From k v
230−pattern NodeInfo { _nodeInfo_to, _nodeInfo_from } = NodeInfo'
231− PM.NodeInfo
232− { PM._nodeInfo_to = _nodeInfo_to
233− , PM._nodeInfo_from = Coerce _nodeInfo_from
234− }
235−
236−_NodeInfo
237− :: Iso
238− (NodeInfo k0 v0)
239− (NodeInfo k1 v1)
240− (PM.NodeInfo k0 (Proxy v0))
241− (PM.NodeInfo k1 (Proxy v1))
242−_NodeInfo = iso unNodeInfo' NodeInfo'
243−
244−instance Functor (NodeInfo k) where
245− fmap f = runIdentity . traverse (Identity . f)
246−
247−instance Foldable (NodeInfo k) where
248− foldMap = foldMapDefault
249−
250−instance Traversable (NodeInfo k) where
251− traverse = bitraverseNodeInfo pure
252−
253−bitraverseNodeInfo
254− :: Applicative f
255− => (k0 -> f k1)
256− -> (v0 -> f v1)
257− -> NodeInfo k0 v0 -> f (NodeInfo k1 v1)
258−bitraverseNodeInfo fk fv = fmap NodeInfo'
259− . PM.bitraverseNodeInfo fk (\ ~Proxy -> pure Proxy) fv
260− . coerce
189+patchThatChangesMap oldByIndex newByIndex = patch
190+ where oldByValue = Map.fromListWith Set.union $ swap . first Set.singleton <$> Map.toList oldByIndex
191+ (insertsAndMoves, unusedValuesByValue) = flip runState oldByValue $ do
192+ let f k v = do
193+ remainingValues <- get
194+ let putRemainingKeys remainingKeys = put $ if Set.null remainingKeys
195+ then Map.delete v remainingValues
196+ else Map.insert v remainingKeys remainingValues
197+ case Map.lookup v remainingValues of
198+ Nothing -> return $ NodeInfo (From_Insert v) $ Just undefined -- There's no existing value we can take
199+ Just fromKs ->
200+ if k `Set.member` fromKs
201+ then do
202+ putRemainingKeys $ Set.delete k fromKs
203+ return $ NodeInfo (From_Move k) $ Just undefined -- There's an existing value, and it's here, so no patch necessary
204+ else do
205+ (fromK, remainingKeys) <- return . fromJust $ Set.minView fromKs -- There's an existing value, but it's not here; move it here
206+ putRemainingKeys remainingKeys
207+ return $ NodeInfo (From_Move fromK) $ Just undefined
208+ Map.traverseWithKey f newByIndex
209+ unusedOldKeys = fold unusedValuesByValue
210+ pointlessMove k = \case
211+ From_Move k' | k == k' -> True
212+ _ -> False
213+ keyWasMoved k = if k `Map.member` oldByIndex && not (k `Set.member` unusedOldKeys)
214+ then Just undefined
215+ else Nothing
216+ patch = unsafePatchMapWithMove $ Map.filterWithKey (\k -> not . pointlessMove k . _nodeInfo_from) $ Map.mergeWithKey (\k a _ -> Just $ nodeInfoSetTo (keyWasMoved k) a) (Map.mapWithKey $ \k -> nodeInfoSetTo $ keyWasMoved k) (Map.mapWithKey $ \k _ -> NodeInfo From_Delete $ keyWasMoved k) insertsAndMoves oldByIndex
261217
262218-- | Change the 'From' value of a 'NodeInfo'
263219nodeInfoMapFrom :: (From k v -> From k v) -> NodeInfo k v -> NodeInfo k v
264−nodeInfoMapFrom f = coerce $ PM.nodeInfoMapFrom (unFrom' . f . From')
220+nodeInfoMapFrom f ni = ni { _nodeInfo_from = f $ _nodeInfo_from ni }
265221
266222-- | Change the 'From' value of a 'NodeInfo', using a 'Functor' (or
267223-- 'Applicative', 'Monad', etc.) action to get the new value
268−nodeInfoMapMFrom
269− :: Functor f
270− => (From k v -> f (From k v))
271− -> NodeInfo k v -> f (NodeInfo k v)
272−nodeInfoMapMFrom f = fmap NodeInfo'
273− . PM.nodeInfoMapMFrom (fmap unFrom' . f . From')
274− . coerce
224+nodeInfoMapMFrom :: Functor f => (From k v -> f (From k v)) -> NodeInfo k v -> f (NodeInfo k v)
225+nodeInfoMapMFrom f ni = fmap (\result -> ni { _nodeInfo_from = result }) $ f $ _nodeInfo_from ni
275226
276227-- | Set the 'To' field of a 'NodeInfo'
277228nodeInfoSetTo :: To k -> NodeInfo k v -> NodeInfo k v
278−nodeInfoSetTo = coerce . PM.nodeInfoSetTo
279−
280−--
281−-- From
282−--
283−
284−-- | Describe how a key's new value should be produced
285−newtype From k (v :: Type) = From' { unFrom' :: PM.From k (Proxy v) }
286−
287−{-# COMPLETE From_Insert, From_Delete, From_Move #-}
288−
289−-- | Insert the given value here
290−pattern From_Insert :: v -> From k v
291−pattern From_Insert v = From' (PM.From_Insert v)
292−
293−-- | Delete the existing value, if any, from here
294−pattern From_Delete :: From k v
295−pattern From_Delete = From' PM.From_Delete
296−
297−-- | Move the value here from the given key
298−pattern From_Move :: k -> From k v
299−pattern From_Move k = From' (PM.From_Move k Proxy)
300−
301−bitraverseFrom
302− :: Applicative f
303− => (k0 -> f k1)
304− -> (v0 -> f v1)
305− -> From k0 v0 -> f (From k1 v1)
306−bitraverseFrom fk fv = fmap From'
307− . PM.bitraverseFrom fk (\ ~Proxy -> pure Proxy) fv
308− . coerce
309−
310−makeWrapped ''PatchMapWithMove
311−makeWrapped ''NodeInfo
312−makeWrapped ''From
229+nodeInfoSetTo to ni = ni { _nodeInfo_to = to }
230+
231+-- |Helper data structure used for composing patches using the monoid instance.
232+data Fixup k v
233+ = Fixup_Delete
234+ | Fixup_Update (These (From k v) (To k))
235+
236+-- |Compose patches having the same effect as applying the patches in turn: @'applyAlways' (p <> q) == 'applyAlways' p . 'applyAlways' q@
237+instance Ord k => Semigroup (PatchMapWithMove k v) where
238+ PatchMapWithMove ma <> PatchMapWithMove mb = PatchMapWithMove m
239+ where
240+ connections = Map.toList $ Map.intersectionWithKey (\_ a b -> (_nodeInfo_to a, _nodeInfo_from b)) ma mb
241+ h :: (k, (Maybe k, From k v)) -> [(k, Fixup k v)]
242+ h (_, (mToAfter, editBefore)) = case (mToAfter, editBefore) of
243+ (Just toAfter, From_Move fromBefore)
244+ | fromBefore == toAfter
245+ -> [(toAfter, Fixup_Delete)]
246+ | otherwise
247+ -> [ (toAfter, Fixup_Update (This editBefore))
248+ , (fromBefore, Fixup_Update (That mToAfter))
249+ ]
250+ (Nothing, From_Move fromBefore) -> [(fromBefore, Fixup_Update (That mToAfter))] -- The item is destroyed in the second patch, so indicate that it is destroyed in the source map
251+ (Just toAfter, _) -> [(toAfter, Fixup_Update (This editBefore))]
252+ (Nothing, _) -> []
253+ mergeFixups _ Fixup_Delete Fixup_Delete = Fixup_Delete
254+ mergeFixups _ (Fixup_Update a) (Fixup_Update b)
255+ | This x <- a, That y <- b
256+ = Fixup_Update $ These x y
257+ | That y <- a, This x <- b
258+ = Fixup_Update $ These x y
259+ mergeFixups _ _ _ = error "PatchMapWithMove: incompatible fixups"
260+ fixups = Map.fromListWithKey mergeFixups $ concatMap h connections
261+ combineNodeInfos _ nia nib = NodeInfo
262+ { _nodeInfo_from = _nodeInfo_from nia
263+ , _nodeInfo_to = _nodeInfo_to nib
264+ }
265+ applyFixup _ ni = \case
266+ Fixup_Delete -> Nothing
267+ Fixup_Update u -> Just $ NodeInfo
268+ { _nodeInfo_from = fromMaybe (_nodeInfo_from ni) $ getHere u
269+ , _nodeInfo_to = fromMaybe (_nodeInfo_to ni) $ getThere u
270+ }
271+ m = Map.differenceWithKey applyFixup (Map.unionWithKey combineNodeInfos ma mb) fixups
272+ getHere :: These a b -> Maybe a
273+ getHere = \case
274+ This a -> Just a
275+ These a _ -> Just a
276+ That _ -> Nothing
277+ getThere :: These a b -> Maybe b
278+ getThere = \case
279+ This _ -> Nothing
280+ These _ b -> Just b
281+ That b -> Just b
282+
283+--TODO: Figure out how to implement this in terms of PatchDMapWithMove rather than duplicating it here
284+-- |Compose patches having the same effect as applying the patches in turn: @'applyAlways' (p <> q) == 'applyAlways' p . 'applyAlways' q@
285+instance Ord k => Monoid (PatchMapWithMove k v) where
286+ mempty = PatchMapWithMove mempty
287+ mappend = (<>)