Define a patch helper to eliminate Patches.

src/Patch.hs

@@ -4,9 +4,7 @@ module Patch
 ( Patch(..)
 , after
 , before
-, unPatch
-, maybeFst
-, maybeSnd
+, patch
 ) where
 
 import Data.Aeson
@@ -29,25 +27,17 @@ data Patch a b
 
 -- | Return the item from the after side of the patch.
 after :: Patch before after -> Maybe after
-after = maybeSnd . unPatch
+after = patch (const Nothing) Just (\ _ b -> Just b)
 
 -- | Return the item from the before side of the patch.
 before :: Patch before after -> Maybe before
-before = maybeFst . unPatch
+before = patch Just (const Nothing) (\ a _ -> Just a)
 
 -- | Return both sides of a patch.
-unPatch :: Patch before after -> These before after
-unPatch (Replace a b) = These a b
-unPatch (Insert b) = That b
-unPatch (Delete a) = This a
-
--- | Return Just the value in This, or the first value in These, if any.
-maybeFst :: These a b -> Maybe a
-maybeFst = these Just (const Nothing) ((Just .) . const)
-
--- | Return Just the value in That, or the second value in These, if any.
-maybeSnd :: These a b -> Maybe b
-maybeSnd = these (const Nothing) Just ((Just .) . flip const)
+patch :: (before -> result) -> (after -> result) -> (before -> after -> result) -> Patch before after -> result
+patch ifDelete _ _ (Delete a) = ifDelete a
+patch _ ifInsert _ (Insert b) = ifInsert b
+patch _ _ ifReplace (Replace a b) = ifReplace a b
 
 
 -- Instances

src/Renderer/Patch.hs

@@ -23,7 +23,6 @@ import Data.Semigroup ((<>))
 import Data.Source
 import Data.These
 import Diff
-import Patch
 import Prelude hiding (fst, snd)
 import SplitDiff
 
@@ -181,3 +180,6 @@ changeIncludingContext leadingContext rows = case changes of
 -- | Whether a row has changes on either side.
 rowHasChanges :: (Foldable f, Functor f) => Join These (SplitDiff f annotation) -> Bool
 rowHasChanges row = or (hasChanges <$> row)
+
+maybeSnd :: These a b -> Maybe b
+maybeSnd = these (const Nothing) Just (\ _ a -> Just a)

src/Renderer/TOC.hs

@@ -156,7 +156,7 @@ tableOfContentsBy selector = fromMaybe [] . cata (\ r -> case r of
     (Just a, Just []) -> Just [Changed a]
     (_     , entries) -> entries)
 
-  where patchEntry = these Deleted Inserted (const Replaced) . unPatch
+  where patchEntry = patch Deleted Inserted (const Replaced)
 
 termTableOfContentsBy :: (Foldable f, Functor f)
                       => (forall b. TermF f annotation b -> Maybe a)

test/TOCSpec.hs

@@ -51,9 +51,9 @@ spec = parallel $ do
       \ term -> tableOfContentsBy (Just . termAnnotation) (diffTerms term term) `shouldBe` [Unchanged (lastValue (term :: Term Syntax (Record '[Category])))]
 
     prop "produces inserted/deleted/replaced entries for relevant nodes within patches" $
-      \ patch -> tableOfContentsBy (Just . termAnnotation) (these deleting inserting replacing (unPatch patch))
+      \ p -> tableOfContentsBy (Just . termAnnotation) (patch deleting inserting replacing p)
       `shouldBe`
-      these (fmap Deleted) (fmap Inserted) (const (fmap Replaced)) (unPatch (bimap (foldMap pure) (foldMap pure) (patch :: Patch (Term Syntax Int) (Term Syntax Int))))
+      patch (fmap Deleted) (fmap Inserted) (const (fmap Replaced)) (bimap (foldMap pure) (foldMap pure) (p :: Patch (Term Syntax Int) (Term Syntax Int)))
 
     prop "produces changed entries for relevant nodes containing irrelevant patches" $
       \ diff -> let diff' = merge (0, 0) (Indexed [1 <$ (diff :: Diff Syntax Int)]) in