Textual representation of different values at the top level

generic-diff-instances/test/Generics/Diff/UnitTestsSpec.hs

@@ -107,7 +107,7 @@ mapTestSets =
     value1 = Map.fromList [(1, "one"), (3, "three")]
 
     value2 = Map.fromList [(1, "one"), (3, "THREE")]
-    error2 = DiffSpecial $ Map.DiffAtKey 3 TopLevelNotEqual
+    error2 = DiffSpecial $ Map.DiffAtKey 3 $ TopLevelNotEqualShow "\"three\"" "\"THREE\""
 
     value3 = Map.fromList [(1, "one"), (2, "two"), (3, "three")]
     error3 = DiffSpecial $ Map.LeftMissingKey 2
@@ -143,7 +143,7 @@ seqTestSets =
     error2 = DiffSpecial $ WrongLengths 2 3
 
     value3 = Seq.fromList [1, 2]
-    error3 = DiffSpecial $ DiffAtIndex 1 TopLevelNotEqual
+    error3 = DiffSpecial $ DiffAtIndex 1 $ TopLevelNotEqualShow "3" "2"
 
 treeTestSets :: [TestSet (Tree Int)]
 treeTestSets =
@@ -176,7 +176,7 @@ treeTestSets =
     value1 = Tree.Node 1 [Tree.Node 2 [], Tree.Node 3 [Tree.Node 4 [], Tree.Node 5 []]]
 
     value2 = Tree.Node 2 []
-    error2 = DiffSpecial $ FieldMismatch $ DiffAtField $ Z $ nodeInfo :*: Z TopLevelNotEqual
+    error2 = DiffSpecial $ FieldMismatch $ DiffAtField $ Z $ nodeInfo :*: Z (TopLevelNotEqualShow "1" "2")
 
     value3 = Tree.Node 1 [Tree.Node 2 []]
     error3 =
@@ -185,7 +185,7 @@ treeTestSets =
 
     value4 = Tree.Node 1 [Tree.Node 2 [], Tree.Node 4 []]
     error4 =
-      let e = DiffSpecial $ DiffAtIndex 1 $ DiffSpecial $ FieldMismatch $ DiffAtField $ Z $ nodeInfo :*: Z TopLevelNotEqual
+      let e = DiffSpecial $ DiffAtIndex 1 $ DiffSpecial $ FieldMismatch $ DiffAtField $ Z $ nodeInfo :*: Z (TopLevelNotEqualShow "3" "4")
       in  DiffSpecial $ FieldMismatch $ DiffAtField $ Z $ nodeInfo :*: S (Z e)
 
     nodeInfo :: ConstructorInfo '[Int, [Tree Int]]
@@ -222,10 +222,10 @@ customTreeTestSets =
     value1 = CustomTree $ Tree.Node 1 [Tree.Node 2 [], Tree.Node 3 [Tree.Node 4 [], Tree.Node 5 []]]
 
     value2 = CustomTree $ Tree.Node 2 []
-    error2 = DiffSpecial $ DiffAtNode (TreePath []) TopLevelNotEqual
+    error2 = DiffSpecial $ DiffAtNode (TreePath []) $ TopLevelNotEqualShow "1" "2"
 
     value3 = CustomTree $ Tree.Node 1 [Tree.Node 2 []]
     error3 = DiffSpecial $ WrongLengthsOfChildren (TreePath []) 2 1
 
     value4 = CustomTree $ Tree.Node 1 [Tree.Node 2 [], Tree.Node 4 []]
-    error4 = DiffSpecial $ DiffAtNode (TreePath [1]) TopLevelNotEqual
+    error4 = DiffSpecial $ DiffAtNode (TreePath [1]) $ TopLevelNotEqualShow "3" "4"

generic-diff/CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Haskell Package Versioning Policy](https://pvp.hask
 
 ## [Unreleased]
 
+### Changed
+
+- New `DiffError` constructor that includes a `Text` showing the two values compared at the top level in [#18](https://github.com/fpringle/generic-diff/pull/18).
+
 ## [0.1.0.1] - 01.02.2026
 
 ### Changed

generic-diff/README.md

@@ -42,4 +42,6 @@ In field right:
     Both values use constructor Atom but fields don't match
     In field 0 (0-indexed):
       Not equal
+      Left value:  1
+      Right value: 2
 ```

generic-diff/src/Generics/Diff.hs

@@ -127,13 +127,16 @@ uses the @Right@ constructor"! And of course, once we have one step of recursion
 
 The 'Diff' class encapsulates the above behaviour with 'diff'. It's very strongly recommended that you don't
 implement 'diff' yourself, but use the default implementation using 'Generics.SOP.Generic', which is just 'gdiff'.
-In case you might want to implement 'diff' yourself, there are three other functions you might want to use.
+In case you might want to implement 'diff' yourself, there are five other functions you might want to use.
 
-- 'eqDiff' simply delegates the entire process to '(==)', and will only ever give 'Equal' or 'TopLevelNotEqual'. This is
-no more useful than 'Eq', and should only be used for primitive types (e.g. all numeric types like 'Char' and 'Int')
-use 'eqDiff', since they don't really have ADTs or recursion.
+- 'eqDiffShow' simply delegates the entire process to '(==)', and will only ever give 'Equal' or 'TopLevelNotEqualShow'. This is
+no more useful than 'Eq', and should only be used for primitive types (e.g. all numeric types like 'Char' and 'Int'
+use 'eqDiff', since they don't really have ADTs or recursion).
 
-- 'gdiffTopLevel' does the above process, but without recursion. In other words each pair of fields is compared using
+- 'eqDiff' is the same as 'eqDiffShow', but uses 'TopLevelNotEqual'. This should only be used for primitive types that
+do not have 'Show' instances.
+
+- 'gdiffTopLevelShow' does the above process, but without recursion. In other words each pair of fields is compared using
 '(==)'. This is definitely better than 'Eq', by one "level". One situation when this might be useful is when your
 type refers to types from other libraries, and you want to avoid orphan 'Diff' instances for those types. Another
 is when the types of the fields are "small" enough that we don't care about recursing into them. For example:
@@ -159,6 +162,9 @@ instance 'Diff' Request where
   'diff' = 'gdiffTopLevel'
 @
 
+- 'gdiffTopLevel' is the same as 'gdiffTopLevelShow', but uses 'eqDiffShow' instead of 'eqDiff'. This should only be used
+for primitive types that do not have 'Show' instances.
+
 - 'diffWithSpecial' lets us handle edge cases for funky types with unusual 'Eq' instances or preserved
 invariants. See "Generics.Diff.Special".
 
@@ -172,8 +178,10 @@ module Generics.Diff
 
     -- ** Implementing diff
   , gdiff
+  , gdiffTopLevelShow
   , gdiffTopLevel
   , gdiffWith
+  , eqDiffShow
   , eqDiff
 
     -- * Types

generic-diff/src/Generics/Diff/Class.hs

@@ -7,8 +7,10 @@ module Generics.Diff.Class
 
     -- ** Implementing diff
   , gdiff
+  , gdiffTopLevelShow
   , gdiffTopLevel
   , gdiffWith
+  , eqDiffShow
   , eqDiff
   , diffWithSpecial
   , gspecialDiffNested
@@ -20,6 +22,7 @@ where
 
 import Data.SOP
 import Data.SOP.NP
+import qualified Data.Text as T
 import qualified GHC.Generics as G
 import Generics.Diff.Render
 import Generics.Diff.Type
@@ -68,7 +71,7 @@ ghci> diff Plus Minus
 Error (Nested (WrongConstructor (Z (Constructor \"Plus\")) (S (Z (Constructor \"Minus\")))))
 
 ghci> diff (Atom 1) (Atom 2)
-Error (Nested (FieldMismatch (AtLoc (Z (Constructor \"Atom\" :*: Z (Nested TopLevelNotEqual))))))
+Error (Nested (FieldMismatch (AtLoc (Z (Constructor \"Atom\" :*: Z (Nested $ TopLevelNotEqualShow \"1\" \"2\"))))))
 
 ghci> diff (Bin (Atom 1) Plus (Atom 1)) (Atom 2)
 Error (Nested (WrongConstructor (S (Z (Constructor \"Bin\"))) (Z (Constructor \"Atom\"))))
@@ -77,7 +80,7 @@ ghci> diff (Bin (Atom 1) Plus (Atom 1)) (Bin (Atom 1) Minus (Atom 1))
 Error (Nested (FieldMismatch (AtLoc (S (Z (Constructor \"Bin\" :*: S (Z (Nested (WrongConstructor (Z (Constructor \"Plus\")) (S (Z (Constructor \"Minus\"))))))))))))
 
 ghci> diff (Bin (Atom 1) Plus (Atom 1)) (Bin (Atom 1) Plus (Atom 2))
-Error (Nested (FieldMismatch (DiffAtField (S (Z (Record \"Bin\" (FieldInfo \"left\" :* FieldInfo \"op\" :* FieldInfo \"right\" :* Nil) :*: S (S (Z (Nested (FieldMismatch (DiffAtField (Z (Constructor \"Atom\" :*: Z TopLevelNotEqual)))))))))))))
+Error (Nested (FieldMismatch (DiffAtField (S (Z (Record \"Bin\" (FieldInfo \"left\" :* FieldInfo \"op\" :* FieldInfo \"right\" :* Nil) :*: S (S (Z (Nested (FieldMismatch (DiffAtField (Z (Constructor \"Atom\" :*: Z $ TopLevelNotEqualShow "1" "2")))))))))))))
 @
 
 Of course, these are just as difficult to understand as derived 'Show' instances, or more so. Fortunately we can
@@ -96,6 +99,8 @@ ghci> printDiffResult $ diff (Atom 1) (Atom 2)
 Both values use constructor Atom but fields don't match
 In field 0 (0-indexed):
   Not equal
+  Left value:  1
+  Right value: 2
 
 ghci> printDiffResult $ diff (Bin (Atom 1) Plus (Atom 1)) (Atom 2)
 Wrong constructor
@@ -115,6 +120,8 @@ In field right:
   Both values use constructor Atom but fields don't match
   In field 0 (0-indexed):
     Not equal
+    Left value:  1
+    Right value: 2
 @
 
 = Laws
@@ -127,7 +134,8 @@ x == y   \<=\>  x \`diff\` y == 'Equal'
 -}
 class Diff a where
   -- | Detailed comparison of two values. It is strongly recommended to only use the
-  -- default implementation, or one of 'eqDiff' or 'gdiffTopLevel'.
+  -- default implementation, or one of 'eqDiffShow' or 'gdiffTopLevelShow'
+  -- (or 'eqDiff' or 'gdiffTopLevel' if the type doesn't have a 'Show' instance).
   diff :: a -> a -> DiffResult a
   default diff :: (Generic a, HasDatatypeInfo a, All2 Diff (Code a)) => a -> a -> DiffResult a
   diff = gdiff
@@ -162,13 +170,29 @@ diffListWith d = go 0
 
 {- | The most basic 'Differ' possible. If the two values are equal, return 'Equal';
 otherwise, return 'TopLevelNotEqual'.
+
+Note that if @a@ has a 'Show' instance, then 'eqDiffShow' is more informative since
+it actually displays the compared values.
 -}
 eqDiff :: (Eq a) => a -> a -> DiffResult a
 eqDiff a b =
   if a == b
     then Equal
     else Error TopLevelNotEqual
 
+tshow :: (Show a) => a -> T.Text
+tshow = T.pack . show
+{-# INLINE tshow #-}
+
+{- | Like 'eqDiff', 'eqDiffShow' only compares the values at the top-level. It also includes
+a textual representation of the compared values.
+-}
+eqDiffShow :: (Eq a, Show a) => a -> a -> DiffResult a
+eqDiffShow a b =
+  if a == b
+    then Equal
+    else Error $ TopLevelNotEqualShow (tshow a) (tshow b)
+
 {- | The default implementation of 'diff'. Follows the procedure described above. We keep recursing
 into the 'Diff' instances of the field types, as far as we can.
 -}
@@ -182,6 +206,9 @@ gdiff = gdiffWithPure @a @Diff (Differ diff)
 
 {- | Alternate implementation of 'diff' - basically one level of 'gdiff'. To compare individual fields of the
 top-level values, we just use '(==)'.
+
+Note that if @a@ has a 'Show' instance, then 'gdiffTopLevelShow' is more informative since
+it actually displays the compared values.
 -}
 gdiffTopLevel ::
   forall a.
@@ -191,6 +218,15 @@ gdiffTopLevel ::
   DiffResult a
 gdiffTopLevel = gdiffWithPure @a @Eq (Differ eqDiff)
 
+-- | Same as 'gdiffTopLevel', but includes a textual representation of the compared values.
+gdiffTopLevelShow ::
+  forall a.
+  (Generic a, HasDatatypeInfo a, All2 (And Eq Show) (Code a)) =>
+  a ->
+  a ->
+  DiffResult a
+gdiffTopLevelShow = gdiffWithPure @a @(And Eq Show) (Differ eqDiffShow)
+
 {- | Follow the same algorithm as 'gdiff', but the caller can provide their own 'POP' grid of 'Differ's
 specifying how to compare each field we might come across.
 -}