Added forgotten Diff.hs

test/Diff.hs

@@ -0,0 +1,14 @@
+module Diff where
+
+import           Data.Algorithm.Diff       (Diff (..), getGroupedDiff)
+import           Data.Algorithm.DiffOutput (ppDiff)
+import           Data.Monoid               ((<>))
+import           Test.Tasty.HUnit          (Assertion, assertBool)
+import           Text.Groom                (groom)
+
+-- | Like '@=?' but producing a diff on failure.
+(@==?) :: (Eq a, Show a) => a -> a -> Assertion
+x @==? y =
+    assertBool ("Expected:\n" <> groom x <> "\nDifference:\n" <> msg) (x == y)
+  where
+    msg = ppDiff $ getGroupedDiff (lines . groom $ x) (lines . groom $ y)