FB2 reader test: better diagnostics on failure.

test/Tests/Readers/FB2.hs

@@ -15,13 +15,40 @@ module Tests.Readers.FB2 (tests) where
 import Prelude
 import Test.Tasty
 import Tests.Helpers
-import Test.Tasty.Golden (goldenVsString)
+-- import Test.Tasty.Golden (goldenVsString)
+import Test.Tasty.Golden.Advanced (goldenTest)
+import Text.Printf
+import qualified Data.ByteString.Lazy as LBS
+
 import qualified Data.ByteString as BS
 import Text.Pandoc
 import Text.Pandoc.UTF8 (toText, fromStringLazy)
 import Data.Text (Text, unpack)
 import System.FilePath (replaceExtension)
 
+simpleCmp :: Eq a => String -> a -> a -> IO (Maybe String)
+simpleCmp e x y =
+  return $ if x == y then Nothing else Just e
+
+
+goldenVsString
+  :: TestName -- ^ test name
+  -> FilePath -- ^ path to the «golden» file (the file that contains correct output)
+  -> IO LBS.ByteString -- ^ action that returns a string
+  -> TestTree -- ^ the test verifies that the returned string is the same as the golden file contents
+goldenVsString name ref act =
+  goldenTest
+    name
+    (BS.readFile ref)
+    (LBS.toStrict <$> act)
+    cmp
+    upd
+  where
+  cmp x y = simpleCmp msg x y
+    where
+    msg = printf "Test output from %s was different from %s. It was: %s" ref (show x) (show y)
+  upd = BS.writeFile ref
+
 fb2ToNative :: Text -> Text
 fb2ToNative = purely (writeNative def{ writerTemplate = Just mempty }) . purely (readFB2 def)