Markdown reader: Fixed regression on left-biased union for metadata.

When multiple YAML metadata blocks are used, and two define
the same field, the value defined first takes precedence,
according to the manual.  This was changed briefly in
ba3ee62323644134f2a5dd3277e3349d3681111a.  This commit
reverts to the original behavior and adds a test case.

1 files changed, 9 insertions, 6 deletions

diff --git a/src/Text/Pandoc/Readers/Markdown.hs b/src/Text/Pandoc/Readers/Markdown.hs
index 42d719e28..4790f83ff 100644
--- a/src/Text/Pandoc/Readers/Markdown.hs
+++ b/src/Text/Pandoc/Readers/Markdown.hs
@@ -254,12 +254,15 @@ yamlMetaBlock = try $ do
                 then return ()
                 else do
                   v' <- yamlToMeta v
-                  updateState $ \st ->
-                      let smeta = stateMeta' st
-                      in  st{ stateMeta' =
-                                     (do v'' <- v'
-                                         m <- smeta
-                                         return $ B.setMeta (T.unpack k) v'' m)}
+                  let k' = T.unpack k
+                  updateState $ \st -> st{ stateMeta' =
+                     (do m <- stateMeta' st
+                         -- if there's already a value, leave it unchanged
+                         case lookupMeta k' m of
+                              Just _ -> return m
+                              Nothing -> do
+                                v'' <- v'
+                                return $ B.setMeta (T.unpack k) v'' m)}
            ) alist
        Right Yaml.Null -> return ()
        Right _ -> do