From 7fc6919f9039a3c71028b807d4372f18cf35bee8 Mon Sep 17 00:00:00 2001
From: John MacFarlane <jgm@berkeley.edu>
Date: Sun, 5 Mar 2017 09:28:44 +0100
Subject: 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.
---
 src/Text/Pandoc/Readers/Markdown.hs      | 15 +++++++++------
 test/command/multiple-metadata-blocks.md | 15 +++++++++++++++
 2 files changed, 24 insertions(+), 6 deletions(-)
 create mode 100644 test/command/multiple-metadata-blocks.md

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
diff --git a/test/command/multiple-metadata-blocks.md b/test/command/multiple-metadata-blocks.md
new file mode 100644
index 000000000..27c5e3dae
--- /dev/null
+++ b/test/command/multiple-metadata-blocks.md
@@ -0,0 +1,15 @@
+If multiple blocks define a field, the first is used.
+
+```
+% pandoc -s -t native
+---
+foo: bar
+...
+
+---
+foo: bim
+...
+^D
+Pandoc (Meta {unMeta = fromList [("foo",MetaInlines [Str "bar"])]})
+[]
+```
-- 
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