Fix regression with --metadata.

It should replace a metadata value set in the document
itself, rather than creating a list including a new value.

Closes #4054.

src/Text/Pandoc/App.hs

@@ -76,7 +76,7 @@ import qualified System.IO as IO (Newline (..))
 import System.IO.Error (isDoesNotExistError)
 import Text.Pandoc
 import Text.Pandoc.BCP47 (Lang (..), parseBCP47)
-import Text.Pandoc.Builder (setMeta)
+import Text.Pandoc.Builder (setMeta, deleteMeta)
 import Text.Pandoc.Highlighting (highlightingStyles)
 import Text.Pandoc.Lua (LuaException (..), runLuaFilter)
 import Text.Pandoc.PDF (makePDF)
@@ -494,7 +494,7 @@ convertWithOpts opts = do
               (   (if isJust (optExtractMedia opts)
                       then fillMediaBag
                       else return)
-              >=> return . flip (foldr addMetadata) metadata
+              >=> return . addMetadata metadata
               >=> applyLuaFilters datadir (optLuaFilters opts) format
               >=> maybe return extractMedia (optExtractMedia opts)
               >=> applyTransforms transforms
@@ -722,8 +722,11 @@ defaultOpts = Opt
     , optStripComments          = False
     }
 
-addMetadata :: (String, String) -> Pandoc -> Pandoc
-addMetadata (k, v) (Pandoc meta bs) = Pandoc meta' bs
+addMetadata :: [(String, String)] -> Pandoc -> Pandoc
+addMetadata kvs pdc = foldr addMeta (removeMetaKeys kvs pdc) kvs
+
+addMeta :: (String, String) -> Pandoc -> Pandoc
+addMeta (k, v) (Pandoc meta bs) = Pandoc meta' bs
   where meta' = case lookupMeta k meta of
                       Nothing -> setMeta k v' meta
                       Just (MetaList xs) ->
@@ -731,6 +734,9 @@ addMetadata (k, v) (Pandoc meta bs) = Pandoc meta' bs
                       Just x  -> setMeta k (MetaList [x, v']) meta
         v' = readMetaValue v
 
+removeMetaKeys :: [(String,String)] -> Pandoc -> Pandoc
+removeMetaKeys kvs pdc = foldr (deleteMeta . fst) pdc kvs
+
 readMetaValue :: String -> MetaValue
 readMetaValue s = case decode (UTF8.fromString s) of
                        Just (Yaml.String t) -> MetaString $ T.unpack t

test/command/4054.md

@@ -0,0 +1,14 @@
+```
+% pandoc -t native -s -M title=New
+% Old
+^D
+Pandoc (Meta {unMeta = fromList [("title",MetaString "New")]})
+[]
+```
+
+```
+% pandoc -t native -s -M foo=1 -M foo=2
+^D
+Pandoc (Meta {unMeta = fromList [("foo",MetaList [MetaString "1",MetaString "2"])]})
+[]
+```