introduce --metadata-file option

closes #1960

API change: Text.Pandoc.Readers.Markdown exports now `yamlToMeta`

2 files changed, 31 insertions, 2 deletions

diff --git a/src/Text/Pandoc/App.hs b/src/Text/Pandoc/App.hs
index 44bb30223..cb1db4f89 100644
--- a/src/Text/Pandoc/App.hs
+++ b/src/Text/Pandoc/App.hs
@@ -89,6 +89,7 @@ import Text.Pandoc.Builder (setMeta, deleteMeta)
 import Text.Pandoc.Filter (Filter (JSONFilter, LuaFilter), applyFilters)
 import Text.Pandoc.Highlighting (highlightingStyles)
 import Text.Pandoc.PDF (makePDF)
+import Text.Pandoc.Readers.Markdown (yamlToMeta)
 import Text.Pandoc.SelfContained (makeDataURI, makeSelfContained)
 import Text.Pandoc.Shared (eastAsianLineBreakFilter, stripEmptyParagraphs,
          headerShift, isURI, ordNub, safeRead, tabFilter, uriPathToPath)
@@ -399,6 +400,10 @@ convertWithOpts opts = do
                                          ("application/xml", jatsCSL)
                      return $ ("csl", jatsEncoded) : optMetadata opts
                    else return $ optMetadata opts
+    metadataFromFile <-
+      case optMetadataFile opts of
+        Nothing   -> return mempty
+        Just file -> readFileLazy file >>= yamlToMeta
 
     case lookup "lang" (optMetadata opts) of
            Just l  -> case parseBCP47 l of
@@ -491,6 +496,7 @@ convertWithOpts opts = do
               (   (if isJust (optExtractMedia opts)
                       then fillMediaBag
                       else return)
+              >=> return . addNonPresentMetadata metadataFromFile
               >=> return . addMetadata metadata
               >=> applyTransforms transforms
               >=> applyFilters readerOpts filters' [format]
@@ -556,6 +562,7 @@ data Opt = Opt
     , optTemplate              :: Maybe FilePath  -- ^ Custom template
     , optVariables             :: [(String,String)] -- ^ Template variables to set
     , optMetadata              :: [(String, String)] -- ^ Metadata fields to set
+    , optMetadataFile          :: Maybe FilePath  -- ^ Name of YAML metadata file
     , optOutputFile            :: Maybe FilePath  -- ^ Name of output file
     , optInputFiles            :: [FilePath] -- ^ Names of input files
     , optNumberSections        :: Bool    -- ^ Number sections in LaTeX
@@ -628,6 +635,7 @@ defaultOpts = Opt
     , optTemplate              = Nothing
     , optVariables             = []
     , optMetadata              = []
+    , optMetadataFile          = Nothing
     , optOutputFile            = Nothing
     , optInputFiles            = []
     , optNumberSections        = False
@@ -687,6 +695,9 @@ defaultOpts = Opt
     , optStripComments          = False
     }
 
+addNonPresentMetadata :: Text.Pandoc.Meta -> Pandoc -> Pandoc
+addNonPresentMetadata newmeta (Pandoc meta bs) = Pandoc (meta <> newmeta) bs
+
 addMetadata :: [(String, String)] -> Pandoc -> Pandoc
 addMetadata kvs pdc = foldr addMeta (removeMetaKeys kvs pdc) kvs
 
@@ -963,6 +974,12 @@ options =
                   "KEY[:VALUE]")
                  ""
 
+    , Option "" ["metadata-file"]
+                 (ReqArg
+                  (\arg opt -> return opt{ optMetadataFile = Just arg })
+                  "FILE")
+                 ""
+
     , Option "V" ["variable"]
                  (ReqArg
                   (\arg opt -> do
diff --git a/src/Text/Pandoc/Readers/Markdown.hs b/src/Text/Pandoc/Readers/Markdown.hs
index 50780b379..502abae9a 100644
--- a/src/Text/Pandoc/Readers/Markdown.hs
+++ b/src/Text/Pandoc/Readers/Markdown.hs
@@ -31,7 +31,7 @@ Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
 
 Conversion of markdown-formatted plain text to 'Pandoc' document.
 -}
-module Text.Pandoc.Readers.Markdown ( readMarkdown ) where
+module Text.Pandoc.Readers.Markdown ( readMarkdown, yamlToMeta ) where
 
 import Prelude
 import Control.Monad
@@ -246,11 +246,23 @@ yamlMetaBlock = try $ do
   updateState $ \st -> st{ stateMeta' = (stateMeta' st) <> newMetaF }
   return mempty
 
+-- | Read a YAML string and convert it to pandoc metadata.
+-- String scalars in the YAML are parsed as Markdown.
+yamlToMeta :: PandocMonad m => BS.ByteString -> m Meta
+yamlToMeta bstr = do
+  let parser = do
+        meta <- yamlBsToMeta bstr
+        return $ runF meta defaultParserState
+  parsed <- readWithM parser def ""
+  case parsed of
+    Right result -> return result
+    Left e       -> throwError e
+
 yamlBsToMeta :: PandocMonad m => BS.ByteString -> MarkdownParser m (F Meta)
 yamlBsToMeta bstr = do
   pos <- getPosition
   case YAML.decodeNode' YAML.failsafeSchemaResolver False False bstr of
-       Right [YAML.Doc (YAML.Mapping _ o)] -> (fmap Meta) <$> yamlMap o
+       Right ((YAML.Doc (YAML.Mapping _ o)):_) -> (fmap Meta) <$> yamlMap o
        Right [] -> return . return $ mempty
        Right [YAML.Doc (YAML.Scalar YAML.SNull)] -> return . return $ mempty
        Right _ -> do