introduce --metadata-file option

closes #1960

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

MANUAL.txt

@@ -578,6 +578,16 @@ Reader options
     printed in some output formats) and metadata values will be escaped
     when inserted into the template.
 
+`--metadata-file=`*FILE*
+
+:   Read metadata from the supplied YAML (or JSON) file.
+    This option can be used with every input format, but string
+    scalars in the YAML file will always be parsed as Markdown.
+    Generally, the input will be handled the same as in
+    [YAML metadata blocks][Extension: `yaml_metadata_block`].
+    Metadata values specified inside the document, or by using `-M`,
+    overwrite values specified with this option.
+
 `-p`, `--preserve-tabs`
 
 :   Preserve tabs instead of converting them to spaces (the default).
@@ -3061,7 +3071,9 @@ and pass it to pandoc as an argument, along with your Markdown files:
     pandoc chap1.md chap2.md chap3.md metadata.yaml -s -o book.html
 
 Just be sure that the YAML file begins with `---` and ends with `---` or
-`...`.)
+`...`.) Alternatively, you can use the `--metadata-file` option. Using
+that approach however, you cannot reference content (like footnotes)
+from the main markdown input document.
 
 Metadata will be taken from the fields of the YAML object and added to any
 existing document metadata.  Metadata can contain lists and objects (nested

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

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