Add biblatex, bibtex as output formats (closes #7040).

* `biblatex` and `bibtex` are now supported as output
  as well as input formats.

* New module Text.Pandoc.Writers.BibTeX, exporting
  writeBibTeX and writeBibLaTeX. [API change]

* New unexported function `writeBibtexString` in
  Text.Pandoc.Citeproc.BibTeX.

1 files changed, 48 insertions, 0 deletions

diff --git a/src/Text/Pandoc/Writers/BibTeX.hs b/src/Text/Pandoc/Writers/BibTeX.hs
new file mode 100644
index 000000000..e1cb47ca1
--- /dev/null
+++ b/src/Text/Pandoc/Writers/BibTeX.hs
@@ -0,0 +1,48 @@
+{-# LANGUAGE OverloadedStrings #-}
+{- |
+   Module      : Text.Pandoc.Writers.BibTeX
+   Copyright   : Copyright (C) 2021 John MacFarlane
+   License     : GNU GPL, version 2 or above
+
+   Maintainer  : John MacFarlane <jgm@berkeley.edu>
+   Stability   : alpha
+   Portability : portable
+
+Writes a BibTeX or BibLaTeX bibliographies based on the
+'references' metadata in a Pandoc document.
+-}
+module Text.Pandoc.Writers.BibTeX
+  ( writeBibTeX
+  , writeBibLaTeX
+  )
+where
+
+import Text.Pandoc.Options
+import Text.Pandoc.Definition
+import Data.Text (Text)
+import Data.Maybe (mapMaybe)
+import Citeproc (parseLang)
+import Text.Pandoc.Class (PandocMonad)
+import Text.Pandoc.Citeproc.BibTeX as BibTeX
+import Text.Pandoc.Citeproc.MetaValue (metaValueToReference)
+import Text.Pandoc.Writers.Shared (lookupMetaString)
+
+-- | Write BibTeX based on the references metadata from a Pandoc document.
+writeBibTeX :: PandocMonad m => WriterOptions -> Pandoc -> m Text
+writeBibTeX = writeBibTeX' BibTeX.Bibtex
+
+-- | Write BibLaTeX based on the references metadata from a Pandoc document.
+writeBibLaTeX :: PandocMonad m => WriterOptions -> Pandoc -> m Text
+writeBibLaTeX = writeBibTeX' BibTeX.Biblatex
+
+writeBibTeX' :: PandocMonad m => Variant -> WriterOptions -> Pandoc -> m Text
+writeBibTeX' variant opts (Pandoc meta _) = do
+  let mblang = case lookupMetaString "lang" meta of
+                 "" -> Nothing
+                 t  -> Just $ parseLang t
+  let refs = case lookupMeta "references" meta of
+               Just (MetaList xs) -> mapMaybe metaValueToReference xs
+               _ -> []
+  return $ mconcat $
+    map (BibTeX.writeBibtexString opts variant mblang) refs
+