Support for PDF generation via `weasyprint` and `prince` (#3909)

* Rename --latex-engine to --pdf-engine
* In `Text.Pandoc.Options.WriterOptions`, rename `writerLaTeXEngine` to `writerPdfEngine` and `writerLaTeXArgs` to `writerPdfArgs`.
 * Add support for `weasyprint` and `prince`, in addition to `wkhtmltopdf`, for PDF generation via HTML (closes #3906).
* `Text.Pandoc.PDF.html2pdf`: use stdin instead of intermediate HTML file

4 files changed, 108 insertions, 62 deletions

diff --git a/src/Text/Pandoc/App.hs b/src/Text/Pandoc/App.hs
index 367a1f550..93a2a9da6 100644
--- a/src/Text/Pandoc/App.hs
+++ b/src/Text/Pandoc/App.hs
@@ -121,6 +121,53 @@ parseOptions options' defaults = do
   opts <- foldl (>>=) (return defaults) actions
   return (opts{ optInputFiles = args })
 
+latexEngines :: [String]
+latexEngines  = ["pdflatex", "lualatex", "xelatex"]
+
+htmlEngines :: [String]
+htmlEngines  = ["wkhtmltopdf", "weasyprint", "prince"]
+
+pdfEngines :: [String]
+pdfEngines = latexEngines ++ htmlEngines ++ ["context", "pdfroff"]
+
+pdfWriterAndProg :: Maybe String              -- ^ user-specified writer name
+                 -> Maybe String              -- ^ user-specified pdf-engine
+                 -> IO (String, Maybe String) -- ^ IO (writerName, maybePdfEngineProg)
+pdfWriterAndProg mWriter mEngine = do
+  let panErr msg = liftIO $ E.throwIO $ PandocAppError msg
+  case go mWriter mEngine of
+      (Right writ, Right prog) -> return (writ, Just prog)
+      (Left err, _)            -> panErr err
+      (_, Left err)            -> panErr err
+    where
+      go Nothing Nothing       = (Right "latex", Right $ head latexEngines)
+      go (Just writer) Nothing = (Right writer, engineForWriter writer)
+      go Nothing (Just engine) = (writerForEngine engine, Right engine)
+      go (Just writer) (Just engine) =
+           let (Right shouldFormat) = writerForEngine engine
+               userFormat = case map toLower writer of
+                              "html5" -> "html"
+                              x       -> x
+           in  if userFormat == shouldFormat
+               then (Right writer, Right engine)
+               else (Left $ "pdf-engine " ++ engine ++ " is not compatible with output format "
+                      ++ writer ++ ", please use `-t " ++ shouldFormat ++ "`", Left "")
+
+      writerForEngine "context"  = Right "context"
+      writerForEngine "pdfroff"  = Right "ms"
+      writerForEngine en
+        | takeBaseName en `elem` latexEngines = Right "latex"
+        | takeBaseName en `elem` htmlEngines  = Right "html"
+      writerForEngine _          = Left "pdf-engine not known"
+
+      engineForWriter "context" = Right "context"
+      engineForWriter "ms"      = Right "pdfroff"
+      engineForWriter "latex"   = Right $ head latexEngines
+      engineForWriter format
+        | format `elem` ["html", "html5"] = Right $ head htmlEngines
+        | otherwise = Left $ "cannot produce pdf output with output format " ++ format
+
+
 convertWithOpts :: Opt -> IO ()
 convertWithOpts opts = do
   let args = optInputFiles opts
@@ -171,18 +218,16 @@ convertWithOpts opts = do
                                           else "markdown") sources
                           Just x  -> map toLower x
 
-  let writerName = case optWriter opts of
-                          Nothing -> defaultWriterName outputFile
-                          Just x  -> map toLower x
-  let format = takeWhile (`notElem` ['+','-'])
-                       $ takeFileName writerName  -- in case path to lua script
+  let nonPdfWriterName Nothing  = defaultWriterName outputFile
+      nonPdfWriterName (Just x) = map toLower x
 
   let pdfOutput = map toLower (takeExtension outputFile) == ".pdf"
+  (writerName, maybePdfProg) <- if pdfOutput
+                                then pdfWriterAndProg (optWriter opts) (optPdfEngine opts)
+                                else return (nonPdfWriterName $ optWriter opts, Nothing)
 
-  let laTeXOutput = format `elem` ["latex", "beamer"]
-  let conTeXtOutput = format == "context"
-  let html5Output = format == "html5" || format == "html"
-  let msOutput = format == "ms"
+  let format = takeWhile (`notElem` ['+','-'])
+                       $ takeFileName writerName  -- in case path to lua script
 
   -- disabling the custom writer for now
   (writer, writerExts) <-
@@ -417,7 +462,7 @@ convertWithOpts opts = do
           , writerEpubChapterLevel = optEpubChapterLevel opts
           , writerTOCDepth         = optTOCDepth opts
           , writerReferenceDoc     = optReferenceDoc opts
-          , writerLaTeXArgs        = optLaTeXEngineArgs opts
+          , writerPdfArgs          = optPdfEngineArgs opts
           , writerSyntaxMap        = syntaxMap
           }
 
@@ -475,27 +520,14 @@ convertWithOpts opts = do
 
     case writer of
       ByteStringWriter f -> f writerOptions doc >>= writeFnBinary outputFile
-      TextWriter f
-        | pdfOutput -> do
-                -- make sure writer is latex, beamer, context, html5 or ms
-                unless (laTeXOutput || conTeXtOutput || html5Output ||
-                        msOutput) $
-                  liftIO $ E.throwIO $ PandocAppError $
-                     "cannot produce pdf output with " ++ format ++ " writer"
-
-                let pdfprog = case () of
-                                _ | conTeXtOutput -> "context"
-                                  | html5Output   -> "wkhtmltopdf"
-                                  | html5Output   -> "wkhtmltopdf"
-                                  | msOutput      -> "pdfroff"
-                                  | otherwise     -> optLaTeXEngine opts
-
-                res <- makePDF pdfprog f writerOptions verbosity media doc
+      TextWriter f -> case maybePdfProg of
+        Just pdfProg -> do
+                res <- makePDF pdfProg f writerOptions verbosity media doc
                 case res of
                      Right pdf -> writeFnBinary outputFile pdf
                      Left err' -> liftIO $
                        E.throwIO $ PandocPDFError (UTF8.toStringLazy err')
-        | otherwise -> do
+        Nothing -> do
                 let htmlFormat = format `elem`
                       ["html","html4","html5","s5","slidy","slideous","dzslides","revealjs"]
                     handleEntities = if (htmlFormat ||
@@ -605,8 +637,8 @@ data Opt = Opt
     , optDataDir               :: Maybe FilePath
     , optCiteMethod            :: CiteMethod -- ^ Method to output cites
     , optListings              :: Bool       -- ^ Use listings package for code blocks
-    , optLaTeXEngine           :: String     -- ^ Program to use for latex -> pdf
-    , optLaTeXEngineArgs       :: [String]   -- ^ Flags to pass to the latex-engine
+    , optPdfEngine             :: Maybe String -- ^ Program to use for latex/html -> pdf
+    , optPdfEngineArgs         :: [String]   -- ^ Flags to pass to the engine
     , optSlideLevel            :: Maybe Int  -- ^ Header level that creates slides
     , optSetextHeaders         :: Bool       -- ^ Use atx headers for markdown level 1-2
     , optAscii                 :: Bool       -- ^ Use ascii characters only in html
@@ -681,8 +713,8 @@ defaultOpts = Opt
     , optDataDir               = Nothing
     , optCiteMethod            = Citeproc
     , optListings              = False
-    , optLaTeXEngine           = "pdflatex"
-    , optLaTeXEngineArgs       = []
+    , optPdfEngine             = Nothing
+    , optPdfEngineArgs         = []
     , optSlideLevel            = Nothing
     , optSetextHeaders         = True
     , optAscii                 = False
@@ -778,7 +810,6 @@ defaultWriterName x =
     ".org"      -> "org"
     ".asciidoc" -> "asciidoc"
     ".adoc"     -> "asciidoc"
-    ".pdf"      -> "latex"
     ".fb2"      -> "fb2"
     ".opml"     -> "opml"
     ".icml"     -> "icml"
@@ -1314,23 +1345,24 @@ options =
                  "NUMBER")
                  "" -- "Header level at which to split chapters in EPUB"
 
-    , Option "" ["latex-engine"]
+    , Option "" ["pdf-engine"]
                  (ReqArg
                   (\arg opt -> do
                      let b = takeBaseName arg
-                     if b `elem` ["pdflatex", "lualatex", "xelatex"]
-                        then return opt { optLaTeXEngine = arg }
-                        else E.throwIO $ PandocOptionError "latex-engine must be pdflatex, lualatex, or xelatex.")
+                     if b `elem` pdfEngines
+                        then return opt { optPdfEngine = Just arg }
+                        else E.throwIO $ PandocOptionError $ "pdf-engine must be one of "
+                               ++ intercalate ", " pdfEngines)
                   "PROGRAM")
-                 "" -- "Name of latex program to use in generating PDF"
+                 "" -- "Name of program to use in generating PDF"
 
-    , Option "" ["latex-engine-opt"]
+    , Option "" ["pdf-engine-opt"]
                  (ReqArg
                   (\arg opt -> do
-                      let oldArgs = optLaTeXEngineArgs opt
-                      return opt { optLaTeXEngineArgs = arg : oldArgs })
+                      let oldArgs = optPdfEngineArgs opt
+                      return opt { optPdfEngineArgs = arg : oldArgs })
                   "STRING")
-                 "" -- "Flags to pass to the LaTeX engine, all instances of this option are accumulated and used"
+                 "" -- "Flags to pass to the PDF-engine, all instances of this option are accumulated and used"
 
     , Option "" ["bibliography"]
                  (ReqArg
@@ -1590,6 +1622,10 @@ handleUnrecognizedOption "--old-dashes" =
   ("--old-dashes has been removed.  Use +old_dashes extension instead." :)
 handleUnrecognizedOption "--no-wrap" =
   ("--no-wrap has been removed.  Use --wrap=none instead." :)
+handleUnrecognizedOption "--latex-engine" =
+  ("--latex-engine has been removed.  Use --pdf-engine instead." :)
+handleUnrecognizedOption "--latex-engine-opt" =
+  ("--latex-engine-opt has been removed.  Use --pdf-engine-opt instead." :)
 handleUnrecognizedOption "--chapters" =
   ("--chapters has been removed. Use --top-level-division=chapter instead." :)
 handleUnrecognizedOption "--reference-docx" =
diff --git a/src/Text/Pandoc/Error.hs b/src/Text/Pandoc/Error.hs
index 24186720c..2bd8bef0a 100644
--- a/src/Text/Pandoc/Error.hs
+++ b/src/Text/Pandoc/Error.hs
@@ -96,7 +96,7 @@ handleError (Left e) =
     PandocSyntaxMapError s -> err 67 s
     PandocFailOnWarningError -> err 3 "Failing because there were warnings."
     PandocPDFProgramNotFoundError pdfprog -> err 47 $
-        pdfprog ++ " not found. " ++ pdfprog ++ " is needed for pdf output."
+        pdfprog ++ " not found. Please select a different --pdf-engine or install " ++ pdfprog
     PandocPDFError logmsg -> err 43 $ "Error producing PDF.\n" ++ logmsg
     PandocFilterError filtername msg -> err 83 $ "Error running filter " ++
         filtername ++ ":\n" ++ msg
diff --git a/src/Text/Pandoc/Options.hs b/src/Text/Pandoc/Options.hs
index 89b26deb0..0bd66d54d 100644
--- a/src/Text/Pandoc/Options.hs
+++ b/src/Text/Pandoc/Options.hs
@@ -216,7 +216,7 @@ data WriterOptions = WriterOptions
   , writerEpubChapterLevel  :: Int            -- ^ Header level for chapters (separate files)
   , writerTOCDepth          :: Int            -- ^ Number of levels to include in TOC
   , writerReferenceDoc      :: Maybe FilePath -- ^ Path to reference document if specified
-  , writerLaTeXArgs         :: [String]       -- ^ Flags to pass to latex-engine
+  , writerPdfArgs           :: [String]       -- ^ Flags to pass to pdf-engine
   , writerReferenceLocation :: ReferenceLocation    -- ^ Location of footnotes and references for writing markdown
   , writerSyntaxMap         :: SyntaxMap
   } deriving (Show, Data, Typeable, Generic)
@@ -252,7 +252,7 @@ instance Default WriterOptions where
                       , writerEpubChapterLevel = 1
                       , writerTOCDepth         = 3
                       , writerReferenceDoc     = Nothing
-                      , writerLaTeXArgs        = []
+                      , writerPdfArgs          = []
                       , writerReferenceLocation = EndOfDocument
                       , writerSyntaxMap        = defaultSyntaxMap
                       }
diff --git a/src/Text/Pandoc/PDF.hs b/src/Text/Pandoc/PDF.hs
index 4e4c0b2c1..b2b7da54f 100644
--- a/src/Text/Pandoc/PDF.hs
+++ b/src/Text/Pandoc/PDF.hs
@@ -37,6 +37,7 @@ import qualified Control.Exception as E
 import Control.Monad (unless, when)
 import Control.Monad.Trans (MonadIO (..))
 import qualified Data.Text as T
+import qualified Data.Text.IO as TextIO
 import Data.Text (Text)
 import qualified Data.ByteString as BS
 import Data.ByteString.Lazy (ByteString)
@@ -78,8 +79,8 @@ changePathSeparators :: FilePath -> FilePath
 changePathSeparators = intercalate "/" . splitDirectories
 #endif
 
-makePDF :: String              -- ^ pdf creator (pdflatex, lualatex,
-                               -- xelatex, context, wkhtmltopdf, pdfroff)
+makePDF :: String              -- ^ pdf creator (pdflatex, lualatex, xelatex,
+                               -- wkhtmltopdf, weasyprint, prince, context, pdfroff)
         -> (WriterOptions -> Pandoc -> PandocIO Text)  -- ^ writer
         -> WriterOptions       -- ^ options
         -> Verbosity           -- ^ verbosity level
@@ -94,7 +95,7 @@ makePDF "wkhtmltopdf" writer opts verbosity _ doc@(Pandoc meta _) = do
                       _ -> []
   meta' <- metaToJSON opts (return . stringify) (return . stringify) meta
   let toArgs (f, mbd) = maybe [] (\d -> ['-':'-':f, d]) mbd
-  let args   = mathArgs ++
+  let args   = writerPdfArgs opts ++ mathArgs ++
                concatMap toArgs
                  [("page-size", getField "papersize" meta')
                  ,("title", getField "title" meta')
@@ -108,11 +109,19 @@ makePDF "wkhtmltopdf" writer opts verbosity _ doc@(Pandoc meta _) = do
                             (getField "margin-left" meta'))
                  ]
   source <- writer opts doc
-  liftIO $ html2pdf verbosity args source
+  liftIO $ html2pdf verbosity "wkhtmltopdf" args source
+makePDF "weasyprint" writer opts verbosity _ doc = do
+  let args = writerPdfArgs opts
+  source <- writer opts doc
+  liftIO $ html2pdf verbosity "weasyprint" args source
+makePDF "prince" writer opts verbosity _ doc = do
+  let args = writerPdfArgs opts
+  source <- writer opts doc
+  liftIO $ html2pdf verbosity "prince" args source
 makePDF "pdfroff" writer opts verbosity _mediabag doc = do
   source <- writer opts doc
   let args   = ["-ms", "-mpdfmark", "-e", "-t", "-k", "-KUTF-8", "-i",
-                "--no-toc-relocation"]
+                "--no-toc-relocation"] ++ writerPdfArgs opts
   liftIO $ ms2pdf verbosity args source
 makePDF program writer opts verbosity mediabag doc = do
   let withTemp = if takeBaseName program == "context"
@@ -124,7 +133,7 @@ makePDF program writer opts verbosity mediabag doc = do
     source <- runIOorExplode $ do
                 setVerbosity verbosity
                 writer opts doc'
-    let args   = writerLaTeXArgs opts
+    let args = writerPdfArgs opts
     case takeBaseName program of
        "context" -> context2pdf verbosity tmpdir source
        prog | prog `elem` ["pdflatex", "lualatex", "xelatex"]
@@ -212,7 +221,7 @@ tex2pdf' verbosity args tmpDir program source = do
                 case logmsg of
                      x | "! Package inputenc Error" `BC.isPrefixOf` x
                            && program /= "xelatex"
-                       -> "\nTry running pandoc with --latex-engine=xelatex."
+                       -> "\nTry running pandoc with --pdf-engine=xelatex."
                      _ -> ""
           return $ Left $ logmsg <> extramsg
        (ExitSuccess, Nothing)  -> return $ Left ""
@@ -347,32 +356,33 @@ ms2pdf verbosity args source = do
              ExitSuccess   -> Right out
 
 html2pdf  :: Verbosity    -- ^ Verbosity level
-          -> [String]     -- ^ Args to wkhtmltopdf
+          -> String       -- ^ Program (wkhtmltopdf, weasyprint or prince)
+          -> [String]     -- ^ Args to program
           -> Text         -- ^ HTML5 source
           -> IO (Either ByteString ByteString)
-html2pdf verbosity args source = do
-  file <- withTempFile "." "html2pdf.html" $ \fp _ -> return fp
+html2pdf verbosity program args source = do
   pdfFile <- withTempFile "." "html2pdf.pdf" $ \fp _ -> return fp
-  BS.writeFile file $ UTF8.fromText source
-  let programArgs = args ++ [file, pdfFile]
+  let pdfFileArgName = if program == "prince"
+                       then ["-o"]
+                       else []
+  let programArgs = args ++ ["-"] ++ pdfFileArgName ++ [pdfFile]
   env' <- getEnvironment
   when (verbosity >= INFO) $ do
     putStrLn "[makePDF] Command line:"
-    putStrLn $ "wkhtmltopdf" ++ " " ++ unwords (map show programArgs)
+    putStrLn $ program ++ " " ++ unwords (map show programArgs)
     putStr "\n"
     putStrLn "[makePDF] Environment:"
     mapM_ print env'
     putStr "\n"
-    putStrLn $ "[makePDF] Contents of " ++ file ++ ":"
-    BL.readFile file >>= BL.putStr
+    putStrLn $ "[makePDF] Contents of intermediate HTML:"
+    TextIO.putStr source
     putStr "\n"
   (exit, out) <- E.catch
-    (pipeProcess (Just env') "wkhtmltopdf" programArgs BL.empty)
+    (pipeProcess (Just env') program programArgs $ BL.fromStrict $ UTF8.fromText source)
     (\(e :: IOError) -> if isDoesNotExistError e
                            then E.throwIO $
-                                  PandocPDFProgramNotFoundError "wkhtml2pdf"
+                                  PandocPDFProgramNotFoundError program
                            else E.throwIO e)
-  removeFile file
   when (verbosity >= INFO) $ do
     BL.hPutStr stdout out
     putStr "\n"