Raise error on unsupported extensions. Closes #4338.

+ An error is now raised if you try to specify (enable or
  disable) an extension that does not affect the given
  format, e.g. `docx+pipe_tables`.

+ The `--list-extensions[=FORMAT]` option now lists only
  extensions that affect the given FORMAT.

+ Text.Pandoc.Error: Add constructors `PandocUnknownReaderError`,
  `PandocUnknownWriterError`, `PandocUnsupportedExtensionError`.
  [API change]

+ Text.Pandoc.Extensions now exports `getAllExtensions`,
  which returns the extensions that affect a given format
  (whether enabled by default or not). [API change]

+ Text.Pandoc.Extensions: change type of `parseFormatSpec`
  from `Either ParseError (String, Extensions -> Extensions)`
  to `Either ParseError (String, [Extension], [Extension])`
  [API change].

+ Text.Pandoc.Readers: change type of `getReader` so it returns
  a value in the PandocMonad instance rather than an Either
  [API change].  Exceptions for unknown formats and unsupported
  extensions are now raised by this function and need not be handled by
  the calling function.

+ Text.Pandoc.Writers: change type of `getWriter` so it returns
  a value in the PandocMonad instance rather than an Either
  [API change].  Exceptions for unknown formats and unsupported
  extensions are now raised by this function and need not be handled by
  the calling function.

2 files changed, 23 insertions, 21 deletions

diff --git a/src/Text/Pandoc/App/CommandLineOptions.hs b/src/Text/Pandoc/App/CommandLineOptions.hs
index cffe69eca..abb73ec92 100644
--- a/src/Text/Pandoc/App/CommandLineOptions.hs
+++ b/src/Text/Pandoc/App/CommandLineOptions.hs
@@ -768,12 +768,25 @@ options =
     , Option "" ["list-extensions"]
                  (OptArg
                   (\arg _ -> do
-                     let exts = getDefaultExtensions (fromMaybe "markdown" arg)
-                     let showExt x = (if extensionEnabled x exts
-                                         then '+'
-                                         else '-') : drop 4 (show x)
+                     let extList :: [Extension]
+                         extList = [minBound..maxBound]
+                     let allExts =
+                           case arg of
+                             Nothing  -> extensionsFromList extList
+                             Just fmt -> getAllExtensions fmt
+                     let defExts =
+                           case arg of
+                             Nothing   -> getDefaultExtensions
+                                           "markdown"
+                             Just fmt  -> getDefaultExtensions fmt
+                     let showExt x =
+                           (if extensionEnabled x defExts
+                               then '+'
+                               else if extensionEnabled x allExts
+                                       then '-'
+                                       else ' ') : drop 4 (show x)
                      mapM_ (UTF8.hPutStrLn stdout . showExt)
-                               ([minBound..maxBound] :: [Extension])
+                       [ex | ex <- extList, extensionEnabled ex allExts]
                      exitSuccess )
                   "FORMAT")
                  ""
diff --git a/src/Text/Pandoc/App/OutputSettings.hs b/src/Text/Pandoc/App/OutputSettings.hs
index 744f4591f..cfb6f7ec2 100644
--- a/src/Text/Pandoc/App/OutputSettings.hs
+++ b/src/Text/Pandoc/App/OutputSettings.hs
@@ -85,18 +85,12 @@ optToOutputSettings opts = do
                   then writerName
                   else map toLower $ baseWriterName writerName
 
-  (writer, writerExts) <-
+  (writer :: Writer PandocIO, writerExts) <-
             if ".lua" `isSuffixOf` format
                then return (TextWriter
                        (\o d -> writeCustom writerName o d)
                                :: Writer PandocIO, mempty)
-               else case getWriter (map toLower writerName) of
-                         Left e  -> throwError $ PandocAppError $
-                           if format == "pdf"
-                              then e ++ "\n" ++ pdfIsNoWriterErrorMsg
-                              else e
-                         Right (w, es) -> return (w :: Writer PandocIO, es)
-
+               else getWriter (map toLower writerName)
 
   let standalone = optStandalone opts || not (isTextFormat format) || pdfOutput
 
@@ -249,13 +243,6 @@ optToOutputSettings opts = do
 baseWriterName :: String -> String
 baseWriterName = takeWhile (\c -> c /= '+' && c /= '-')
 
-pdfIsNoWriterErrorMsg :: String
-pdfIsNoWriterErrorMsg =
-  "To create a pdf using pandoc, use " ++
-  "-t latex|beamer|context|ms|html5" ++
-  "\nand specify an output file with " ++
-  ".pdf extension (-o filename.pdf)."
-
 pdfWriterAndProg :: Maybe String              -- ^ user-specified writer name
                  -> Maybe String              -- ^ user-specified pdf-engine
                  -> IO (String, Maybe String) -- ^ IO (writerName, maybePdfEngineProg)
@@ -263,6 +250,8 @@ pdfWriterAndProg mWriter mEngine = do
   let panErr msg = liftIO $ E.throwIO $ PandocAppError msg
   case go mWriter mEngine of
       Right (writ, prog) -> return (writ, Just prog)
+      Left "pdf writer"  -> liftIO $ E.throwIO $
+                               PandocUnknownWriterError "pdf"
       Left err           -> panErr err
     where
       go Nothing Nothing       = Right ("latex", "pdflatex")
@@ -279,7 +268,7 @@ pdfWriterAndProg mWriter mEngine = do
                                  []      -> Left $
                                    "pdf-engine " ++ eng ++ " not known"
 
-      engineForWriter "pdf" = Left pdfIsNoWriterErrorMsg
+      engineForWriter "pdf" = Left "pdf writer"
       engineForWriter w = case [e |  (f,e) <- engines, f == baseWriterName w] of
                                 eng : _ -> Right eng
                                 []      -> Left $