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.

1 files changed, 23 insertions, 0 deletions

diff --git a/src/Text/Pandoc/Error.hs b/src/Text/Pandoc/Error.hs
index ae66162b3..113ab9d6e 100644
--- a/src/Text/Pandoc/Error.hs
+++ b/src/Text/Pandoc/Error.hs
@@ -54,6 +54,9 @@ data PandocError = PandocIOError String IOError
                  | PandocMacroLoop String
                  | PandocUTF8DecodingError String Int Word8
                  | PandocIpynbDecodingError String
+                 | PandocUnknownReaderError String
+                 | PandocUnknownWriterError String
+                 | PandocUnsupportedExtensionError String String
                  deriving (Show, Typeable, Generic)
 
 instance Exception PandocError
@@ -112,6 +115,26 @@ handleError (Left e) =
       "The input must be a UTF-8 encoded text."
     PandocIpynbDecodingError w -> err 93 $
       "ipynb decoding error: " ++ w
+    PandocUnknownReaderError r -> err 21 $
+      "Unknown input format " ++ r ++
+      case r of
+        "doc" -> "\nPandoc can convert from DOCX, but not from DOC." ++
+                 "\nTry using Word to save your DOC file as DOCX," ++
+                 " and convert that with pandoc."
+        "pdf" -> "\nPandoc can convert to PDF, but not from PDF."
+        _     -> ""
+    PandocUnknownWriterError w -> err 22 $
+       "Unknown output format " ++ w ++
+       case w of
+         "pdf" -> "To create a pdf using pandoc, use" ++
+                  " -t latex|beamer|context|ms|html5" ++
+                 "\nand specify an output file with " ++
+                 ".pdf extension (-o filename.pdf)."
+         "doc" -> "\nPandoc can convert to DOCX, but not from DOC."
+         _     -> ""
+    PandocUnsupportedExtensionError ext f -> err 23 $
+      "The extension " ++ ext ++ " is not supported " ++
+      "for " ++ f
 
 err :: Int -> String -> IO a
 err exitCode msg = do