Issue warning for duplicate header identifiers.

As noted in the previous commit, an autogenerated identifier
may still coincide with an explicit identifier that is given
for a header later in the document, or with an identifier on
a div, span, link, or image. This commit adds a warning
in this case, so users can supply an explicit identifier.

* Added `DuplicateIdentifier` to LogMessage.
* Modified HTML, Org, MediaWiki readers so their custom
  state type is an instance of HasLogMessages.  This is necessary
  for `registerHeader` to issue warnings.

See #1745.

1 files changed, 11 insertions, 5 deletions

diff --git a/src/Text/Pandoc/Readers/HTML.hs b/src/Text/Pandoc/Readers/HTML.hs
index 7e7d505ac..0af369469 100644
--- a/src/Text/Pandoc/Readers/HTML.hs
+++ b/src/Text/Pandoc/Readers/HTML.hs
@@ -83,14 +83,15 @@ readHtml opts inp = do
         blocks <- (fixPlains False) . mconcat <$> manyTill block eof
         meta <- stateMeta . parserState <$> getState
         bs' <- replaceNotes (B.toList blocks)
+        reportLogMessages
         return $ Pandoc meta bs'
       getError (errorMessages -> ms) = case ms of
                                          []    -> ""
                                          (m:_) -> messageString m
   result <- flip runReaderT def $
-         runParserT parseDoc
-         (HTMLState def{ stateOptions = opts } [] Nothing Set.empty M.empty)
-         "source" tags
+       runParserT parseDoc
+       (HTMLState def{ stateOptions = opts } [] Nothing Set.empty M.empty [])
+       "source" tags
   case result of
     Right doc -> return doc
     Left  err -> throwError $ PandocParseError $ getError err
@@ -110,7 +111,8 @@ data HTMLState =
      noteTable   :: [(String, Blocks)],
      baseHref    :: Maybe URI,
      identifiers :: Set.Set String,
-     headerMap   :: M.Map Inlines String
+     headerMap   :: M.Map Inlines String,
+     logMessages :: [LogMessage]
   }
 
 data HTMLLocal = HTMLLocal { quoteContext :: QuoteContext
@@ -376,7 +378,7 @@ ignore raw = do
   -- raw can be null for tags like <!DOCTYPE>; see paRawTag
   -- in this case we don't want a warning:
   unless (null raw) $
-    report $ SkippedContent raw pos
+    logMessage $ SkippedContent raw pos
   return mempty
 
 pHtmlBlock :: PandocMonad m => String -> TagParser m String
@@ -1092,6 +1094,10 @@ instance HasHeaderMap HTMLState where
   extractHeaderMap = headerMap
   updateHeaderMap  f s = s{ headerMap = f (headerMap s) }
 
+instance HasLogMessages HTMLState where
+  addLogMessage m s = s{ logMessages = m : logMessages s }
+  getLogMessages = reverse . logMessages
+
 -- This signature should be more general
 -- MonadReader HTMLLocal m => HasQuoteContext st m
 instance PandocMonad m => HasQuoteContext HTMLState (ReaderT HTMLLocal m) where