content: {-| Scans page of Markdown looking for http links. When it finds them, it submits them
  to webcitation.org / https://secure.wikimedia.org/wikipedia/en/wiki/WebCite
  (It will also submit them to Alexa (the source for the Internet Archive), but Alexa says that
  its bots take weeks to visit and may not ever.)
  
  This module employs the archiver daemon <http://hackage.haskell.org/package/archiver> as a library; `cabal install archiver` will install it.
  
  Limitations:
  * Only parses Markdown, not ReST or any other format; this is because 'readMarkdown'
  is hardwired into it.
  * No rate limitation or choking; will fire off all requests as fast as possible.
    If pages have more than 20 external links or so, this may result in your IP being temporarily
    banned by WebCite. To avoid this, you can use WebArchiverBot.hs instead, which will parse & dump
    URLs into a file processed by the archiver daemon (which *is* rate-limited).
  
  By: Gwern Branwen; placed in the public domain -}
  
  module WebArchiver (plugin) where
  
  import Control.Concurrent (forkIO)
  import Network.URL.Archiver as A (checkArchive)
  import Network.Gitit.Interface (askUser, bottomUpM, liftIO, uEmail, Plugin(PreCommitTransform), Inline(Link))
  import Text.Pandoc (defaultParserState, readMarkdown)
  
  plugin :: Plugin
  plugin = PreCommitTransform archivePage
  
  -- archivePage :: String -> ReaderT PluginData (StateT Context IO) String
  archivePage x = do mbUser <- askUser
                     let email = case mbUser of
                          Nothing -> "nobody@mailinator.com"
                          Just u  -> uEmail u
                     let p = readMarkdown defaultParserState x
                     -- force evaluation and archiving side-effects
                     _p' <- liftIO $ bottomUpM (archiveLinks email) p
                     return x -- note: this is read-only - don't actually change page!
  
  archiveLinks :: String -> Inline -> IO Inline
  archiveLinks e x@(Link _ (uln, _)) = forkIO (A.checkArchive e uln) >> return x
  archiveLinks _ x                   = return x
  