content: {-# LANGUAGE CPP #-}
  {-# LANGUAGE OverloadedStrings #-}
  {-# LANGUAGE TypeApplications #-}
  {-
  Copyright (C) 2009 John MacFarlane <jgm@berkeley.edu>,
  Anton van Straaten <anton@appsolutions.com>
  
  This program is free software; you can redistribute it and/or modify
  it under the terms of the GNU General Public License as published by
  the Free Software Foundation; either version 2 of the License, or
  (at your option) any later version.
  
  This program is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  GNU General Public License for more details.
  
  You should have received a copy of the GNU General Public License
  along with this program; if not, write to the Free Software
  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  -}
  
  {- Functions for content conversion.
  -}
  
  module Network.Gitit.ContentTransformer
    (
    -- * ContentTransformer runners
      runPageTransformer
    , runFileTransformer
    -- * Gitit responders
    , showRawPage
    , showFileAsText
    , showPage
    , showHighlightedSource
    , showFile
    , preview
    , applyPreCommitPlugins
    -- * Cache support for transformers
    , cacheHtml
    , cachedHtml
    -- * Content retrieval combinators
    , rawContents
    -- * Response-generating combinators
    , textResponse
    , mimeFileResponse
    , mimeResponse
    , applyWikiTemplate
    -- * Content-type transformation combinators
    , pageToWikiPandoc
    , pageToPandoc
    , pandocToHtml
    , highlightSource
    -- * Content or context augmentation combinators
    , applyPageTransforms
    , wikiDivify
    , addPageTitleToPandoc
    , addMathSupport
    , addScripts
    -- * ContentTransformer context API
    , getFileName
    , getPageName
    , getLayout
    , getParams
    , getCacheable
    -- * Pandoc and wiki content conversion support
    , inlinesToURL
    , inlinesToString
    )
  where
  
  import qualified Control.Exception as E
  import Control.Monad
  import Control.Monad.State
  import Control.Monad.Reader (ask)
  import Control.Monad.Except (throwError)
  import Data.Foldable (traverse_)
  import Data.List (stripPrefix)
  import Data.Maybe (isNothing, mapMaybe)
  import Network.Gitit.Cache (lookupCache, cacheContents)
  import Network.Gitit.Framework hiding (uriPath)
  import Network.Gitit.Layout
  import Network.Gitit.Page (stringToPage)
  import Network.Gitit.Server
  import Network.Gitit.State
  import Network.Gitit.Types
  import Network.Gitit.Util (getPageTypeDefaultExtensions)
  import Network.HTTP (urlDecode)
  import Network.URI (isUnescapedInURI)
  import Network.URL (encString)
  import System.FilePath
  import qualified Text.Pandoc.Builder as B
  import Text.HTML.SanitizeXSS (sanitizeBalance)
  import Skylighting hiding (Context)
  import Text.Pandoc hiding (MathML, WebTeX, MathJax)
  import URI.ByteString (Query(Query), URIRef(uriPath), laxURIParserOptions,
                         parseURI, uriQuery)
  import qualified Data.Text as T
  import qualified Data.ByteString as S (concat)
  import qualified Data.ByteString.Char8 as SC (pack, unpack)
  import qualified Data.ByteString.Lazy as L (toChunks, fromChunks)
  import qualified Data.FileStore as FS
  import qualified Text.Pandoc as Pandoc
  import Data.String (IsString(fromString))
  import Text.Blaze.Html.Renderer.String as Blaze ( renderHtml )
  import Text.Blaze.Html5 hiding (u, s, contents, source, html, title, map)
  import Text.Blaze.Html5.Attributes hiding (id)
  import qualified Text.Blaze.Html5 as Html5
  import qualified Text.Blaze.Html5.Attributes as Html5.Attr
  
  --
  -- ContentTransformer runners
  --
  
  runPageTransformer :: ToMessage a
                 => ContentTransformer a
                 -> GititServerPart a
  runPageTransformer xform = withData $ \params -> do
    page <- getPage
    cfg <- getConfig
    evalStateT xform  Context{ ctxFile = pathForPage page (defaultExtension cfg)
                             , ctxLayout = defaultPageLayout{
                                               pgPageName = page
                                             , pgTitle = page
                                             , pgPrintable = pPrintable params
                                             , pgMessages = pMessages params
                                             , pgRevision = pRevision params
                                             , pgLinkToFeed = useFeed cfg }
                             , ctxCacheable = True
                             , ctxTOC = tableOfContents cfg
                             , ctxBirdTracks = showLHSBirdTracks cfg
                             , ctxCategories = []
                             , ctxMeta = [] }
  
  runFileTransformer :: ToMessage a
                 => ContentTransformer a
                 -> GititServerPart a
  runFileTransformer xform = withData $ \params -> do
    page <- getPage
    cfg <- getConfig
    evalStateT xform  Context{ ctxFile = id page
                             , ctxLayout = defaultPageLayout{
                                               pgPageName = page
                                             , pgTitle = page
                                             , pgPrintable = pPrintable params
                                             , pgMessages = pMessages params
                                             , pgRevision = pRevision params
                                             , pgLinkToFeed = useFeed cfg }
                             , ctxCacheable = True
                             , ctxTOC = tableOfContents cfg
                             , ctxBirdTracks = showLHSBirdTracks cfg
                             , ctxCategories = []
                             , ctxMeta = [] }
  
  -- | Converts a @ContentTransformer@ into a @GititServerPart@;
  -- specialized to wiki pages.
  -- runPageTransformer :: ToMessage a
  --                    => ContentTransformer a
  --                    -> GititServerPart a
  -- runPageTransformer = runTransformer pathForPage
  
  -- | Converts a @ContentTransformer@ into a @GititServerPart@;
  -- specialized to non-pages.
  -- runFileTransformer :: ToMessage a
  --                    => ContentTransformer a
  --                    -> GititServerPart a
  -- runFileTransformer = runTransformer id
  
  --
  -- Gitit responders
  --
  
  -- | Responds with raw page source.
  showRawPage :: Handler
  showRawPage = runPageTransformer rawTextResponse
  
  -- | Responds with raw source (for non-pages such as source
  -- code files).
  showFileAsText :: Handler
  showFileAsText = runFileTransformer rawTextResponse
  
  -- | Responds with rendered wiki page.
  showPage :: Handler
  showPage = runPageTransformer htmlViaPandoc
  
  -- | Responds with highlighted source code.
  showHighlightedSource :: Handler
  showHighlightedSource = runFileTransformer highlightRawSource
  
  -- | Responds with non-highlighted source code.
  showFile :: Handler
  showFile = runFileTransformer (rawContents >>= mimeFileResponse)
  
  -- | Responds with rendered page derived from form data.
  preview :: Handler
  preview = runPageTransformer $
            liftM (filter (/= '\r') . pRaw) getParams >>=
            contentsToPage >>=
            pageToWikiPandoc >>=
            pandocToHtml >>=
            return . toResponse . renderHtml
  
  -- | Applies pre-commit plugins to raw page source, possibly
  -- modifying it.
  applyPreCommitPlugins :: String -> GititServerPart String
  applyPreCommitPlugins = runPageTransformer . applyPreCommitTransforms
  
  --
  -- Top level, composed transformers
  --
  
  -- | Responds with raw source.
  rawTextResponse :: ContentTransformer Response
  rawTextResponse = rawContents >>= textResponse
  
  -- | Responds with a wiki page. Uses the cache when
  -- possible and caches the rendered page when appropriate.
  htmlViaPandoc :: ContentTransformer Response
  htmlViaPandoc = cachedHtml `mplus`
                    (rawContents >>=
                     maybe mzero return >>=
                     contentsToPage >>=
                     handleRedirects >>=
                     either return
                       (pageToWikiPandoc >=>
                        addMathSupport >=>
                        pandocToHtml >=>
                        wikiDivify >=>
                        applyWikiTemplate >=>
                        cacheHtml))
  
  -- | Responds with highlighted source code in a wiki
  -- page template.  Uses the cache when possible and
  -- caches the rendered page when appropriate.
  highlightRawSource :: ContentTransformer Response
  highlightRawSource =
    cachedHtml `mplus`
      (updateLayout (\l -> l { pgTabs = [ViewTab,HistoryTab] }) >>
       rawContents >>=
       highlightSource >>=
       applyWikiTemplate >>=
       cacheHtml)
  
  --
  -- Cache support for transformers
  --
  
  -- | Caches a response (actually just the response body) on disk,
  -- unless the context indicates that the page is not cacheable.
  cacheHtml :: Response -> ContentTransformer Response
  cacheHtml resp' = do
    params <- getParams
    file <- getFileName
    cacheable <- getCacheable
    cfg <- lift getConfig
    when (useCache cfg && cacheable && isNothing (pRevision params) && not (pPrintable params)) $
      lift $ cacheContents file $ S.concat $ L.toChunks $ rsBody resp'
    return resp'
  
  -- | Returns cached page if available, otherwise mzero.
  cachedHtml :: ContentTransformer Response
  cachedHtml = do
    file <- getFileName
    params <- getParams
    cfg <- lift getConfig
    if useCache cfg && not (pPrintable params) && isNothing (pRevision params)
       then do mbCached <- lift $ lookupCache file
               let emptyResponse = setContentType "text/html; charset=utf-8" . toResponse $ ()
               maybe mzero (\(_modtime, contents) -> lift . ok $ emptyResponse{rsBody = L.fromChunks [contents]}) mbCached
       else mzero
  
  --
  -- Content retrieval combinators
  --
  
  -- | Returns raw file contents.
  rawContents :: ContentTransformer (Maybe String)
  rawContents = do
    params <- getParams
    file <- getFileName
    fs <- lift getFileStore
    let rev = pRevision params
    liftIO $ E.catch (liftM Just $ FS.retrieve fs file rev)
                 (\e -> if e == FS.NotFound then return Nothing else E.throwIO e)
  
  --
  -- Response-generating combinators
  --
  
  -- | Converts raw contents to a text/plain response.
  textResponse :: Maybe String -> ContentTransformer Response
  textResponse Nothing  = mzero  -- fail quietly if file not found
  textResponse (Just c) = mimeResponse c "text/plain; charset=utf-8"
  
  -- | Converts raw contents to a response that is appropriate with
  -- a mime type derived from the page's extension.
  mimeFileResponse :: Maybe String -> ContentTransformer Response
  mimeFileResponse Nothing = error "Unable to retrieve file contents."
  mimeFileResponse (Just c) =
    mimeResponse c =<< lift . getMimeTypeForExtension . takeExtension =<< getFileName
  
  mimeResponse :: Monad m
               => String        -- ^ Raw contents for response body
               -> String        -- ^ Mime type
               -> m Response
  mimeResponse c mimeType =
    return . setContentType mimeType . toResponse $ c
  
  -- | Adds the sidebar, page tabs, and other elements of the wiki page
  -- layout to the raw content.
  applyWikiTemplate :: Html -> ContentTransformer Response
  applyWikiTemplate c = do
    Context { ctxLayout = layout } <- get
    lift $ formattedPage layout c
  
  --
  -- Content-type transformation combinators
  --
  
  -- | Converts Page to Pandoc, applies page transforms, and adds page
  -- title.
  pageToWikiPandoc :: Page -> ContentTransformer Pandoc
  pageToWikiPandoc page' =
    pageToWikiPandoc' page' >>= addPageTitleToPandoc (pageTitle page')
  
  pageToWikiPandoc' :: Page -> ContentTransformer Pandoc
  pageToWikiPandoc' = applyPreParseTransforms >=>
                       pageToPandoc >=> applyPageTransforms
  
  -- | Converts source text to Pandoc using default page type.
  pageToPandoc :: Page -> ContentTransformer Pandoc
  pageToPandoc page' = do
    modifyContext $ \ctx -> ctx{ ctxTOC = pageTOC page'
                               , ctxCategories = pageCategories page'
                               , ctxMeta = pageMeta page' }
    either (liftIO . E.throwIO) return $ readerFor (pageFormat page') (pageLHS page') (pageText page')
  
  data WasRedirect = WasRedirect | WasNoRedirect
  
  -- | Detects if the page is a redirect page and handles accordingly. The exact
  -- behaviour is as follows:
  --
  -- If the page is /not/ a redirect page (the most common case), then check the
  -- referer to see if the client came to this page as a result of a redirect
  -- from another page. If so, then add a notice to the messages to notify the
  -- user that they were redirected from another page, and provide a link back
  -- to the original page, with an extra parameter to disable redirection
  -- (e.g., to allow the original page to be edited).
  --
  -- If the page /is/ a redirect page, then check the query string for the
  -- @redirect@ parameter. This can modify the behaviour of the redirect as
  -- follows:
  --
  -- 1. If the @redirect@ parameter is unset, then check the referer to see if
  --    client came to this page as a result of a redirect from another page. If
  --    so, then do not redirect, and add a notice to the messages explaining
  --    that this page is a redirect page, that would have redirected to the
  --    destination given in the metadata (and provide a link thereto), but this
  --    was stopped because a double-redirect was detected. This is a simple way
  --    to prevent cyclical redirects and other abuses enabled by redirects.
  --    redirect to the same page. If the client did /not/ come to this page as
  --    a result of a redirect, then redirect back to the same page, except with
  --    the redirect parameter set to @\"yes\"@.
  --
  -- 2. If the @redirect@ parameter is set to \"yes\", then redirect to the
  --    destination specified in the metadata. This uses a client-side (meta
  --    refresh + javascript backup) redirect to make sure the referer is set to
  --    this URL.
  --
  -- 3. If the @redirect@ parameter is set to \"no\", then do not redirect, but
  --    add a notice to the messages that this page /would/ have redirected to
  --    the destination given in the metadata had it not been disabled, and
  --    provide a link to the destination given in the metadata. This behaviour
  --    is the @revision@ parameter is present in the query string.
  handleRedirects :: Page -> ContentTransformer (Either Response Page)
  handleRedirects page = case lookup "redirect" (pageMeta page) of
      Nothing -> isn'tRedirect
      Just destination -> isRedirect destination
    where
      addMessage message = modifyContext $ \context -> context
          { ctxLayout = (ctxLayout context)
              { pgMessages = pgMessages (ctxLayout context) ++ [renderHtml message]
              }
          }
      redirectedFrom source = do
          (url, html) <- processSource source
          return $ mconcat
              [ "Redirected from ",
                 a ! href (url WasNoRedirect) ! title "Go to original page" $ html
              ]
      doubleRedirect source destination = do
          (url, html) <- processSource source
          (url', html') <- processDestination destination
          return $ mconcat
              [ "This page normally redirects to "
              , a ! href (fromString $ url' WasRedirect) ! title "Continue to destination" $ html'
              , ", but as you were already redirected from "
              , a ! href (url WasNoRedirect) ! title "Go to original page" $ html
              , ", this was stopped to prevent a double-redirect."
              ]
      cancelledRedirect destination = do
          (url', html') <- processDestination destination
          return $ mconcat
              [ "This page redirects to "
              , a ! href (fromString $ url' WasRedirect) ! title "Continue to destination" $ html'
              ]
      processSource source = do
          base' <- getWikiBase
          let url redir = fromString @AttributeValue $
                base' ++ urlForPage source ++ case redir of
                  WasNoRedirect -> "?redirect=no"
                  WasRedirect -> ""
          let html = fromString @Html source
          return (url, html)
      processDestination destination = do
          base' <- getWikiBase
          let (page', fragment) = break (== '#') destination
          let url redir = concat
               [ base'
               , urlForPage page'
               , fragment
  
               ] ++ case redir of
                  WasNoRedirect -> "?redirect=no"
                  WasRedirect -> ""
          let html = fromString @Html page'
          return (url, html)
      getSource = do
          cfg <- lift getConfig
          base' <- getWikiBase
          request <- askRq
          return $ do
              referer <- getHeader "referer" request
              uri <- case parseURI laxURIParserOptions referer of
                  Left _ -> Nothing
                  Right uri -> Just uri
              let Query params = uriQuery uri
              redirect' <- lookup (SC.pack "redirect") params
              guard $ redirect' == SC.pack "yes"
              path' <- stripPrefix (base' ++ "/") (SC.unpack (uriPath uri))
              let path'' = if null path' then frontPage cfg else urlDecode path'
              guard $ isPage path''
              return path''
      withBody = setContentType "text/html; charset=utf-8" . toResponse
      isn'tRedirect = do
          getSource >>= traverse_ (redirectedFrom >=> addMessage)
          return (Right page)
      isRedirect destination = do
          params <- getParams
          case maybe (pRedirect params) (\_ -> Just False) (pRevision params) of
               Nothing -> do
                  source <- getSource
                  case source of
                       Just source' -> do
                          doubleRedirect source' destination >>= addMessage
                          return (Right page)
                       Nothing -> fmap Left $ do
                          base' <- getWikiBase
                          let url' = concat
                               [ base'
                               , urlForPage (pageName page)
                               , "?redirect=yes"
                               ]
                          lift $ seeOther url' $ withBody $ renderHtml $ docTypeHtml $ mconcat
                              [ Html5.head $ Html5.title "307 Redirect"
                              , Html5.body $ p $ mconcat [
                                  "You are being",
                                  a ! href (fromString url') $ "redirected."
                                ]
                              ]
               Just True -> fmap Left $ do
                  (url', html') <- processDestination destination
                  lift $ ok $ withBody $ renderHtml $ docTypeHtml $ mconcat
                      [ Html5.head $ mconcat
                          [ Html5.title $ "Redirecting to" <> html'
                          , meta ! httpEquiv "refresh" ! content (fromString $ "0; url=" <> url' WasRedirect)
                          , script ! type_ "text/javascript" $ fromString $ "window.location=\"" <> url' WasRedirect <> "\""
                          ],
                        Html5.body $ p $ mconcat
                          [ "Redirecting to "
                          , a ! href (fromString $ url' WasRedirect) $ html'
                          ]
                      ]
               Just False -> do
                  cancelledRedirect destination >>= addMessage
                  return (Right page)
  
  -- | Converts contents of page file to Page object.
  contentsToPage :: String -> ContentTransformer Page
  contentsToPage s = do
    cfg <- lift getConfig
    pn <- getPageName
    return $ stringToPage cfg pn s
  
  -- | Converts pandoc document to HTML.
  pandocToHtml :: Pandoc -> ContentTransformer Html
  pandocToHtml pandocContents = do
    toc <- liftM ctxTOC get
    bird <- liftM ctxBirdTracks get
    cfg <- lift getConfig
    let tpl = "$if(toc)$<div id=\"TOC\">\n$toc$\n</div>\n$endif$\n$body$"
    compiledTemplate <- liftIO $ runIOorExplode $ do
      res <- runWithDefaultPartials $ compileTemplate "toc" tpl
      case res of
        Right t -> return t
        Left e  -> throwError $ PandocTemplateError $ T.pack e
    return $ preEscapedToHtml @T.Text .
             (if xssSanitize cfg then sanitizeBalance else id) $
             either E.throw id . runPure $ writeHtml5String def{
                          writerTemplate = Just compiledTemplate
                        , writerHTMLMathMethod =
                              case mathMethod cfg of
                                   MathML -> Pandoc.MathML
                                   WebTeX u -> Pandoc.WebTeX $ T.pack u
                                   MathJax u -> Pandoc.MathJax $ T.pack u
                                   RawTeX -> Pandoc.PlainMath
                        , writerTableOfContents = toc
                        , writerHighlightStyle = Just pygments
                        , writerExtensions = if bird
                                                then enableExtension Ext_literate_haskell
                                                     $ writerExtensions def
                                                else writerExtensions def
                        -- note: javascript obfuscation gives problems on preview
                        , writerEmailObfuscation = ReferenceObfuscation
                        } pandocContents
  
  -- | Returns highlighted source code.
  highlightSource :: Maybe String -> ContentTransformer Html
  highlightSource Nothing = mzero
  highlightSource (Just source) = do
    file <- getFileName
    let formatOpts = defaultFormatOpts { numberLines = True, lineAnchors = True }
    case syntaxesByFilename defaultSyntaxMap file of
          []    -> mzero
          (l:_) -> case tokenize TokenizerConfig{
                                syntaxMap = defaultSyntaxMap
                              , traceOutput = False} l
                          $ T.pack $ filter (/='\r') source of
                      Left e ->  fail (show e)
                      Right r -> return $ formatHtmlBlock formatOpts r
  
  --
  -- Plugin combinators
  --
  
  getPageTransforms :: ContentTransformer [Pandoc -> PluginM Pandoc]
  getPageTransforms = liftM (mapMaybe pageTransform) $ queryGititState plugins
    where pageTransform (PageTransform x) = Just x
          pageTransform _                 = Nothing
  
  getPreParseTransforms :: ContentTransformer [String -> PluginM String]
  getPreParseTransforms = liftM (mapMaybe preParseTransform) $
                            queryGititState plugins
    where preParseTransform (PreParseTransform x) = Just x
          preParseTransform _                     = Nothing
  
  getPreCommitTransforms :: ContentTransformer [String -> PluginM String]
  getPreCommitTransforms = liftM (mapMaybe preCommitTransform) $
                            queryGititState plugins
    where preCommitTransform (PreCommitTransform x) = Just x
          preCommitTransform _                      = Nothing
  
  -- | @applyTransform a t@ applies the transform @t@ to input @a@.
  applyTransform :: a -> (a -> PluginM a) -> ContentTransformer a
  applyTransform inp transform = do
    context <- get
    conf <- lift getConfig
    user <- lift getLoggedInUser
    fs <- lift getFileStore
    req <- lift askRq
    let pluginData = PluginData{ pluginConfig = conf
                               , pluginUser = user
                               , pluginRequest = req
                               , pluginFileStore = fs }
    (result', context') <- liftIO $ runPluginM (transform inp) pluginData context
    put context'
    return result'
  
  -- | Applies all the page transform plugins to a Pandoc document.
  applyPageTransforms :: Pandoc -> ContentTransformer Pandoc
  applyPageTransforms c = do
    xforms <- getPageTransforms
    foldM applyTransform c (wikiLinksTransform : xforms)
  
  -- | Applies all the pre-parse transform plugins to a Page object.
  applyPreParseTransforms :: Page -> ContentTransformer Page
  applyPreParseTransforms page' = getPreParseTransforms >>= foldM applyTransform (pageText page') >>=
                                  (\t -> return page'{ pageText = t })
  
  -- | Applies all the pre-commit transform plugins to a raw string.
  applyPreCommitTransforms :: String -> ContentTransformer String
  applyPreCommitTransforms c = getPreCommitTransforms >>= foldM applyTransform c
  
  --
  -- Content or context augmentation combinators
  --
  
  -- | Puts rendered page content into a wikipage div, adding
  -- categories.
  wikiDivify :: Html -> ContentTransformer Html
  wikiDivify c = do
    categories <- liftM ctxCategories get
    base' <- lift getWikiBase
    let categoryLink ctg = li (a ! href (fromString $ base' ++ "/_category/" ++ ctg) $ fromString ctg)
    let htmlCategories = if null categories
                            then mempty
                            else Html5.div ! Html5.Attr.id "categoryList" $ ul $ foldMap categoryLink categories
    return $ Html5.div ! Html5.Attr.id "wikipage" $ c <> htmlCategories
  
  -- | Adds page title to a Pandoc document.
  addPageTitleToPandoc :: String -> Pandoc -> ContentTransformer Pandoc
  addPageTitleToPandoc title' (Pandoc _ blocks) = do
    updateLayout $ \layout -> layout{ pgTitle = title' }
    return $ if null title'
                then Pandoc nullMeta blocks
                else Pandoc
                      (B.setMeta "title" (B.str (T.pack title')) nullMeta)
                      blocks
  
  -- | Adds javascript links for math support.
  addMathSupport :: a -> ContentTransformer a
  addMathSupport c = do
    conf <- lift getConfig
    updateLayout $ \l ->
      case mathMethod conf of
           MathML       -> addScripts l ["MathMLinHTML.js"]
           WebTeX _     -> l
           MathJax u    -> addScripts l [u]
           RawTeX       -> l
    return c
  
  -- | Adds javascripts to page layout.
  addScripts :: PageLayout -> [String] -> PageLayout
  addScripts layout scriptPaths =
    layout{ pgScripts = scriptPaths ++ pgScripts layout }
  
  --
  -- ContentTransformer context API
  --
  
  getParams :: ContentTransformer Params
  getParams = lift (withData return)
  
  getFileName :: ContentTransformer FilePath
  getFileName = liftM ctxFile get
  
  getPageName :: ContentTransformer String
  getPageName = liftM (pgPageName . ctxLayout) get
  
  getLayout :: ContentTransformer PageLayout
  getLayout = liftM ctxLayout get
  
  getCacheable :: ContentTransformer Bool
  getCacheable = liftM ctxCacheable get
  
  -- | Updates the layout with the result of applying f to the current layout
  updateLayout :: (PageLayout -> PageLayout) -> ContentTransformer ()
  updateLayout f = do
    ctx <- get
    let l = ctxLayout ctx
    put ctx { ctxLayout = f l }
  
  --
  -- Pandoc and wiki content conversion support
  --
  
  readerFor :: PageType -> Bool -> String -> Either PandocError Pandoc
  readerFor pt lhs =
    let defExts = getDefaultExtensions $ T.toLower $ T.pack $ show pt
        defPS = def{ readerExtensions = defExts
                                        <> extensionsFromList [Ext_emoji]
                                        <> getPageTypeDefaultExtensions pt lhs
                                        <> readerExtensions def }
    in runPure . (case pt of
         RST        -> readRST defPS
         Markdown   -> readMarkdown defPS
         CommonMark -> readCommonMark defPS
         LaTeX      -> readLaTeX defPS
         HTML       -> readHtml defPS
         Textile    -> readTextile defPS
         Org        -> readOrg defPS
         DocBook    -> readDocBook defPS
         MediaWiki  -> readMediaWiki defPS) . T.pack
  
  wikiLinksTransform :: Pandoc -> PluginM Pandoc
  wikiLinksTransform pandoc
    = do cfg <- liftM pluginConfig ask -- Can't use askConfig from Interface due to circular dependencies.
         return (bottomUp (convertWikiLinks cfg) pandoc)
  
  -- | Convert links with no URL to wikilinks.
  convertWikiLinks :: Config -> Inline -> Inline
  convertWikiLinks cfg (Link attr ref ("", "")) | useAbsoluteUrls cfg =
    Link attr ref (T.pack ("/" </> baseUrl cfg </> inlinesToURL ref),
                   "Go to wiki page")
  convertWikiLinks _cfg (Link attr ref ("", "")) =
    Link attr ref (T.pack (inlinesToURL ref), "Go to wiki page")
  convertWikiLinks _cfg x = x
  
  -- | Derives a URL from a list of Pandoc Inline elements.
  inlinesToURL :: [Inline] -> String
  inlinesToURL = encString False isUnescapedInURI . inlinesToString
  
  -- | Convert a list of inlines into a string.
  inlinesToString :: [Inline] -> String
  inlinesToString = T.unpack . mconcat . map go
    where go :: Inline -> T.Text
          go x = case x of
                 Str s                   -> s
                 Emph xs                 -> mconcat $ map go xs
                 Strong xs               -> mconcat $ map go xs
                 Strikeout xs            -> mconcat $ map go xs
                 Superscript xs          -> mconcat $ map go xs
                 Subscript xs            -> mconcat $ map go xs
                 SmallCaps xs            -> mconcat $ map go xs
  #if MIN_VERSION_pandoc(2,10,0)
                 Underline xs            -> mconcat $ map go xs
  #endif
                 Quoted DoubleQuote xs   -> "\"" <> mconcat (map go xs) <> "\""
                 Quoted SingleQuote xs   -> "'" <> mconcat (map go xs) <> "'"
                 Cite _ xs               -> mconcat $ map go xs
                 Code _ s                -> s
                 Space                   -> " "
                 SoftBreak               -> " "
                 LineBreak               -> " "
                 Math DisplayMath s      -> "$$" <> s <> "$$"
                 Math InlineMath s       -> "$" <> s <> "$"
                 RawInline (Format "tex") s -> s
                 RawInline _ _           -> ""
                 Link _ xs _             -> mconcat $ map go xs
                 Image _ xs _            -> mconcat $ map go xs
                 Note _                  -> ""
                 Span _ xs               -> mconcat $ map go xs
  
  