    {-# OPTIONS_HADDOCK hide #-}
    -- #hide
    #if __GLASGOW_HASKELL__ >= 701
    {-# LANGUAGE Safe #-}
    #endif

    -- | This module contains functions for displaying
    --   HTML as a pretty tree.
    module Text.XHtml.Debug ( HtmlTree(..), treeHtml, treeColors, debugHtml ) where

    import Text.XHtml.Internals
    import Text.XHtml.Extras
    import Text.XHtml.Table
    import Text.XHtml.Strict.Elements
    import Text.XHtml.Strict.Attributes

    --
    -- * Tree Displaying Combinators
    --

    -- | The basic idea is you render your structure in the form
    -- of this tree, and then use treeHtml to turn it into a Html
    -- object with the structure explicit.
    data HtmlTree
          = HtmlLeaf Html
          | HtmlNode Html [HtmlTree] Html

    treeHtml :: [String] -> HtmlTree -> Html
    treeHtml colors h = table ! [
                        border 0,
                        cellpadding 0,
                        cellspacing 2] << treeHtml' colors h
         where
          manycolors = scanr (:) []

          treeHtmls :: [[String]] -> [HtmlTree] -> HtmlTable
          treeHtmls c ts = aboves (zipWith treeHtml' c ts)

          treeHtml' :: [String] -> HtmlTree -> HtmlTable
          treeHtml' _ (HtmlLeaf leaf) = cell
                                             (td ! [width "100%"]
                                                << bold
                                                   << leaf)
          treeHtml' (c:cs@(c2:_)) (HtmlNode hopen ts hclose) =
              if null ts && isNoHtml hclose
              then
                  cell hd
              else if null ts
              then
                  hd </> bar `beside` (td ! [bgcolor' c2] << spaceHtml)
                     </> tl
              else
                  hd </> (bar `beside` treeHtmls morecolors ts)
                     </> tl
            where
                  -- This stops a column of colors being the same
                  -- color as the immeduately outside nesting bar.
                  morecolors = filter ((/= c).head) (manycolors cs)
                  bar = td ! [bgcolor' c,width "10"] << spaceHtml
                  hd = td ! [bgcolor' c] << hopen
                  tl = td ! [bgcolor' c] << hclose
          treeHtml' _ _ = error "The imposible happens"

    instance HTML HtmlTree where
          toHtml x = treeHtml treeColors x

    -- type "length treeColors" to see how many colors are here.
    treeColors :: [String]
    treeColors = ["#88ccff","#ffffaa","#ffaaff","#ccffff"] ++ treeColors

    --
    -- * Html Debugging Combinators
    --

    -- | This uses the above tree rendering function, and displays the
    -- Html as a tree structure, allowing debugging of what is
    -- actually getting produced.
    debugHtml :: (HTML a) => a -> Html
    debugHtml obj = table ! [border 0] <<
                      ( th ! [bgcolor' "#008888"]
                         << underline'
                           << "Debugging Output"
                   </>  td << (toHtml (debug' (toHtml obj)))
                  )
      where

          debug' :: Html -> [HtmlTree]
          debug' (Html markups) = map debug markups

          debug :: HtmlElement -> HtmlTree
          debug (HtmlString str) = HtmlLeaf (spaceHtml +++
                                                  linesToHtml (lines str))
          debug (HtmlTag {
                  markupTag = tag',
                  markupContent = content',
                  markupAttrs  = attrs
                  }) =
                  case content' of
                    Html [] -> HtmlNode hd [] noHtml
                    Html xs -> HtmlNode hd (map debug xs) tl
            where
                  args = if null attrs
                         then ""
                         else "  " ++ unwords (map show attrs)
                  hd = xsmallFont << ("<" ++ tag' ++ args ++ ">")
                  tl = xsmallFont << ("</" ++ tag' ++ ">")

    bgcolor' :: String -> HtmlAttr
    bgcolor' c = thestyle ("background-color:" ++ c)

    underline' :: Html -> Html
    underline' = thespan ! [thestyle ("text-decoration:underline")]

    xsmallFont :: Html -> Html
    xsmallFont  = thespan ! [thestyle ("font-size:x-small")]