    -- #hide

    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Text.XHtml.internals
    -- Copyright   :  (c) Andy Gill, and the Oregon Graduate Institute of
    --                Science and Technology, 1999-2001,
    --                (c) Bjorn Bringert, 2004-2006
    -- License     :  BSD-style (see the file LICENSE)
    -- Maintainer  :  Bjorn Bringert <bjorn@bringert.net>
    -- Stability   :  experimental
    -- Portability :  portable
    --
    -- Internals of the XHTML combinator library.
    -----------------------------------------------------------------------------
    module Text.XHtml.Internals where

    import Data.Char
    import Data.Monoid

    infixr 2 +++  -- combining Html
    infixr 7 <<   -- nesting Html
    infixl 8 !    -- adding optional arguments

    --
    -- * Data types
    --

    -- | A important property of Html is that all strings inside the
    -- structure are already in Html friendly format.
    data HtmlElement
          = HtmlString String
            -- ^ ..just..plain..normal..text... but using &copy; and &amb;, etc.
          | HtmlTag {
                  markupTag      :: String,
                  markupAttrs    :: [HtmlAttr],
                  markupContent  :: Html
                  }
            -- ^ tag with internal markup

    -- | Attributes with name and value.
    data HtmlAttr = HtmlAttr String String

    newtype Html = Html { getHtmlElements :: [HtmlElement] }

    --
    -- * Classes
    --

    instance Show Html where
          showsPrec _ html = showString (renderHtmlFragment html)
          showList htmls   = foldr (.) id (map shows htmls)

    instance Show HtmlAttr where
          showsPrec _ (HtmlAttr str val) =
                  showString str .
                  showString "=" .
                  shows val

    instance Monoid Html where
        mempty = noHtml
        mappend = (+++)

    -- | HTML is the class of things that can be validly put
    -- inside an HTML tag. So this can be one or more 'Html' elements,
    -- or a 'String', for example.
    class HTML a where
          toHtml     :: a -> Html
          toHtmlFromList :: [a] -> Html

          toHtmlFromList xs = Html (concat [ x | (Html x) <- map toHtml xs])

    instance HTML Html where
          toHtml a    = a

    instance HTML Char where
          toHtml       a = toHtml [a]
          toHtmlFromList []  = Html []
          toHtmlFromList str = Html [HtmlString (stringToHtmlString str)]

    instance (HTML a) => HTML [a] where
          toHtml xs = toHtmlFromList xs

    instance HTML a => HTML (Maybe a) where
          toHtml = maybe noHtml toHtml

    class ADDATTRS a where
          (!) :: a -> [HtmlAttr] -> a

    instance (ADDATTRS b) => ADDATTRS (a -> b) where
          fn ! attr = \ arg -> fn arg ! attr

    instance ADDATTRS Html where
          (Html htmls) ! attr = Html (map addAttrs htmls)
            where
                  addAttrs (html@(HtmlTag { markupAttrs = markupAttrs }) )
                                  = html { markupAttrs = markupAttrs ++ attr }
                  addAttrs html = html

    --
    -- * Html primitives and basic combinators
    --

    -- | Put something inside an HTML element.
    (<<) :: (HTML a) =>
            (Html -> b) -- ^ Parent
         -> a -- ^ Child
         -> b
    fn << arg = fn (toHtml arg)

    concatHtml :: (HTML a) => [a] -> Html
    concatHtml as = Html (concat (map (getHtmlElements.toHtml) as))

    -- | Create a piece of HTML which is the concatenation
    --   of two things which can be made into HTML.
    (+++) :: (HTML a,HTML b) => a -> b -> Html
    a +++ b = Html (getHtmlElements (toHtml a) ++ getHtmlElements (toHtml b))

    -- | An empty piece of HTML.
    noHtml :: Html
    noHtml = Html []

    -- | Checks whether the given piece of HTML is empty.
    isNoHtml :: Html -> Bool
    isNoHtml (Html xs) = null xs

    -- | Constructs an element with a custom name.
    tag :: String -- ^ Element name
        -> Html -- ^ Element contents
        -> Html
    tag str       htmls = Html [
          HtmlTag {
                  markupTag = str,
                  markupAttrs = [],
                  markupContent = htmls }]

    -- | Constructs an element with a custom name, and
    --   without any children.
    itag :: String -> Html
    itag str = tag str noHtml

    emptyAttr :: String -> HtmlAttr
    emptyAttr s = HtmlAttr s s

    intAttr :: String -> Int -> HtmlAttr
    intAttr s i = HtmlAttr s (show i)

    strAttr :: String -> String -> HtmlAttr
    strAttr s t = HtmlAttr s (stringToHtmlString t)

    htmlAttr :: String -> Html -> HtmlAttr
    htmlAttr s t = HtmlAttr s (show t)

    {-
    foldHtml :: (String -> [HtmlAttr] -> [a] -> a)
          -> (String -> a)
          -> Html
          -> a
    foldHtml f g (HtmlTag str attr fmls)
          = f str attr (map (foldHtml f g) fmls)
    foldHtml f g (HtmlString  str)
          = g str

    -}

    -- | Processing Strings into Html friendly things.
    stringToHtmlString :: String -> String
    stringToHtmlString = concatMap fixChar
        where
          fixChar '<' = "&lt;"
          fixChar '>' = "&gt;"
          fixChar '&' = "&amp;"
          fixChar '"' = "&quot;"
          fixChar c | ord c < 0x80 = [c]
          fixChar c = "&#" ++ show (ord c) ++ ";"

    -- | This is not processed for special chars.
    -- use stringToHtml or lineToHtml instead, for user strings,
    -- because they understand special chars, like @'<'@.
    primHtml :: String -> Html
    primHtml x | null x    = Html []
               | otherwise = Html [HtmlString x]

    --
    -- * Html Rendering
    --

    mkHtml :: HTML html => html -> Html
    mkHtml = (tag "html" ! [strAttr "xmlns" "http://www.w3.org/1999/xhtml"] <<)

    -- | Output the HTML without adding newlines or spaces within the markup.
    --   This should be the most time and space efficient way to
    --   render HTML, though the ouput is quite unreadable.
    showHtmlInternal :: HTML html =>
                        String -- ^ DOCTYPE declaration
                     -> html -> String
    showHtmlInternal docType theHtml =
        docType ++ showHtmlFragment (mkHtml theHtml)

    -- | Outputs indented HTML. Because space matters in
    --   HTML, the output is quite messy.
    renderHtmlInternal :: HTML html =>
                          String  -- ^ DOCTYPE declaration
                       -> html -> String
    renderHtmlInternal docType theHtml =
          docType ++ "\n" ++ renderHtmlFragment (mkHtml theHtml) ++ "\n"

    -- | Outputs indented HTML, with indentation inside elements.
    --   This can change the meaning of the HTML document, and
    --   is mostly useful for debugging the HTML output.
    --   The implementation is inefficient, and you are normally
    --   better off using 'showHtml' or 'renderHtml'.
    prettyHtmlInternal :: HTML html =>
                          String -- ^ DOCTYPE declaration
                       -> html -> String
    prettyHtmlInternal docType theHtml =
        docType ++ "\n" ++ prettyHtmlFragment (mkHtml theHtml)

    -- | Render a piece of HTML without adding a DOCTYPE declaration
    --   or root element. Does not add any extra whitespace.
    showHtmlFragment :: HTML html => html -> String
    showHtmlFragment h =
        (foldr (.) id $ map showHtml' $ getHtmlElements $ toHtml h) ""

    -- | Render a piece of indented HTML without adding a DOCTYPE declaration
    --   or root element. Only adds whitespace where it does not change
    --   the meaning of the document.
    renderHtmlFragment :: HTML html => html -> String
    renderHtmlFragment h =
        (foldr (.) id $ map (renderHtml' 0) $ getHtmlElements $ toHtml h) ""

    -- | Render a piece of indented HTML without adding a DOCTYPE declaration
    --   or a root element.
    --   The indentation is done inside elements.
    --   This can change the meaning of the HTML document, and
    --   is mostly useful for debugging the HTML output.
    --   The implementation is inefficient, and you are normally
    --   better off using 'showHtmlFragment' or 'renderHtmlFragment'.
    prettyHtmlFragment :: HTML html => html -> String
    prettyHtmlFragment =
        unlines . concat . map prettyHtml' . getHtmlElements . toHtml

    -- | Show a single HTML element, without adding whitespace.
    showHtml' :: HtmlElement -> ShowS
    showHtml' (HtmlString str) = (++) str
    showHtml'(HtmlTag { markupTag = name,
                        markupContent = html,
                        markupAttrs = markupAttrs })
        = if isNoHtml html && elem name validHtmlITags
          then renderTag True name markupAttrs ""
          else (renderTag False name markupAttrs ""
                . foldr (.) id (map showHtml' (getHtmlElements html))
                . renderEndTag name "")

    renderHtml' :: Int -> HtmlElement -> ShowS
    renderHtml' _ (HtmlString str) = (++) str
    renderHtml' n (HtmlTag
                  { markupTag = name,
                    markupContent = html,
                    markupAttrs = markupAttrs })
          = if isNoHtml html && elem name validHtmlITags
            then renderTag True name markupAttrs (nl n)
            else (renderTag False name markupAttrs (nl n)
                 . foldr (.) id (map (renderHtml' (n+2)) (getHtmlElements html))
                 . renderEndTag name (nl n))
        where
          nl n = "\n" ++ replicate (n `div` 8) '\t'
                 ++ replicate (n `mod` 8) ' '

    prettyHtml' :: HtmlElement -> [String]
    prettyHtml' (HtmlString str) = [str]
    prettyHtml' (HtmlTag
                  { markupTag = name,
                    markupContent = html,
                    markupAttrs = markupAttrs })
          = if isNoHtml html && elem name validHtmlITags
            then
             [rmNL (renderTag True name markupAttrs "" "")]
            else
             [rmNL (renderTag False name markupAttrs "" "")] ++
              shift (concat (map prettyHtml' (getHtmlElements html))) ++
             [rmNL (renderEndTag name "" "")]
      where
          shift = map (\x -> "   " ++ x)
          rmNL = filter (/= '\n')

    -- | Show a start tag
    renderTag :: Bool       -- ^ 'True' if the empty tag shorthand should be used
    	  -> String     -- ^ Tag name
    	  -> [HtmlAttr] -- ^ Attributes
    	  -> String     -- ^ Whitespace to add after attributes
    	  -> ShowS
    renderTag empty name attrs nl r
          = "<" ++ name ++ showAttrs attrs ++ nl ++ close ++ r
      where
          close = if empty then " />" else ">"

          showAttrs attrs = concat [' ':showPair attr | attr <- attrs ]

          showPair :: HtmlAttr -> String
          showPair (HtmlAttr tag val)
                  = tag ++ "=\"" ++ val  ++ "\""

    -- | Show an end tag
    renderEndTag :: String -- ^ Tag name
    	     -> String -- ^ Whitespace to add after tag name
    	     -> ShowS
    renderEndTag name nl r = "</" ++ name ++ nl ++ ">" ++ r

    -- | The names of all elements which can represented using the empty tag
    --   short-hand.
    validHtmlITags :: [String]
    validHtmlITags = [
    		  "area",
    		  "base",
    		  "basefont",
    		  "br",
                      "col",
                      "frame",
    		  "hr",
    		  "img",
    		  "input",
                      "isindex",
                      "link",
    		  "meta",
    		  "param"
    		 ]