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diff --git a/haddock-api/src/Haddock/Backends/Xhtml/Themes.hs b/haddock-api/src/Haddock/Backends/Xhtml/Themes.hs
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+-----------------------------------------------------------------------------
+-- |
+-- Module      :  Haddock.Backends.Html.Themes
+-- Copyright   :  (c) Mark Lentczner 2010
+-- License     :  BSD-like
+--
+-- Maintainer  :  haddock@projects.haskell.org
+-- Stability   :  experimental
+-- Portability :  portable
+-----------------------------------------------------------------------------
+module Haddock.Backends.Xhtml.Themes (
+    Themes,
+    getThemes,
+
+    cssFiles, styleSheet
+    )
+    where
+
+import Haddock.Options
+
+import Control.Applicative
+import Control.Monad (liftM)
+import Data.Char (toLower)
+import Data.Either (lefts, rights)
+import Data.List (nub)
+import Data.Maybe (isJust, listToMaybe)
+
+import System.Directory
+import System.FilePath
+import Text.XHtml hiding ( name, title, p, quote, (</>) )
+import qualified Text.XHtml as XHtml
+
+
+--------------------------------------------------------------------------------
+-- * CSS Themes
+--------------------------------------------------------------------------------
+
+data Theme = Theme {
+  themeName :: String,
+  themeHref :: String,
+  themeFiles :: [FilePath]
+  }
+
+type Themes = [Theme]
+
+type PossibleTheme = Either String Theme
+type PossibleThemes = Either String Themes
+
+
+-- | Find a theme by name (case insensitive match)
+findTheme :: String -> Themes -> Maybe Theme
+findTheme s = listToMaybe . filter ((== ls).lower.themeName)
+  where lower = map toLower
+        ls = lower s
+
+
+-- | Standard theme used by default
+standardTheme :: FilePath -> IO PossibleThemes
+standardTheme libDir = liftM (liftEither (take 1)) (defaultThemes libDir)
+
+
+-- | Default themes that are part of Haddock; added with --default-themes
+-- The first theme in this list is considered the standard theme.
+-- Themes are "discovered" by scanning the html sub-dir of the libDir,
+-- and looking for directories with the extension .theme or .std-theme.
+-- The later is, obviously, the standard theme.
+defaultThemes :: FilePath -> IO PossibleThemes
+defaultThemes libDir = do
+  themeDirs <- getDirectoryItems (libDir </> "html")
+  themes <- mapM directoryTheme $ discoverThemes themeDirs
+  return $ sequenceEither themes
+  where
+    discoverThemes paths =
+      filterExt ".std-theme" paths ++ filterExt ".theme" paths
+    filterExt ext = filter ((== ext).takeExtension)
+
+
+-- | Build a theme from a single .css file
+singleFileTheme :: FilePath -> IO PossibleTheme
+singleFileTheme path =
+  if isCssFilePath path
+      then retRight $ Theme name file [path]
+      else errMessage "File extension isn't .css" path
+  where
+    name = takeBaseName path
+    file = takeFileName path
+
+
+-- | Build a theme from a directory
+directoryTheme :: FilePath -> IO PossibleTheme
+directoryTheme path = do
+  items <- getDirectoryItems path
+  case filter isCssFilePath items of
+    [cf] -> retRight $ Theme (takeBaseName path) (takeFileName cf) items
+    [] -> errMessage "No .css file in theme directory" path
+    _ -> errMessage "More than one .css file in theme directory" path
+
+
+-- | Check if we have a built in theme
+doesBuiltInExist :: IO PossibleThemes -> String -> IO Bool
+doesBuiltInExist pts s = fmap (either (const False) test) pts
+  where test = isJust . findTheme s
+
+
+-- | Find a built in theme
+builtInTheme :: IO PossibleThemes -> String -> IO PossibleTheme
+builtInTheme pts s = either Left fetch <$> pts
+  where fetch = maybe (Left ("Unknown theme: " ++ s)) Right . findTheme s
+
+
+--------------------------------------------------------------------------------
+-- * CSS Theme Arguments
+--------------------------------------------------------------------------------
+
+-- | Process input flags for CSS Theme arguments
+getThemes :: FilePath -> [Flag] -> IO PossibleThemes
+getThemes libDir flags =
+  liftM concatEither (mapM themeFlag flags) >>= someTheme
+  where
+    themeFlag :: Flag -> IO (Either String Themes)
+    themeFlag (Flag_CSS path) = (liftM . liftEither) (:[]) (theme path)
+    themeFlag (Flag_BuiltInThemes) = builtIns
+    themeFlag _ = retRight []
+
+    theme :: FilePath -> IO PossibleTheme
+    theme path = pick path
+      [(doesFileExist,              singleFileTheme),
+       (doesDirectoryExist,         directoryTheme),
+       (doesBuiltInExist builtIns,  builtInTheme builtIns)]
+      "Theme not found"
+
+    pick :: FilePath
+      -> [(FilePath -> IO Bool, FilePath -> IO PossibleTheme)] -> String
+      -> IO PossibleTheme
+    pick path [] msg = errMessage msg path
+    pick path ((test,build):opts) msg = do
+      pass <- test path
+      if pass then build path else pick path opts msg
+
+
+    someTheme :: Either String Themes -> IO (Either String Themes)
+    someTheme (Right []) = standardTheme libDir
+    someTheme est = return est
+
+    builtIns = defaultThemes libDir
+
+
+errMessage :: String -> FilePath -> IO (Either String a)
+errMessage msg path = return (Left msg')
+  where msg' = "Error: " ++ msg ++ ": \"" ++ path ++ "\"\n"
+
+
+retRight :: a -> IO (Either String a)
+retRight = return . Right
+
+
+--------------------------------------------------------------------------------
+-- * File Utilities
+--------------------------------------------------------------------------------
+
+
+getDirectoryItems :: FilePath -> IO [FilePath]
+getDirectoryItems path =
+  map (combine path) . filter notDot <$> getDirectoryContents path
+  where notDot s = s /= "." && s /= ".."
+
+
+isCssFilePath :: FilePath -> Bool
+isCssFilePath path = takeExtension path == ".css"
+
+
+--------------------------------------------------------------------------------
+-- * Style Sheet Utilities
+--------------------------------------------------------------------------------
+
+cssFiles :: Themes -> [String]
+cssFiles ts = nub $ concatMap themeFiles ts
+
+
+styleSheet :: Themes -> Html
+styleSheet ts = toHtml $ zipWith mkLink rels ts
+  where
+    rels = "stylesheet" : repeat "alternate stylesheet"
+    mkLink aRel t =
+      thelink
+        ! [ href (themeHref t),  rel aRel, thetype "text/css",
+            XHtml.title (themeName t)
+          ]
+        << noHtml
+
+--------------------------------------------------------------------------------
+-- * Either Utilities
+--------------------------------------------------------------------------------
+
+-- These three routines are here because Haddock does not have access to the
+-- Control.Monad.Error module which supplies the Functor and Monad instances
+-- for Either String.
+
+sequenceEither :: [Either a b] -> Either a [b]
+sequenceEither es = maybe (Right $ rights es) Left (listToMaybe (lefts es))
+
+
+liftEither :: (b -> c) -> Either a b -> Either a c
+liftEither f = either Left (Right . f)
+
+
+concatEither :: [Either a [b]] -> Either a [b]
+concatEither = liftEither concat . sequenceEither
+