    -----------------------------------------------------------------------------
    -- |
    -- Module      :  Distribution.Simple.PreProcess.Unlit
    -- Copyright   :  ...
    --
    -- Maintainer  :  cabal-devel@haskell.org
    -- Portability :  portable
    --
    -- Remove the \"literal\" markups from a Haskell source file, including
    -- \"@>@\", \"@\\begin{code}@\", \"@\\end{code}@\", and \"@#@\"

    -- This version is interesting because instead of striping comment lines, it
    -- turns them into "-- " style comments. This allows using haddock markup
    -- in literate scripts without having to use "> --" prefix.

    module Distribution.Simple.PreProcess.Unlit (unlit,plain) where

    import Data.Char
    import Data.List

    data Classified = BirdTrack String | Blank String | Ordinary String
                    | Line !Int String | CPP String
                    | BeginCode | EndCode
                    -- output only:
                    | Error String | Comment String

    -- | No unliteration.
    plain :: String -> String -> String
    plain _ hs = hs

    classify :: String -> Classified
    classify ('>':s) = BirdTrack s
    classify ('#':s) = case tokens s of
                         (line:file:_) | all isDigit line
                                      && length file >= 2
                                      && head file == '"'
                                      && last file == '"'
                                    -> Line (read line) (tail (init file))
                         _          -> CPP s
      where tokens = unfoldr $ \str -> case lex str of
                                       (t@(_:_), str'):_ -> Just (t, str')
                                       _                 -> Nothing
    classify ('\\':s)
      | s `isPrefixOf` "begin{code}" = BeginCode
      | s `isPrefixOf` "end{code}"   = EndCode
    classify s | all isSpace s       = Blank s
    classify s                       = Ordinary s

    -- So the weird exception for comment indenting is to make things work with
    -- haddock, see classifyAndCheckForBirdTracks below.
    unclassify :: Bool -> Classified -> String
    unclassify _     (BirdTrack s) = ' ':s
    unclassify _     (Blank s)     = s
    unclassify _     (Ordinary s)  = s
    unclassify _     (Line n file) = "# " ++ show n ++ " " ++ show file
    unclassify _     (CPP s)       = '#':s
    unclassify True  (Comment "")  = "  --"
    unclassify True  (Comment s)   = "  -- " ++ s
    unclassify False (Comment "")  = "--"
    unclassify False (Comment s)   = "-- " ++ s
    unclassify _     _             = internalError

    -- | 'unlit' takes a filename (for error reports), and transforms the
    --   given string, to eliminate the literate comments from the program text.
    unlit :: FilePath -> String -> Either String String
    unlit file input =
      let (usesBirdTracks, classified) = classifyAndCheckForBirdTracks
                                       . inlines
                                       $ input
       in either (Left . unlines . map (unclassify usesBirdTracks))
                  Right
        . checkErrors
        . reclassify
        $ classified

      where
        -- So haddock requires comments and code to align, since it treats comments
        -- as following the layout rule. This is a pain for us since bird track
        -- style literate code typically gets indented by two since ">" is replaced
        -- by " " and people usually use one additional space of indent ie
        -- "> then the code". On the other hand we cannot just go and indent all
        -- the comments by two since that does not work for latex style literate
        -- code. So the hacky solution we use here is that if we see any bird track
        -- style code then we'll indent all comments by two, otherwise by none.
        -- Of course this will not work for mixed latex/bird track .lhs files but
        -- nobody does that, it's silly and specifically recommended against in the
        -- H98 unlit spec.
        --
        classifyAndCheckForBirdTracks =
          flip mapAccumL False $ \seenBirdTrack line ->
            let classification = classify line
             in (seenBirdTrack || isBirdTrack classification, classification)

        isBirdTrack (BirdTrack _) = True
        isBirdTrack _             = False

        checkErrors ls = case [ e | Error e <- ls ] of
          []          -> Left  ls
          (message:_) -> Right (f ++ ":" ++ show n ++ ": " ++ message)
            where (f, n) = errorPos file 1 ls
        errorPos f n []              = (f, n)
        errorPos f n (Error _:_)     = (f, n)
        errorPos _ _ (Line n' f':ls) = errorPos f' n' ls
        errorPos f n (_         :ls) = errorPos f  (n+1) ls

    -- Here we model a state machine, with each state represented by
    -- a local function. We only have four states (well, five,
    -- if you count the error state), but the rules
    -- to transition between then are not so simple.
    -- Would it be simpler to have more states?
    --
    -- Each state represents the type of line that was last read
    -- i.e. are we in a comment section, or a latex-code section,
    -- or a bird-code section, etc?
    reclassify :: [Classified] -> [Classified]
    reclassify = blank -- begin in blank state
      where
        latex []               = []
        latex (EndCode    :ls) = Blank "" : comment ls
        latex (BeginCode  :_ ) = [Error "\\begin{code} in code section"]
        latex (BirdTrack l:ls) = Ordinary ('>':l) : latex ls
        latex (          l:ls) = l : latex ls

        blank []               = []
        blank (EndCode    :_ ) = [Error "\\end{code} without \\begin{code}"]
        blank (BeginCode  :ls) = Blank ""    : latex ls
        blank (BirdTrack l:ls) = BirdTrack l : bird ls
        blank (Ordinary  l:ls) = Comment   l : comment ls
        blank (          l:ls) =           l : blank ls

        bird []              = []
        bird (EndCode   :_ ) = [Error "\\end{code} without \\begin{code}"]
        bird (BeginCode :ls) = Blank "" : latex ls
        bird (Blank l   :ls) = Blank l  : blank ls
        bird (Ordinary _:_ ) = [Error "program line before comment line"]
        bird (         l:ls) = l : bird ls

        comment []               = []
        comment (EndCode    :_ ) = [Error "\\end{code} without \\begin{code}"]
        comment (BeginCode  :ls) = Blank "" : latex ls
        comment (CPP l      :ls) = CPP l : comment ls
        comment (BirdTrack _:_ ) = [Error "comment line before program line"]
        -- a blank line and another ordinary line following a comment
        -- will be treated as continuing the comment. Otherwise it's
        -- then end of the comment, with a blank line.
        comment (Blank     l:ls@(Ordinary  _:_)) = Comment l : comment ls
        comment (Blank     l:ls) = Blank l   : blank ls
        comment (Line n f   :ls) = Line n f  : comment ls
        comment (Ordinary  l:ls) = Comment l : comment ls
        comment (Comment   _: _) = internalError
        comment (Error     _: _) = internalError

    -- Re-implementation of 'lines', for better efficiency (but decreased laziness).
    -- Also, importantly, accepts non-standard DOS and Mac line ending characters.
    inlines :: String -> [String]
    inlines xs = lines' xs id
      where
      lines' []             acc = [acc []]
      lines' ('\^M':'\n':s) acc = acc [] : lines' s id    -- DOS
      lines' ('\^M':s)      acc = acc [] : lines' s id    -- MacOS
      lines' ('\n':s)       acc = acc [] : lines' s id    -- Unix
      lines' (c:s)          acc = lines' s (acc . (c:))

    internalError :: a
    internalError = error "unlit: internal error"