    module Language.Haskell.Lexer.Layout (layoutPre,PosToken) where

    import Language.Haskell.Lexer.Tokens
    import Language.Haskell.Lexer.Position

    type PosToken = (Token,(Pos,String))

    -- | This is an implementation of Haskell layout, as specified in
    -- section 9.3 of the revised Haskell 98 report.
    -- This preprocessor inserts the extra \<n\> and {n} tokens.
    layoutPre :: [PosToken] -> [PosToken]
    layoutPre = indent . open

    open :: [PosToken] -> [PosToken]
    open = open1

    {-+
    If the first lexeme of a module is not { or module, then it is preceded
    by {n} where n is the indentation of the lexeme.
    -}
    open1 :: [PosToken] -> [PosToken]
    open1 (t1@(Reservedid,(_,"module")):ts) = t1:open2 ts
    open1 (t1@(Special,(_,"{")):ts)         = t1:open2 ts
    open1 ts@((_,(p,_)):_)                  = (Open (column p),(p,"")):open2 ts
    open1 []                                = []

    {-+
    If a let, where, do, or of keyword is not followed by the lexeme {,
    the token {n} is inserted after the keyword, where n is the indentation of
    the next lexeme if there is one, or 0 if the end of file has been reached.
    -}
    open2 :: [PosToken] -> [PosToken]
    open2 (t1:ts1) | isLtoken t1 =
        case ts1 of
          t2@(_,(p,_)):ts2 ->
            if notLBrace t2
            then t1:(Open (column p),(p,"")):open2 ts1
            else t1:t2:open2 ts2
          [] -> t1:(Open 0,(fst (snd t1),"")):[]
      where
        isLtoken (Reservedid,(_,s)) = s `elem` ["let","where","do","of"]
        isLtoken _ = False

        notLBrace (Special,(_,"{")) = False
        notLBrace _ = True
    open2 (t:ts) = t:open2 ts
    open2 [] = []

    {-+
    (This is from the original Haskell 98 report.)
    The first token on each line (not including tokens already annotated) is
    preceeded by &lt;n&gt;, where n is the indentation of the token.
    -}
    indent :: [PosToken] -> [PosToken]
    indent (t1@(Open _,(p,_)):ts) = t1:indent2 (line p) ts
    indent (t1@(_,(p,_)):ts)    = (Indent (column p),(p,"")):t1:indent2 (line p) ts
    indent [] = []

    indent2 :: Int -> [PosToken] -> [PosToken]
    indent2 r (t1@(_,(p,_)):ts) | line p==r = t1:indent2 r ts
    indent2 _ ts = indent ts