    {-# LANGUAGE RecordWildCards #-}
    module System.Console.CmdArgs.Explicit.ExpandArgsAt(expandArgsAt) where

    import System.FilePath

    -- | Expand @\@@ directives in a list of arguments, usually obtained from 'getArgs'.
    --   As an example, given the file @test.txt@ with the lines @hello@ and @world@:
    --
    -- > expandArgsAt ["@test.txt","!"] == ["hello","world","!"]
    --
    --   Any @\@@ directives in the files will be recursively expanded (raising an error
    --   if there is infinite recursion).
    --
    --   To supress @\@@ expansion, pass any @\@@ arguments after @--@.
    expandArgsAt :: [String] -> IO [String]
    expandArgsAt args = do
            ebefore <- mapM (f [] ".") before
            return $ concat ebefore ++ after
        where
            (before,after) = break (== "--") args

            f seen dir ('@':x)
                | x `elem` seen = error $ unlines $
                    "System.Console.CmdArgs.Explicit.expandArgsAt, recursion in @ directives:" :
                    map ("  "++) (reverse $ x:seen)
                | length seen > 15 = error $ unlines $
                    "System.Console.CmdArgs.Explicit.expandArgsAt, over 15 @ directives deep:" :
                    map ("  "++) (reverse seen)
                | otherwise = do
                    src <- readFile $ dir </> x
                    fmap concat $ mapM (f (x:seen) (takeDirectory x)) $ lines src
            f _ _ x = return [x]