    {-# LANGUAGE NoImplicitPrelude #-}
    {-
    Copyright (C) 2006-2018 John MacFarlane <jgm@berkeley.edu>

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
    -}

    {- |
       Module      : Text.Pandoc.Filter.Path
       Copyright   : Copyright (C) 2006-2018 John MacFarlane
       License     : GNU GPL, version 2 or above

       Maintainer  : John MacFarlane <jgm@berkeley@edu>
       Stability   : alpha
       Portability : portable

    Expand paths of filters, searching the data directory.
    -}
    module Text.Pandoc.Filter.Path
      ( expandFilterPath
      ) where

    import Prelude
    import Text.Pandoc.Class (PandocMonad, fileExists, getUserDataDir)
    import System.FilePath ((</>), isRelative)

      -- First we check to see if a filter is found.  If not, and if it's
      -- not an absolute path, we check to see whether it's in `userdir/filters`.
      -- If not, we leave it unchanged.
    expandFilterPath :: PandocMonad m => FilePath -> m FilePath
    expandFilterPath fp = do
      mbDatadir <- getUserDataDir
      fpExists <- fileExists fp
      if fpExists
         then return fp
         else case mbDatadir of
                   Just datadir | isRelative fp -> do
                     let filterPath = datadir </> "filters" </> fp
                     filterPathExists <- fileExists filterPath
                     if filterPathExists
                        then return filterPath
                        else return fp
                   _ -> return fp