    {-# LANGUAGE DeriveDataTypeable, DeriveGeneric #-}
    {-
    Copyright (C) 2006-2015 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.Error
       Copyright   : Copyright (C) 2006-2015 John MacFarlane
       License     : GNU GPL, version 2 or above

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

    This module provides a standard way to deal with possible errors encounted
    during parsing.

    -}
    module Text.Pandoc.Error (PandocError(..), handleError) where

    import Text.Parsec.Error
    import Text.Parsec.Pos hiding (Line)
    import Text.Pandoc.Compat.Except
    import GHC.Generics (Generic)
    import Data.Generics (Typeable)
    import Control.Exception (Exception)

    type Input = String

    data PandocError = -- | Generic parse failure
                       ParseFailure String
                     -- | Error thrown by a Parsec parser
                     | ParsecError Input ParseError
                     deriving (Show, Typeable, Generic)

    instance Exception PandocError

    instance Error PandocError where
      strMsg = ParseFailure

    -- | An unsafe method to handle `PandocError`s.
    handleError :: Either PandocError a -> a
    handleError (Right r) = r
    handleError (Left err) =
      case err of
        ParseFailure string -> error string
        ParsecError input err' ->
            let errPos = errorPos err'
                errLine = sourceLine errPos
                errColumn = sourceColumn errPos
                theline = (lines input ++ [""]) !! (errLine - 1)
            in  error $ "\nError at " ++ show  err' ++ "\n" ++
                    theline ++ "\n" ++ replicate (errColumn - 1) ' ' ++
                    "^"