{-# LANGUAGE TypeFamilies, ScopedTypeVariables #-}
module Language.Haskell.GhcMod.Error (
    GhcModError(..)
  , modifyError
  , modifyError'
  , tryFix
  , module Control.Monad.Error
  , module Exception
  ) where

import Control.Monad.Error (MonadError(..), Error(..))
import Exception

data GhcModError = GMENoMsg
                 -- ^ Unknown error
                 | GMEString String
                 -- ^ Some Error with a message. These are produced mostly by
                 -- 'fail' calls on GhcModT.
                 | GMECabalConfigure GhcModError
                 -- ^ Configuring a cabal project failed.
                 | GMECabalFlags GhcModError
                 -- ^ Retrieval of the cabal configuration flags failed.
                 | GMEProcess [String] GhcModError
                 -- ^ Launching an operating system process failed. The first
                 -- field is the command.
                   deriving (Eq,Show)

instance Error GhcModError where
    noMsg = GMENoMsg
    strMsg = GMEString

modifyError :: MonadError e m => (e -> e) -> m a -> m a
modifyError f action = action `catchError` \e -> throwError $ f e

infixr 0 `modifyError'`
modifyError' :: MonadError e m => m a -> (e -> e) -> m a
modifyError' = flip modifyError

tryFix :: MonadError e m => m a -> (e -> m ()) -> m a
tryFix action fix = do
  action `catchError` \e -> fix e >> action