symantic-grammar/Language/Symantic/Grammar/Error.hs

   1 {-# LANGUAGE TypeApplications #-}
   2 module Language.Symantic.Grammar.Error where
   3
   4 import Data.Proxy (Proxy)
   5
   6 -- * Class 'Inj_Error'
   7 class Inj_Error a b where
   8         inj_Error :: a -> b
   9 instance Inj_Error err e => Inj_Error err (Either e a) where
  10         inj_Error = Left . inj_Error
  11
  12 liftError ::
  13  forall e0 err e1 a.
  14  Inj_Error e0 e1 =>
  15  Inj_Error e1 err =>
  16  Proxy e1 -> Either e0 a -> Either err a
  17 liftError _e1 (Right a) = Right a
  18 liftError _e1 (Left e)  = Left $ inj_Error @e1 @err $ inj_Error @e0 @e1 e