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

   1 {-# LANGUAGE TypeApplications #-}
   2 -- | Error handling utilities.
   3 module Language.Symantic.Grammar.Error where
   4
   5 import Data.Proxy (Proxy(..))
   6
   7 -- * Class 'Gram_Error'
   8 -- | Symantics for handling errors at the semantic level (not the syntaxic one).
   9 class Gram_Error err g where
  10         catch :: g (Either err a) -> g a
  11
  12 -- * Class 'Inj_Error'
  13 class Inj_Error a b where
  14         inj_Error :: a -> b
  15 instance Inj_Error err e => Inj_Error err (Either e a) where
  16         inj_Error = Left . inj_Error
  17
  18 lift_Error ::
  19  forall e0 err e1 a.
  20  Inj_Error e0 e1 =>
  21  Inj_Error e1 err =>
  22  Proxy e1 -> Either e0 a -> Either err a
  23 lift_Error _e1 (Right a) = Right a
  24 lift_Error _e1 (Left e)  = Left $ inj_Error @e1 @err $ inj_Error @e0 @e1 e