control-dsl-0.2.0.0: src/Control/Dsl/Empty.hs

{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}

module Control.Dsl.Empty where

import Control.Dsl.PolyCont
import Data.Void
import qualified Control.Applicative
import Prelude hiding ((>>), (>>=), return, fail)

data Empty r a where
  Empty :: Empty r Void

instance {-# OVERLAPS #-} Control.Applicative.Alternative m => PolyCont Empty (m a) Void where
  runPolyCont Empty _ = Control.Applicative.empty

{- | Return an empty @a@, similar to 'Control.Alternative.empty'.

This 'empty' function aims to be used as the last statement of a @do@ block.
-}
empty :: PolyCont Empty a Void => a
empty = runPolyCont Empty absurd