Void.hs

{-# language DataKinds #-}
{-# language ScopedTypeVariables #-}
{-# language ViewPatterns #-}
{-# language RankNTypes #-}
{-# language KindSignatures #-}
{-# language PatternSynonyms #-}
{-# language MagicHash #-}
{-# language GADTSyntax #-}
{-# language UnliftedNewtypes #-}
{-# language UnboxedTuples #-}

module Data.Either.Void
  ( EitherVoid#(..)
  , pattern LeftVoid#
  , pattern RightVoid#
  ) where

import GHC.Exts

-- | Unboxed variant of @Either@. The thing possibly contained by @Just@
-- has a void runtime representation. Rather than using a sum, like the
-- more general @Either#@ does, this represents @Left@ with 0 and
-- @Right@ with 1.
--
-- It is recommended that the data constructor not be used directly.
-- Prefer the two pattern synonyms.
newtype EitherVoid# :: TYPE ('TupleRep '[]) -> TYPE ('TupleRep '[]) -> TYPE 'WordRep where
  EitherVoid# :: forall (a :: TYPE ('TupleRep '[])) (b :: TYPE ('TupleRep '[])). Word# -> EitherVoid# a b

{-# COMPLETE RightVoid#, LeftVoid# #-}

pattern RightVoid# :: b -> EitherVoid# a b
pattern RightVoid# a <- (helperRight -> (# 1##, a #)) where
  RightVoid# _ = EitherVoid# 1##

helperRight :: forall (a :: TYPE ('TupleRep '[])) (b :: TYPE ('TupleRep '[])).
  EitherVoid# a b -> (# Word#, b #)
{-# inline helperRight #-}
helperRight (EitherVoid# x) =
  (# x, (unsafeCoerce# :: (# #) -> b) (# #) #)

pattern LeftVoid# :: a -> EitherVoid# a b
pattern LeftVoid# a <- (helperLeft -> (# 0##, a #)) where
  LeftVoid# _ = EitherVoid# 0##

helperLeft :: forall (a :: TYPE ('TupleRep '[])) (b :: TYPE ('TupleRep '[])).
  EitherVoid# a b -> (# Word#, a #)
{-# inline helperLeft #-}
helperLeft (EitherVoid# x) =
  (# x, (unsafeCoerce# :: (# #) -> a) (# #) #)