Either.hs

-- Copyright © 2018 chessai
-- BSD-3-Clause license (see LICENSE for full text)
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE UnboxedSums #-}

{- | Unpacked Either using unboxed sums for zero-allocation branching.

Internal module — public API boundaries convert to standard Either.
Uses GHC's @UnboxedSums@ extension so the Left\/Right tag lives on
the stack, not the heap.
-}
module Proto.Internal.Either (
  Either (Either, Left, Right),
  either,
  fromBaseEither,
  toBaseEither,
) where

import Data.Either qualified as BaseEither
import Prelude ()


data Either a b = Either (# a | b #)


pattern Left :: a -> Either a b
pattern Left a = Either (# a | #)


pattern Right :: b -> Either a b
pattern Right b = Either (# | b #)


{-# COMPLETE Left, Right #-}


either :: (a -> c) -> (b -> c) -> Either a b -> c
either fa fb (Either x) = case x of
  (# a | #) -> fa a
  (# | b #) -> fb b
{-# INLINE either #-}


fromBaseEither :: BaseEither.Either a b -> Either a b
fromBaseEither (BaseEither.Left a) = Left a
fromBaseEither (BaseEither.Right b) = Right b
{-# INLINE fromBaseEither #-}


toBaseEither :: Either a b -> BaseEither.Either a b
toBaseEither = either BaseEither.Left BaseEither.Right
{-# INLINE toBaseEither #-}