LastToFinshSTM without `Semigroup` constraint.

Synchronisation.hs

module Synchronisation where

import           Control.Concurrent
import           Control.Concurrent.STM.TMVar
import           Control.Monad.STM


newtype LastToFinishSTM a = LastToFinishSTM { runLastToFinish :: STM a }

instance Semigroup (LastToFinishSTM a) where
    LastToFinishSTM ma <> LastToFinishSTM mb = LastToFinishSTM $ do
      x <- (Left <$> ma) `orElse` (Right <$> mb)
      case x of
        Left _  -> mb
        Right _ -> ma

-- returns 'True'
test :: IO Bool
test = do
    a <- newEmptyTMVarIO
    b <- newEmptyTMVarIO
    forkIO $ do
      threadDelay 250_000
      atomically $ putTMVar a False
      threadDelay 250_000
      atomically $ putTMVar b True
    atomically $ runLastToFinish $
      LastToFinishSTM (readTMVar a) <> LastToFinishSTM (readTMVar b)