{-# LANGUAGE BlockArguments #-}

module Util
  ( stmChanges,
  )
where

import UnliftIO.STM

stmChanges :: (Eq a) => STM a -> IO (STM a)
stmChanges mkX = do
  x'T <- newTVarIO Nothing
  pure do
    x' <- readTVar x'T
    x <- mkX
    checkSTM (Just x /= x')
    writeTVar x'T (Just x)
    pure x