mode :: Mode (CmdArgs Cmd) memoizes async exceptions, breaking HLS Shake restarts #1663

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1. soulomoon commented on Sep 7, 2025

   

   soulomoon

   on Sep 7, 2025

   ContributorAuthor

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   Minimal reproduction of the same problem:

   {-# LANGUAGE NumericUnderscores #-}
   module Main where
   import System.IO.Unsafe (unsafePerformIO)
   import Control.Concurrent
   import Control.Concurrent.Async
   import Control.Exception
   
   {-# NOINLINE initData #-}
   initData :: String
   initData = unsafePerformIO $ do
     threadDelay 1_000_000
     return "Initialized Data"
   
   threadRun :: IO String
   threadRun = do
     putStrLn "Thread started"
     evaluate initData
   
   main :: IO ()
   main = do
     putStrLn "Main started"
     oldRun <- async threadRun
     threadDelay 500_000
     cancel oldRun
   
     newRun <- async threadRun
     res <- waitCatch newRun
     print "First attempt:"
     print res
   
     newRun <- async threadRun
     res <- waitCatch newRun
     print "Second attempt:"
     print res
   
     putStrLn "Main ended"

   Main started
   Thread started
   Thread started
   "First attempt:"
   Left AsyncCancelled
   Thread started
   "Second attempt:"
   Left AsyncCancelled
   Main ended