Cleaning Async resources upon cancel

test.hs

{-# LANGUAGE ScopedTypeVariables #-}
import Control.Concurrent
import Control.Concurrent.Async
import Control.Exception

main :: IO ()
main = do
    let f = 1000000
    v <- newMVar ()
    let debug = withMVar v . const . putStrLn
    a <- recProc debug (replicate 10 (3 * f))
    debug "Waiting main"
    threadDelay f
    debug "Canceling main"
    cancel a
    debug "Finishing main"
  where
    recProc :: (String -> IO ()) -> [Int] -> IO (Async ())
    recProc _ [] = async (return ())
    recProc debug (n:ns) = do
      let l = show $ length ns
      debug $ "Spawning " ++ l
      async $ do
        debug $ "Spawned " ++ l
        a <- recProc debug ns
        debug $ "Waiting " ++ l
        threadDelay n `catch` \(_ :: SomeException) -> do
          debug $ "Canceling " ++ l
          cancel a
        debug $ "Finishing " ++ l