    -- | Operations for running IO operations asynchronously.

    -- These are the same as in the 'async' package. We do not use
    -- 'async' to avoid its dependencies.

    {- License for the 'async' package
    Copyright (c) 2012, Simon Marlow

    All rights reserved.

    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions are met:

        * Redistributions of source code must retain the above copyright
          notice, this list of conditions and the following disclaimer.

        * Redistributions in binary form must reproduce the above
          copyright notice, this list of conditions and the following
          disclaimer in the documentation and/or other materials provided
          with the distribution.

        * Neither the name of Simon Marlow nor the names of other
          contributors may be used to endorse or promote products derived
          from this software without specific prior written permission.

    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
    A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
    OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
    SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
    LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
    DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
    THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
    (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
    OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
    -}

    {-# LANGUAGE DeriveDataTypeable, MagicHash, UnboxedTuples #-}

    module Control.Concurrent.Async (
      async, withAsync, wait, asyncThreadId, cancel, concurrently
      ) where

    import Control.Concurrent.STM
    import Control.Exception
    import Control.Concurrent
    import Control.Monad
    import Data.IORef
    import Data.Typeable
    import GHC.Conc (ThreadId(..))
    import GHC.Exts
    import GHC.IO hiding (onException)

    -- | An asynchronous action spawned by 'async' or 'withAsync'.
    -- Asynchronous actions are executed in a separate thread, and
    -- operations are provided for waiting for asynchronous actions to
    -- complete and obtaining their results (see e.g. 'wait').
    --
    data Async a = Async
      { forall a. Async a -> ThreadId
    asyncThreadId :: {-# UNPACK #-} !ThreadId
                      -- ^ Returns the 'ThreadId' of the thread running
                      -- the given 'Async'.
      , forall a. Async a -> STM (Either SomeException a)
    _asyncWait    :: STM (Either SomeException a)
      }

    -- | Spawn an asynchronous action in a separate thread.
    async :: IO a -> IO (Async a)
    async :: forall a. IO a -> IO (Async a)
    async = ((IO () -> IO ThreadId) -> IO a -> IO (Async a))
    -> (IO () -> IO ThreadId) -> IO a -> IO (Async a)
    forall a. a -> a
    inline (IO () -> IO ThreadId) -> IO a -> IO (Async a)
    forall a. (IO () -> IO ThreadId) -> IO a -> IO (Async a)
    asyncUsing IO () -> IO ThreadId
    rawForkIO

    asyncUsing :: (IO () -> IO ThreadId)
               -> IO a -> IO (Async a)
    asyncUsing :: forall a. (IO () -> IO ThreadId) -> IO a -> IO (Async a)
    asyncUsing IO () -> IO ThreadId
    doFork = \IO a
    action -> do
       var <- IO (TMVar (Either SomeException a))
    forall a. IO (TMVar a)
    newEmptyTMVarIO
       -- t <- forkFinally action (\r -> atomically $ putTMVar var r)
       -- slightly faster:
       t <- mask $ \forall a. IO a -> IO a
    restore ->
              IO () -> IO ThreadId
    doFork (IO () -> IO ThreadId) -> IO () -> IO ThreadId
    forall a b. (a -> b) -> a -> b
    $ IO a -> IO (Either SomeException a)
    forall e a. Exception e => IO a -> IO (Either e a)
    try (IO a -> IO a
    forall a. IO a -> IO a
    restore IO a
    action) IO (Either SomeException a)
    -> (Either SomeException a -> IO ()) -> IO ()
    forall a b. IO a -> (a -> IO b) -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
    >>= STM () -> IO ()
    forall a. STM a -> IO a
    atomically (STM () -> IO ())
    -> (Either SomeException a -> STM ())
    -> Either SomeException a
    -> IO ()
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . TMVar (Either SomeException a) -> Either SomeException a -> STM ()
    forall a. TMVar a -> a -> STM ()
    putTMVar TMVar (Either SomeException a)
    var
       return (Async t (readTMVar var))

    -- | Spawn an asynchronous action in a separate thread, and pass its
    -- @Async@ handle to the supplied function.  When the function returns
    -- or throws an exception, 'uninterruptibleCancel' is called on the @Async@.
    --
    -- > withAsync action inner = mask $ \restore -> do
    -- >   a <- async (restore action)
    -- >   restore (inner a) `finally` uninterruptibleCancel a
    --
    -- This is a useful variant of 'async' that ensures an @Async@ is
    -- never left running unintentionally.
    --
    -- Note: a reference to the child thread is kept alive until the call
    -- to `withAsync` returns, so nesting many `withAsync` calls requires
    -- linear memory.
    --
    withAsync :: IO a -> (Async a -> IO b) -> IO b
    withAsync :: forall a b. IO a -> (Async a -> IO b) -> IO b
    withAsync = ((IO () -> IO ThreadId) -> IO a -> (Async a -> IO b) -> IO b)
    -> (IO () -> IO ThreadId) -> IO a -> (Async a -> IO b) -> IO b
    forall a. a -> a
    inline (IO () -> IO ThreadId) -> IO a -> (Async a -> IO b) -> IO b
    forall a b.
    (IO () -> IO ThreadId) -> IO a -> (Async a -> IO b) -> IO b
    withAsyncUsing IO () -> IO ThreadId
    rawForkIO

    withAsyncUsing :: (IO () -> IO ThreadId)
                   -> IO a -> (Async a -> IO b) -> IO b
    -- The bracket version works, but is slow.  We can do better by
    -- hand-coding it:
    withAsyncUsing :: forall a b.
    (IO () -> IO ThreadId) -> IO a -> (Async a -> IO b) -> IO b
    withAsyncUsing IO () -> IO ThreadId
    doFork = \IO a
    action Async a -> IO b
    inner -> do
      var <- IO (TMVar (Either SomeException a))
    forall a. IO (TMVar a)
    newEmptyTMVarIO
      mask $ \forall a. IO a -> IO a
    restore -> do
        t <- IO () -> IO ThreadId
    doFork (IO () -> IO ThreadId) -> IO () -> IO ThreadId
    forall a b. (a -> b) -> a -> b
    $ IO a -> IO (Either SomeException a)
    forall e a. Exception e => IO a -> IO (Either e a)
    try (IO a -> IO a
    forall a. IO a -> IO a
    restore IO a
    action) IO (Either SomeException a)
    -> (Either SomeException a -> IO ()) -> IO ()
    forall a b. IO a -> (a -> IO b) -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
    >>= STM () -> IO ()
    forall a. STM a -> IO a
    atomically (STM () -> IO ())
    -> (Either SomeException a -> STM ())
    -> Either SomeException a
    -> IO ()
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . TMVar (Either SomeException a) -> Either SomeException a -> STM ()
    forall a. TMVar a -> a -> STM ()
    putTMVar TMVar (Either SomeException a)
    var
        let a = ThreadId -> STM (Either SomeException a) -> Async a
    forall a. ThreadId -> STM (Either SomeException a) -> Async a
    Async ThreadId
    t (TMVar (Either SomeException a) -> STM (Either SomeException a)
    forall a. TMVar a -> STM a
    readTMVar TMVar (Either SomeException a)
    var)
        r <- restore (inner a) `catchAll` \SomeException
    e -> do
          Async a -> IO ()
    forall a. Async a -> IO ()
    uninterruptibleCancel Async a
    a
          SomeException -> IO b
    forall e a. (HasCallStack, Exception e) => e -> IO a
    throwIO SomeException
    e
        uninterruptibleCancel a
        return r

    -- | Wait for an asynchronous action to complete, and return its
    -- value.  If the asynchronous action threw an exception, then the
    -- exception is re-thrown by 'wait'.
    --
    -- > wait = atomically . waitSTM
    --
    {-# INLINE wait #-}
    wait :: Async a -> IO a
    wait :: forall a. Async a -> IO a
    wait = IO a -> IO a
    forall a. IO a -> IO a
    tryAgain (IO a -> IO a) -> (Async a -> IO a) -> Async a -> IO a
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . STM a -> IO a
    forall a. STM a -> IO a
    atomically (STM a -> IO a) -> (Async a -> STM a) -> Async a -> IO a
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Async a -> STM a
    forall a. Async a -> STM a
    waitSTM
      where
        -- See: https://github.com/simonmar/async/issues/14
        tryAgain :: IO a -> IO a
    tryAgain IO a
    f = IO a
    f IO a -> (BlockedIndefinitelyOnSTM -> IO a) -> IO a
    forall e a. Exception e => IO a -> (e -> IO a) -> IO a
    `Control.Exception.catch` \BlockedIndefinitelyOnSTM
    BlockedIndefinitelyOnSTM -> IO a
    f

    -- | Wait for an asynchronous action to complete, and return either
    -- @Left e@ if the action raised an exception @e@, or @Right a@ if it
    -- returned a value @a@.
    --
    -- > waitCatch = atomically . waitCatchSTM
    --
    {-# INLINE waitCatch #-}
    waitCatch :: Async a -> IO (Either SomeException a)
    waitCatch :: forall a. Async a -> IO (Either SomeException a)
    waitCatch = IO (Either SomeException a) -> IO (Either SomeException a)
    forall a. IO a -> IO a
    tryAgain (IO (Either SomeException a) -> IO (Either SomeException a))
    -> (Async a -> IO (Either SomeException a))
    -> Async a
    -> IO (Either SomeException a)
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . STM (Either SomeException a) -> IO (Either SomeException a)
    forall a. STM a -> IO a
    atomically (STM (Either SomeException a) -> IO (Either SomeException a))
    -> (Async a -> STM (Either SomeException a))
    -> Async a
    -> IO (Either SomeException a)
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Async a -> STM (Either SomeException a)
    forall a. Async a -> STM (Either SomeException a)
    waitCatchSTM
      where
        -- See: https://github.com/simonmar/async/issues/14
        tryAgain :: IO a -> IO a
    tryAgain IO a
    f = IO a
    f IO a -> (BlockedIndefinitelyOnSTM -> IO a) -> IO a
    forall e a. Exception e => IO a -> (e -> IO a) -> IO a
    `Control.Exception.catch` \BlockedIndefinitelyOnSTM
    BlockedIndefinitelyOnSTM -> IO a
    f

    -- | A version of 'wait' that can be used inside an STM transaction.
    --
    waitSTM :: Async a -> STM a
    waitSTM :: forall a. Async a -> STM a
    waitSTM Async a
    a = do
       r <- Async a -> STM (Either SomeException a)
    forall a. Async a -> STM (Either SomeException a)
    waitCatchSTM Async a
    a
       either throwSTM return r

    -- | A version of 'waitCatch' that can be used inside an STM transaction.
    --
    {-# INLINE waitCatchSTM #-}
    waitCatchSTM :: Async a -> STM (Either SomeException a)
    waitCatchSTM :: forall a. Async a -> STM (Either SomeException a)
    waitCatchSTM (Async ThreadId
    _ STM (Either SomeException a)
    w) = STM (Either SomeException a)
    w

    -- | Cancel an asynchronous action by throwing the @AsyncCancelled@
    -- exception to it, and waiting for the `Async` thread to quit.
    -- Has no effect if the 'Async' has already completed.
    --
    -- > cancel a = throwTo (asyncThreadId a) AsyncCancelled <* waitCatch a
    --
    -- Note that 'cancel' will not terminate until the thread the 'Async'
    -- refers to has terminated. This means that 'cancel' will block for
    -- as long said thread blocks when receiving an asynchronous exception.
    --
    -- For example, it could block if:
    --
    -- * It's executing a foreign call, and thus cannot receive the asynchronous
    -- exception;
    -- * It's executing some cleanup handler after having received the exception,
    -- and the handler is blocking.
    {-# INLINE cancel #-}
    cancel :: Async a -> IO ()
    cancel :: forall a. Async a -> IO ()
    cancel a :: Async a
    a@(Async ThreadId
    t STM (Either SomeException a)
    _) = ThreadId -> AsyncCancelled -> IO ()
    forall e. Exception e => ThreadId -> e -> IO ()
    throwTo ThreadId
    t AsyncCancelled
    AsyncCancelled IO () -> IO () -> IO ()
    forall a b. IO a -> IO b -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> m b -> m b
    >> IO (Either SomeException a) -> IO ()
    forall (f :: * -> *) a. Functor f => f a -> f ()
    void (Async a -> IO (Either SomeException a)
    forall a. Async a -> IO (Either SomeException a)
    waitCatch Async a
    a)

    -- | The exception thrown by `cancel` to terminate a thread.
    data AsyncCancelled = AsyncCancelled
      deriving (Int -> AsyncCancelled -> ShowS
    [AsyncCancelled] -> ShowS
    AsyncCancelled -> String
    (Int -> AsyncCancelled -> ShowS)
    -> (AsyncCancelled -> String)
    -> ([AsyncCancelled] -> ShowS)
    -> Show AsyncCancelled
    forall a.
    (Int -> a -> ShowS) -> (a -> String) -> ([a] -> ShowS) -> Show a
    $cshowsPrec :: Int -> AsyncCancelled -> ShowS
    showsPrec :: Int -> AsyncCancelled -> ShowS
    $cshow :: AsyncCancelled -> String
    show :: AsyncCancelled -> String
    $cshowList :: [AsyncCancelled] -> ShowS
    showList :: [AsyncCancelled] -> ShowS
    Show, AsyncCancelled -> AsyncCancelled -> Bool
    (AsyncCancelled -> AsyncCancelled -> Bool)
    -> (AsyncCancelled -> AsyncCancelled -> Bool) -> Eq AsyncCancelled
    forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a
    $c== :: AsyncCancelled -> AsyncCancelled -> Bool
    == :: AsyncCancelled -> AsyncCancelled -> Bool
    $c/= :: AsyncCancelled -> AsyncCancelled -> Bool
    /= :: AsyncCancelled -> AsyncCancelled -> Bool
    Eq, Typeable)

    instance Exception AsyncCancelled where
    #if __GLASGOW_HASKELL__ >= 708
      fromException :: SomeException -> Maybe AsyncCancelled
    fromException = SomeException -> Maybe AsyncCancelled
    forall e. Exception e => SomeException -> Maybe e
    asyncExceptionFromException
      toException :: AsyncCancelled -> SomeException
    toException = AsyncCancelled -> SomeException
    forall e. Exception e => e -> SomeException
    asyncExceptionToException
    #endif

    -- | Cancel an asynchronous action
    --
    -- This is a variant of `cancel`, but it is not interruptible.
    {-# INLINE uninterruptibleCancel #-}
    uninterruptibleCancel :: Async a -> IO ()
    uninterruptibleCancel :: forall a. Async a -> IO ()
    uninterruptibleCancel = IO () -> IO ()
    forall a. IO a -> IO a
    uninterruptibleMask_ (IO () -> IO ()) -> (Async a -> IO ()) -> Async a -> IO ()
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Async a -> IO ()
    forall a. Async a -> IO ()
    cancel

    -- | Run two @IO@ actions concurrently, and return both results.  If
    -- either action throws an exception at any time, then the other
    -- action is 'cancel'led, and the exception is re-thrown by
    -- 'concurrently'.
    --
    -- > concurrently left right =
    -- >   withAsync left $ \a ->
    -- >   withAsync right $ \b ->
    -- >   waitBoth a b
    concurrently :: IO a -> IO b -> IO (a,b)
    concurrently :: forall a b. IO a -> IO b -> IO (a, b)
    concurrently IO a
    left IO b
    right = IO a
    -> IO b
    -> (IO (Either SomeException (Either a b)) -> IO (a, b))
    -> IO (a, b)
    forall a b r.
    IO a
    -> IO b -> (IO (Either SomeException (Either a b)) -> IO r) -> IO r
    concurrently' IO a
    left IO b
    right ([Either a b] -> IO (Either SomeException (Either a b)) -> IO (a, b)
    forall {e} {a} {b}.
    Exception e =>
    [Either a b] -> IO (Either e (Either a b)) -> IO (a, b)
    collect [])
      where
        collect :: [Either a b] -> IO (Either e (Either a b)) -> IO (a, b)
    collect [Left a
    a, Right b
    b] IO (Either e (Either a b))
    _ = (a, b) -> IO (a, b)
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (a
    a,b
    b)
        collect [Right b
    b, Left a
    a] IO (Either e (Either a b))
    _ = (a, b) -> IO (a, b)
    forall a. a -> IO a
    forall (m :: * -> *) a. Monad m => a -> m a
    return (a
    a,b
    b)
        collect [Either a b]
    xs IO (Either e (Either a b))
    m = do
            e <- IO (Either e (Either a b))
    m
            case e of
                Left e
    ex -> e -> IO (a, b)
    forall e a. (HasCallStack, Exception e) => e -> IO a
    throwIO e
    ex
                Right Either a b
    r -> [Either a b] -> IO (Either e (Either a b)) -> IO (a, b)
    collect (Either a b
    rEither a b -> [Either a b] -> [Either a b]
    forall a. a -> [a] -> [a]
    :[Either a b]
    xs) IO (Either e (Either a b))
    m

    concurrently' :: IO a -> IO b
                 -> (IO (Either SomeException (Either a b)) -> IO r)
                 -> IO r
    concurrently' :: forall a b r.
    IO a
    -> IO b -> (IO (Either SomeException (Either a b)) -> IO r) -> IO r
    concurrently' IO a
    left IO b
    right IO (Either SomeException (Either a b)) -> IO r
    collect = do
        done <- IO (MVar (Either SomeException (Either a b)))
    forall a. IO (MVar a)
    newEmptyMVar
        mask $ \forall a. IO a -> IO a
    restore -> do
            -- Note: uninterruptibleMask here is because we must not allow
            -- the putMVar in the exception handler to be interrupted,
            -- otherwise the parent thread will deadlock when it waits for
            -- the thread to terminate.
            lid <- IO () -> IO ThreadId
    forkIO (IO () -> IO ThreadId) -> IO () -> IO ThreadId
    forall a b. (a -> b) -> a -> b
    $ IO () -> IO ()
    forall a. IO a -> IO a
    uninterruptibleMask_ (IO () -> IO ()) -> IO () -> IO ()
    forall a b. (a -> b) -> a -> b
    $
              IO () -> IO ()
    forall a. IO a -> IO a
    restore (IO a
    left IO a -> (a -> IO ()) -> IO ()
    forall a b. IO a -> (a -> IO b) -> IO b
    forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
    >>= MVar (Either SomeException (Either a b))
    -> Either SomeException (Either a b) -> IO ()
    forall a. MVar a -> a -> IO ()
    putMVar MVar (Either SomeException (Either a b))
    done (Either SomeException (Either a b) -> IO ())
    -> (a -> Either SomeException (Either a b)) -> a -> IO ()
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . Either a b -> Either SomeException (Either a b)
    forall a b. b -> Either a b
    Right (Either a b -> Either SomeException (Either a b))
    -> (a -> Either a b) -> a -> Either SomeException (Either a b)
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . a -> Either a b
    forall a b. a -> Either a b
    Left)
                IO () -> (SomeException -> IO ()) -> IO ()
    forall a. IO a -> (SomeException -> IO a) -> IO a
    `catchAll` (MVar (Either SomeException (Either a b))
    -> Either SomeException (Either a b) -> IO ()
    forall a. MVar a -> a -> IO ()
    putMVar MVar (Either SomeException (Either a b))
    done (Either SomeException (Either a b) -> IO ())
    -> (SomeException -> Either SomeException (Either a b))
    -> SomeException
    -> IO ()
    forall b c a. (b -> c) -> (a -> b) -> a -> c
    . SomeException -> Either SomeException (Either a b)
    forall a b. a -> Either a b
    Left)
            rid <- forkIO $ uninterruptibleMask_ $
              restore (right >>= putMVar done . Right . Right)
                `catchAll` (putMVar done . Left)

            count <- newIORef (2 :: Int)
            let takeDone = do
                    r <- MVar (Either SomeException (Either a b))
    -> IO (Either SomeException (Either a b))
    forall a. MVar a -> IO a
    takeMVar MVar (Either SomeException (Either a b))
    done      -- interruptible
                    -- Decrement the counter so we know how many takes are left.
                    -- Since only the parent thread is calling this, we can
                    -- use non-atomic modifications.
                    -- NB. do this *after* takeMVar, because takeMVar might be
                    -- interrupted.
                    modifyIORef count (subtract 1)
                    return r

            let tryAgain IO a
    f = IO a
    f IO a -> (BlockedIndefinitelyOnMVar -> IO a) -> IO a
    forall e a. Exception e => IO a -> (e -> IO a) -> IO a
    `Control.Exception.catch` \BlockedIndefinitelyOnMVar
    BlockedIndefinitelyOnMVar -> IO a
    f

                stop = do
                    -- kill right before left, to match the semantics of
                    -- the version using withAsync. (#27)
                    IO () -> IO ()
    forall a. IO a -> IO a
    uninterruptibleMask_ (IO () -> IO ()) -> IO () -> IO ()
    forall a b. (a -> b) -> a -> b
    $ do
                      count' <- IORef Int -> IO Int
    forall a. IORef a -> IO a
    readIORef IORef Int
    count
                      -- we only need to use killThread if there are still
                      -- children alive.  Note: forkIO here is because the
                      -- child thread could be in an uninterruptible
                      -- putMVar.
                      when (count' > 0) $
                        void $ forkIO $ do
                          throwTo rid AsyncCancelled
                          throwTo lid AsyncCancelled
                      -- ensure the children are really dead
                      replicateM_ count' (tryAgain $ takeMVar done)

            r <- collect (tryAgain $ takeDone) `onException` stop
            stop
            return r

    catchAll :: IO a -> (SomeException -> IO a) -> IO a
    catchAll :: forall a. IO a -> (SomeException -> IO a) -> IO a
    catchAll = IO a -> (SomeException -> IO a) -> IO a
    forall e a. Exception e => IO a -> (e -> IO a) -> IO a
    Control.Exception.catch

    -- A version of forkIO that does not include the outer exception
    -- handler: saves a bit of time when we will be installing our own
    -- exception handler.
    {-# INLINE rawForkIO #-}
    rawForkIO :: IO () -> IO ThreadId
    rawForkIO :: IO () -> IO ThreadId
    rawForkIO IO ()
    action = (State# RealWorld -> (# State# RealWorld, ThreadId #))
    -> IO ThreadId
    forall a. (State# RealWorld -> (# State# RealWorld, a #)) -> IO a
    IO ((State# RealWorld -> (# State# RealWorld, ThreadId #))
     -> IO ThreadId)
    -> (State# RealWorld -> (# State# RealWorld, ThreadId #))
    -> IO ThreadId
    forall a b. (a -> b) -> a -> b
    $ \ State# RealWorld
    s ->
       case ((State# RealWorld -> (# State# RealWorld, () #))
    -> State# RealWorld -> (# State# RealWorld, ThreadId# #)
    forall a.
    (State# RealWorld -> (# State# RealWorld, a #))
    -> State# RealWorld -> (# State# RealWorld, ThreadId# #)
    fork# (IO () -> State# RealWorld -> (# State# RealWorld, () #)
    forall a. IO a -> State# RealWorld -> (# State# RealWorld, a #)
    unIO IO ()
    action) State# RealWorld
    s) of (# State# RealWorld
    s1, ThreadId#
    tid #) -> (# State# RealWorld
    s1, ThreadId# -> ThreadId
    ThreadId ThreadId#
    tid #)