I have a good theory on the latter symptom (the "thread killed" message). Sticking in some traces, as in my appended code, helped me to see what's going on. It seems to be exactly what you describe -- the variable v is permanently bound to the exception it "evaluates" to. Since the right hand True portion of the unamb evaluates more quickly, the spawned threads are killed and the left hand (the v) "evaluates" to "thread killed". This remains the value of its thunk when you access it later. This problem seems sort of innate to a pure unamb utilizing unsafePerformIO and asynchronous exceptions. A clever use of `par` might conceivably help, given that if the par spark fails, the thunk can still be evaluated? Might be a dead end. Here's the code: go = f "f" (f "" True) where f s v = (unamb (s++"f") (s++"g") v True) `seq` v --unamb :: String -> String -> a -> a -> a unamb s s' a b = unsafePerformIO (race s s' (evaluate a) (evaluate b)) --race :: String -> String -> IO a -> IO a -> IO a race s s' a b = do v <- newEmptyMVar let t x = x >>= putMVar v withThread s (t a) $ withThread s' (t b) $ takeMVar v where withThread s u v = bracket (forkIO u) (killNote s) (const $ putStrLn ("in: " ++ s) >> v >>= \x -> putStrLn ("out: " ++ show x ++ " "++ s) >> return x) killNote s tid = throwTo tid (ErrorCall s) And a GHCi session: *Un> go in: ff in: fg in: f in: g out: True fg out: True ff <interactive>: ff *** Exception: ff Cheers, Sterl. On Dec 26, 2008, at 1:15 AM, Conal Elliott wrote:
I'm looking for information about black hole detection with ghc. I'm getting "<<loop>>" where I don't think there is an actual black hole. I get this message sometimes with the unamb package, which is implemented with unsafePerformIO, concurrency, and killThread, as described in http://conal.net/blog/posts/functional-concurrency- with-unambiguous-choice/ and http://conal.net/blog/posts/smarter- termination-for-thread-racing/ .
Suppose I have a definition 'v = unsafePerformIO ...', and v is used more than once. Evaluation (to whnf) of v is begun and the evaluation thread gets killed before evaluation is complete. Then the second use begins. Will the second evaluation be (incorrectly) flagged as a black hole?
I haven't found a simple, reproducible example of incorrect black- hole reporting. My current examples are tied up with the Reactive library. I do have another strange symptom, which is "thread killed" message. I wonder if it's related to the <<loop>> message. Code below.
Thanks, - Conal
import Prelude hiding (catch) import System.IO.Unsafe import Control.Concurrent import Control.Exception
-- *** Exception: thread killed main :: IO () main = print $ f (f True) where f v = (v `unamb` True) `seq` v
-- | Unambiguous choice operator. Equivalent to the ambiguous choice -- operator, but with arguments restricted to be equal where not bottom, -- so that the choice doesn't matter. See also 'amb'. unamb :: a -> a -> a unamb a b = unsafePerformIO (evaluate a `race` evaluate b)
-- | Race two actions against each other in separate threads, and pick -- whichever finishes first. See also 'amb'. race :: IO a -> IO a -> IO a race a b = do v <- newEmptyMVar let t x = x >>= putMVar v withThread (t a) $ withThread (t b) $ takeMVar v where withThread u v = bracket (forkIO u) killThread (const v)
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