[Haskell-cafe] Re: A GHC error message puzzle
wei.hoo at gmail.com
Fri Aug 13 14:02:02 EDT 2010
On Fri, Aug 13, 2010 at 4:53 AM, Simon Marlow <marlowsd at gmail.com> wrote:
> Ah yes, that works too. But other similar versions don't, like this one:
> process :: String -> String
> process _ = let x = x in x
> Hence why I added the "tail" in my version.
Without "-O", this version doesn't work, but with "-O" it does. Why?
> So what happens is this:
> - the recursive definition causes the main thread to block on itself
> (known as a "black hole")
> - the program is deadlocked (no threads to run), so the runtime
> invokes the GC to see if any threads are unreachable
> - the GC finds that
> (a) the main thread is unreachable and blocked on a blackhole, so it
> gets a NonTermination exception
> (b) the Handle is unreachable, so its finalizer is started
> - the finalizer runs first, and closes the Handle
> - the main thread runs next, and the exception handler for writeFile
> tries to close the Handle, which has already been finalized
> Really hClose shouldn't complain about a finalized handle, I'll see if I can
> fix that.
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