Re: [Haskell-cafe] How to ensure code executes in the context of a specific OS thread?

On 07/07/2011 08:41, Simon Marlow wrote:

On 06/07/2011 21:19, Jason Dagit wrote:

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On Wed, Jul 6, 2011 at 12:52 PM, Simon Marlow wrote:

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On 06/07/11 17:14, David Barbour wrote:

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On Wed, Jul 6, 2011 at 8:09 AM, Simon Marlowmailto:marlowsd@gmail.com> wrote:

On 06/07/2011 15:42, Jason Dagit wrote:

How can I make sure my library works from GHC (with arbitrary

user threads) and from GHCI?

Right, but usually the way this is implemented is with some cooperation from the main thread. [...] So you can't just do this from a library - the main thread has to be in on the game. I suppose you might wonder whether the GHC RTS could implement runInMainThread by preempting the main thread and running some different code on it. [...]

I think the real issue is that GHC has a different behavior than GHCi, and I think this causes a lot of difficulties for people working on GUI and other FFI integration.

Perhaps it would be possible to reverse the default roles of threads in GHCi: the main thread run user commands, and a second bound thread will process user interrupts and such.

Well, GHCi has no main, so it doesn't seem surprising (to me) that it's different.

However, if -fno-ghci-sandbox doesn't have any drawbacks we could just make it the default. I don't actually remember why we run each statement in a new thread, I think it just seemed like a prudent thing to do.

+1 for this change. I'm not sure how we would know if there are drawbacks.

Now that I think about it, the original reason may have been that if the computation grows a large stack, having it in a separate thread means GHCi can recover the memory. However we have been able to recover stack memory for some time now, so that is no longer an issue.

I discovered the real reason we run statements in a separate thread: the GHCi debugger. If the computation stops at a breakpoint, then we have to save the context and resume GHCi, which can only be done if the computation was running in a separate thread. The way things are arranged right now, each stopped computation gets a different thread. What you want is for all these to be on the main thread. It might be possible to arrange this, but it would require some non-trivial reorganisation in the implementation of interactive evaluation (compiler/main/InteractiveEval.hs). I'm going to have to leave this for now, sorry. In the meantime you'll still be able to use -fno-ghci-sandbox, but the debugging features in GHCi will be disabled. Cheers, Simon