Re: [Haskell-cafe] idea for avoiding temporaries

Nothing is being done concurrently, so I don't see what STM would gain us. What is it that you're thinking we could gain from STM? David On Thu, Mar 08, 2007 at 03:04:43PM -0800, Dan Weston wrote:

Have you looked into using STM (Software Transactional Memory)? This problem seems like some subset of concurrent programming.

Dan

David Roundy wrote:

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Ah, I was missing your point, I've heard something called copy-on-write, which wasn't what you describe (or I also misunderstood it when I heard it before).

I see. But how would one manage these handles? What's to keep me from accidentally copying a handle? It sounds like it'd require explicit memory management, in order to avoid ever copying a handle, if I were to implment this myself.

Or are you suggesting that if the simons implemented a copy-on-write scheme in ghc's RTS, then I'd be all set?

In short, managing the reader count is exactly the problem that sounds hard, and I still don't have any idea how one would go about it.

David

On Thu, Mar 08, 2007 at 01:31:19PM -0800, Dan Weston wrote:

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I might be missing the point, but I think you are missing mine.

The copy-on-write I am talking about means that it's no longer "your data", so you don't need any knowledge of who has access to it because you don't own it or have a pointer to it. It is owned by some broker from which you request a read-only or write access handle as needed. Requested changes to underlying data already shared by others triggers a copy and reassignment of pointers to it for your handle alone.

The copy cost appears only when there is more than one handle to the same data and one of them changes it.

All this can be wrapped up and hidden away. If you want to escape this broker business and steal back your data, just ask: the broker will duplicate shared data needed by others, change their pointers to it, then disown the pointer it returns to you.

This is copying without writing (unnecessarily). Or am I missing something?

Dan

David Roundy wrote:

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I'm thinking you're missing the point. The point is to copy without writing, and that requires some knowledge (whether static or runtime) of whether anyone else has a reference to my data--which copy-on-write won't give me.

David

On Thu, Mar 08, 2007 at 11:15:25AM -0800, Dan Weston wrote:

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Or possibly more generally copy-on-write, which requires one more level of indirection (handle instead of ptr). Since you are talking about using ForeignPtr, this is already within your power to prototype, I should think.

Dan

Dan Piponi wrote:

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On 3/8/07, David Roundy wrote:

>I started wondering whether there's a solution that would allow us to >write pretty high-level pure functional code, while the RTS can >realize >at run-time that we have the only reference to the input argument and >that it is therefore safe to consume it destructively. I think you're talking about uniqueness typing which is supported by the programming language Clean.