[Haskell-cafe] idea for avoiding temporaries

Dan Weston westondan at imageworks.com
Thu Mar 8 18:04:43 EST 2007

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


David Roundy wrote:
> 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:
>> 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:
>>> 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:
>>>> 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:
>>>>> On 3/8/07, David Roundy <droundy at darcs.net> 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.
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