[Haskell-beginners] Designing complex Haskell programs

Courtney Robinson courtney at crlog.info
Tue Jan 14 17:24:53 UTC 2014

Hi all,
In my case I don't really care which version of the data a thread sees,
from the apps point of view the data is correct at the time it is fetched.
I was playing around with this idea and it's actually what the
multi-version concurrency control technique was developed for* (more or
less anyway).
I ended up writing http://hackage.haskell.org/package/Stasis to experiment
with using an MVCC style in Haskell.
Docs aren't currently being built on hackage, trying to figure out why but
have a look at the source on github to see what I was thinking

On Tue, Jan 14, 2014 at 12:23 PM, Daniel Trstenjak <
daniel.trstenjak at gmail.com> wrote:

> Hi AntC,
> > Out of interest, is there some reason you don't mention TVar's?
> The use case here seemed to be more about having shared data which is
> only read most of the time and updated occasionally. So the MVar seems
> to be a quite good fit for this.
> > I thought they are the (modern) way to "build robust systems"
> > with "different threads using forkIO"(?)
> I think it really depends how your threads are accessing the shared
> data. Sometimes the rollbacks of a STM might hurt you more than
> waiting for the lock of a MVar.
> You could use a TVar here, but you have to be aware, that your
> threads might operate on outdated data - if one thread is currently
> updating the data - and this might be an issue for your system.
> If I'm getting it correctly, then a MVar can't be read by mutliple
> threads at once, even if all threads are only reading from it.
> That's an advantage of the TVar.
> So thinking again about it, having only occasionally writes from
> favorably only one thread and reads from multiple threads, then indeed
> the TVar might be a quite good fit for this use case.
> Greetings,
> Daniel
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Courtney Robinson
courtney at crlog.info
07535691628 (No private #s)
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