Hi,
On Sun, Mar 14, 2010 at 01:26:10AM +0100, Christophe Grand wrote:
> I agree: it can be concurrently computed several times (but a given thread
> would only compute it at most once).
I must confess I was a little insisting on a point which has probably
only little impact in the real life. But a) I prefer to have correct
solution and b) it was a nice exercise. :)
> You're right of course. I apologize.
^^^^^^^^^
Huh? What for? This was a really enlightening discussion! Who would have
thought, that memoize is so non-trivial. :)
> As a minor note: couldn't a future replace your promise? Or couldn't you get
> rid of the other thread and deliver the promise in the same thread but
> outside of the (locking ..) form?
The problem with the future is, that it starts immediately. A similar
issue is with the same thread: the behaviour after the swap! must be
identical to hits and misses. So I needed the locking to really start
the computation only once.
> Hmm or even a delay!?
And of course, this is the solution to get rid of the locking! I should
have thought of delay much earlier.
> Do you see a problem in my latest implementation?
> http://gist.github.com/330644#LID153
No. This one looks fine.
Christophe, Eugen! I will summarise this discussion in blog post. It
really shows, that concurrent programming is not trivial. Not even for
„trivial“ things like a memoised function.
Sincerely
Meikel
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