Software Transactional Memory in Scala (2008) | Hacker News Reader

Software Transactional Memory in Scala (2008)

codecommit.com

16 pointsspacemanaki10 comments

I hate to be a negative Nancy, but if STM was going to take off it would have by now. Is there something I am missing?

Would it really have taken off by now? I really don't know much about STM, and I know there is no one canonical STM, but it's often compared to garbage collection. How long did it take for garbage collection to take off? Wikipedia claims GC was invented in 1959 and STM in 1995.

edited to correct the date

I think we need it but its just like with GC. You cant just not worry about it any more. Developers need to learn about STMs. How the work, what there problems are and they need to learn programming with these implementations STMs.

It depends on what you mean by "take off", and what type of STM you're talking about. I use Clojure's STM regularly, and to great benefit.

Yeah, I agree with wtetzner, I've been using STM in Scala via both ScalaSTM and Akka for almost 2 years now. It's incredibly easy to use and I use it far more than any of the other concurrency controls out there. I've written a blog entry about how to use STM in Scala and some pitfalls to avoid: http://outputstream.io/dear-scalasphere-why-arent-we-talking...

This is way to much simplification. The problem with STM is that it is very hard to integrate in classical languages. In Clojure and Haskell STM works really well and STM is just in production.

Given that Clojure is impure and has IO effects hidden behind seemingly innocuous evalation, how does STM work well in Clojure?

You can still write pure functions even if they aren't enforced by static typing, right? What do you mean by "IO effects hidden behind seemingly innocuous evalation"?

Clojure's data structures are immutable, and I don't think you're supposed to use STM with any mutable objects or data. You pass pure functions to the STM system that take an old (immutable) value and return a new (immutable) value. They are pure though, so they can be called multiple times in a transaction.

I'm sure I'm making a muddle of it explaining it though, and I'm not sure what you concerns are specifically. So I highly recommend reading the ants.clj demo and watching the accompanying video with a detailed explanation by Rich Hickey, it helped me understand how STM works specifically in Clojure better than any other source, including two books on Clojure. I don't have experience with Haskell's or any other STM though, and I'm really just a novice with Clojure, so YMMV, etc.

http://blip.tv/clojure/clojure-concurrency-819147

STM needs a guarantee that all actions within are reversible. If you can launchTheMissiles in the STM transaction, then the STM cannot guarantee the desired properties.

One of the nice things in Haskell is that effects are typed. So STM can require its given actions to only have STM effects, and disallow full-fledged IO effects.

If you're willing to go by convention without compiler guarantees, then I'm not sure what advantage Clojure has over Java here, as an example?

STMs need nothing! They are Software.

There is more then one point to make here:

- There is a IO macro that you can use to signal "Hey IO is going on you cant do this". I have jet to see this beeing used in production.

- Only Refs can change in Clojure (would be diffrent if you would have it in java)

- Refs are not really used that often. (in java everything changes all the time)

- Its easy to see/know if a function is dangerous because of naming conventions and other coding conventions

So why does "by convention" work out in Clojure and not in C#:

- Standard library is written with this in mind.

- Immutable Data structures are standard.

- The conventions where there since the language was created and get inforced both by the community and the language.

So you are right in terms of compiler guarantees you don't have much more in clojure then you have in Java but in the end it on the programmer that he writtes something that works. The stuff mention above seem to make it easy enought to work with them that people don't have a problem creating something that works with the STM.

Reply on news.ycombinator.com