Haskell noob here. How is STM used in Haskell? What is type-driven STM?
In Clojure, I don't recommend it because it's Just Another Place To Stick State. It's cool, but provides little semantic value over compare-and-set! on an atom.
In Clojure, I don't recommend it because it's Just Another Place To Stick State. It's cool, but provides little semantic value over compare-and-set! on an atom.
If your compare-and-set returns false, what do you do?
STM resolves these issues by letting you coordinate updates to more granular, finer-scoped refs
It actually does not have to be, but this is how it is (was) implemented. I built a simple STM (in Java) as part of my bachelor thesis and its performance was comparable to the atomic reference.
I'm not convinced a single atomic reference sufficiently covers enough use cases, however.
Can you compare and set on multiple atoms at the same time? If not, how do you make updates atomic without using either STM, a global lock, or re-implementing STM yourself with an incremental locking and rollback system.
http://chimera.labs.oreilly.com/books/1230000000929/ch10.htm... http://book.realworldhaskell.org/read/software-transactional...
It is pretty much as like this
1. Dereference a variable and keep the old reference.
2. Create a new modified variable.
3. Use CAS on old reference and new variable
to update the reference to point to the new vairable.
In case of failure (some other thread made the update before) go to 1.
It has the same drawbacks and advantages as atomic references: your code does not depend on lock taking order (as they are managed by the transaction manager) and you avoid dead locks as the locks are taken in the same order before applying the update, but now two different threads can start the transaction to update the same variable, but only one of them can finish as the first one and the other one should be cancelled and repeated.
In principle you can get away with a simple atomic reference (that is much faster) when the set of the updated variables is always the same - you make a pair of them and update them together.