STM monad problem

Question

This is just a hypothetical scenario to illustrate my question. Suppose that there are two threads and one TVar shared between them. In one thread there is an atomically b

Answer 1

No, it would work fine. Exactly how the two threads would interact depends on the retry logic.

For example, let's say you have:

ten tv = do
  n <- readTVar tv
  when (n < 7) retry
  writeTVar tv 0
  -- do something that takes about 10 seconds

one tv = do
  modifyTVar tv (+1)
  -- do something that takes about 1 second

So the "ten" thread will be in retry state until the TVar reaches the value 7, then it will proceed.

Note that you can't directly control how long these computations will take inside the STM monad. That would be a side-effect, and side-effects are not allowed in STM calculations. The only way to communicate with the outside world is via values passed through transactional memory.

And that means that if the "baton-passing" logic through transactional memory is correct, the program will work correctly independently of the exact amount of time any part of it takes. That's part of the guarantee of STM.