On Thu, 24 Apr 2008 12:57:56 +0200, apfelmus wrote:
there is also the option of collapsing ReadOnlySTM a and STM a by changing the semantics of writeTVar .
I mean, writeTVar can be used for two different things:
1) communicate with other threads, i.e. crossing atomically boundaries 2) communicating inside a single thread/STM action à la mutable state (IORef).
We only want 1), but 2) is expendable, we can always use parameters to pass state around or wrap the whole thing into a state monad. For 1), it's enough to have a primitive
scheduleWriteTVar :: TVar a -> a -> STM ()
that ensures to write the TVar at the very end of the atomically block.
Unfortunately, though, this breaks the very thing that makes STM attractive: namely, composability. Now in order to work with a TVar, I need to know whether anything that came before me might have modified it, and if so take the current value as a parameter instead of reading it like normal. Or am I misunderstanding something? -- Chris Smith