On Tue, Mar 28, 2006 at 10:14:27AM +0100, Simon Marlow wrote:
On 28 March 2006 00:24, Ross Paterson wrote:
As Malcolm pointed out, using MVars requires some care, even if you were just aiming to be thread-safe.
I don't really understand the problem, maybe I'm missing something. I thought the idea would be that a thread-safe library would simply use MVar instead of IORef.
MVars certainly require more care than IORefs: you have to ensure your takes and puts are matched, for example. And there's the possibility of deadlock when you have more than one variable. I was toying with an interface like newRef :: a -> IO (Ref a) modifyRef :: Ref a -> (a -> (a, r)) -> IO r modifyRef2 :: Ref a -> Ref b -> (a -> b -> (a, b, r)) -> IO r ... where Refs are MVars plus a stable ordering, so all the primitives lock (i.e. take) them in the same order. It's a bit clunky, though. On Tue, Mar 28, 2006 at 10:25:04AM +0100, Simon Marlow wrote:
It just occurred to me that STM isn't completely trivial in a single-threaded implementation, because exceptions have to abort a transaction in progress.
Ah, and it seemed so simple. Still, exception-safety would be a nice property for a state abstraction to have.