On Tue, Aug 26, 2008 at 3:15 AM, Ashley Yakeley
Judah Jacobson wrote:
I've been wondering: is there any benefit to having top-level ACIO'd <- instead of just using runOnce (or perhaps "oneshot") as the primitive for everything?
I don't think oneshots are very good for open witness declarations (such as the open exceptions I mentioned originally), since there are pure functions that do useful things with them.
I think you're saying that you want to write "w <- newIOWitness" at the top level, so that w can then be referenced in a pure function. Fair enough. But newIOWitness's implementation requires writeIORef (or an equivalent), which is not ACIO, right? I suppose you could call unsafeIOToACIO, but if that function is used often it seems to defeat the purpose of defining an ACIO monad in the first place.
Not sure about TVars either, which operate in the STM monad. Would you also need a oneshotSTM (or a class)?
Interesting point; I think you can work around it, but it does make the code a little more complicated. For example: oneshot uniqueVar :: IO (TVar Integer) uniqueVar =< atomically $ newTVar 0 -- alternately, use newTVarIO uniqueIntSTM :: IO (STM Integer) uniqueIntSTM = uniqueVar >>= \v -> return $ do n <- readTVar v writeTVar v (n+1) return n getUniqueInt :: IO Integer getUniqueInt = uniqueIntSTM >>= atomically -Judah