Judah Jacobson wrote:
> 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?
newIOWitness is very like newUnique. In both cases, the internal
implementation updates an MVar to make them unique. Internally the
open-witness package would use unsafeIOtoACIO (just as it already uses
unsafeCoerce), but an exposed newIOWitnessACIO would be safe.
> 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
This complicates the purpose of STM, which is to make composable STM
transactions. I would rather do this:
uniqueVar :: TVar Integer
uniqueVar <- newTVarACIO
uniqueInt :: STM Integer
uniqueInt = do
n <- readTVar uniqueVar
writeTVar uniqueVar (n+1)
return n
AFAICT, one-shots are less powerful and just as complicated as an ACIO
monad.
--
Ashley Yakeley
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