[Haskell-cafe] Re: [Haskell] Top Level <-

[Ashley Yakeley]

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