[Haskell-cafe] shared oneShot IO (was top-level state proposals)

[Dan Weston]

I was wondering why, since IO is an instance of MonadFix [1], and 
therefore of ArrowLoop (Kleisli m), and since "The loop operator 
expresses computations in which an output value is fed back as input, 
even though the computation occurs only once." [2], the MonadFix or 
ArrowLoop class (through use of mfix or loop, respectively) doesn't 
appear in anyone's suggestion, where the top-level state was the thing 
looped over.

Or is this more or less what is going on in the function

oneShot :: IO a -> ACIO (IO a)
oneShot io = mdo mv <- newMVar $ do a <- io
                                     let loop = do putMVar mv loop
                                                   return a
                                     loop
                  return $ do act <- takeMVar mv
                              act

but without explicitly using the MonadFix or ArrowLoop classes?

Dan

[1] 
http://www.haskell.org/ghc/docs/6.4.1/html/libraries/base/Control-Monad-Fix.html
[2] 
http://www.haskell.org/ghc/docs/6.4.1/html/libraries/base/Control-Arrow.html

Claus Reinke wrote:
> 
> what we do not know is how to share IO actions themselves in a
> demand-driven way, ie how to describe an IO action that is executed at
> most once, only on demand, with shared result.