Ross Paterson wrote: On the haskell list it has almost been decided that (ST RealWorld a) and (IO a) _should_ merge and become the same thing. Keean.

On Mon, Dec 20, 2004 at 02:16:52PM +0000, Keean Schupke wrote:

...

Surely if IO is (ST RealWorld a) then (StateT RealWorld m a) is more or less the right thing?

But IO is not ST RealWorld (even if GHC pretends it is): other users of the world are not waiting for the new world produced by your Haskell program.

IO is more accurately modelled by a resumption monad, cf

http://www.haskell.org/pipermail/haskell-cafe/2003-August/004892.html

That suggests a possible monad transformer:

data IOT m a = Return a | Invoke (m (SysCall (IOT m a)))

but with no way to define runIOT :: Monad m => IOT m a -> IO (...), it's not much use.