Add a `PrimBase (IOSim s)` instance

[jorisdral]

IOSim has a PrimMonad instance, so I suppose a PrimBase instance could be added too? This would, for example, allow using keepAlive in IOSim

[coot]

IOSim is essentially a free monad, one of its operations allows to run ST actions, that would fit into PrimBase, but how would you embed all the other actions into it?

[jorisdral]

Right, I posted it here partially to get your input on whether it would be possible. I suppose it isn't, so let's close this issue!

[coot]

We could just implement keepAlive, although it's non-trivial. It would require implementing schedule function which can halt and resume:

class MonadKeepAlive m where
  keepAlive :: a -> m r -> m r

data SimA s a where
  ...
  KeepAlive    :: a -> IOSim s r -> (r -> SimA s b) -> SimA s b
  ...


schedule :: _
schedule thread@Thread { threadControl = ThreadControl action ctl },
         simstate =
  case action of 
    KeepAlive a mr k -> do
      keepAlive a (_ {- here we want to schedule `mr` -})
      _continue {- here we want to continue with `k` -}

Currently, schedule doesn't allow for something like that. It runs the simulation until completion, while we need some sort of scheduling an IOSim computation in a given computation context. Hopefully, this doesn't become more challenging for IOSimPOR.

[coot]

Hmm, another idea would be to implement keepAlive using touch. For it we'd need to push a new kind of frame to the ControlStack.

[jorisdral]

I currently don't immediately have a need for keepAlive myself, but I can imagine it might be nice to have. It was more intended as an example

[coot]

I'm not sure if it has the correct semantics with regards to exceptions (or that it works correctly), but here's a proof of concept: https://github.com/input-output-hk/io-sim/compare/coot/keepAlive