Reify state

[Kyuuhachi]

I have a program in which I want to run an effectful function several times with different arguments, evaluate which is best, and continue as if only that one had happened. As far as I know, this can't be done with NonDet (as there's no way to communicate between branches to choose one) or any other existing effect. Is something like that possible? I have no idea how state threading works, but I would imagine an API akin to

choose :: Ord c => [Sem r (c, a)] -> Sem r a
choose alts = do
  st <- getState
  values <- forM alts $ \alt -> do
    setState st
    (c, v) <- alt
    st' <- getState
    pure (st', c, v)
  let (st', _, v) = maximumBy (comparing (\(_, c, _) -> c)) values
  setState st'
  pure v

[Kyuuhachi]

I've managed to cobble something together, but I have no idea whether I'm doing it right.

data Opti m a where
  Opti :: Ord c => [m (c, a)] -> Opti m a
makeSem ''Opti

runOpti :: Sem (Opti : r) a -> Sem r a
runOpti
  = interpretH \case
    Opti ms -> do
      ms' <- mapM runT ms
      ins <- getInspectorT
      vals <- forM ms' \m' -> do
                x <- raise (runOpti m')
                case inspect ins x of
                  Just (c, v) ->
                    pure (c, v <$ x)
                  Nothing ->
                    error "runOpti: fail"
      pure $ snd $ maximumBy (comparing fst) vals

(Renamed it from choose because that's apparently already used by NonDet.)

[Heimdell]

Just remember that certain effects (Lift IO) cannot be undone anyway, and you have to keep it like Sem (xs ++ [Opt, Final M]) a.