First, there is a higher-order effect for UnliftIO that can be used: Data.Effect.Unlift.UnliftIO. When the UnliftIO higher-order effect is in the effect list, users can use withRunInIO.

On the other hand, handlers involving continuations, such as runNonDet, runThrow, or runState, require that the list of higher-order effects be empty when they’re used: Control.Effect.Interpreter.Heftia.Except.runThrow

runThrow :: Eff '[] (Throw e ': r) a -> Eff '[] r (Either e a)

(I’ve simplified the type signatures for clarity.)

This is the limitation. The UnliftIO higher-order effect remains in the list until the very end, when runUnliftIO is executed, which prevents it from being used alongside continuation-handling effects: Control.Effect.Interpreter.Heftia.Unlift.UnliftIO

runUnliftIO :: Eff '[UnliftIO] '[IO] a -> IO a

The only exception is the Shift_ effect. This is the limit of what Heftia (and probably higher-order effects in general) can do to relax these restrictions and allow higher-order effects and delimited continuations to be used together.

It functions even when higher-order effects remain in the list, and when used alongside higher-order effects like withRunInIO, it behaves such that reset (in the sense of delimited continuations, shift/reset) is applied in places where it is ‘principally impossible to escape the scope.’ This is quite advanced and tricky, but it behaves logically, so advanced users should be able to use it in meaningful ways. Example of Shift_ in use.

Handlers that don’t require continuations, like runReader, or those using runStateIORef, runThrowIO, or runNonDetForkIO, which rely on the IO monad internally, can be used even if higher-order effects remain in the list. These handlers can be used without issue1: Control.Effect.Interpreter.Heftia.Except.runThrowIO

runThrowIO ::
    (IO <| r, ForallHFunctor eh) =>
    Eff eh (Throw s ': r) a -> Eff eh r a

In short, Heftia separates the world into UnliftIO-based and continuation-based realms, and they can’t typically be mixed. This is a type-level safeguard to prevent weird things from happening, and the only breakthrough is the Shift_ effect, which pushes the limits of what can be achieved while adhering to the restrictions2.

So, because Heftia can handle the UnliftIO-based realm, it essentially includes everything that can be achieved by other IO-based libraries. It’s just that they can’t be mixed with the continuation-based realm. Heftia gradually expands the feasible area within safe boundaries, maximizing user freedom while preventing weird things from happening.

So, would it be a practical problem if they are incompatible? If you want to use fork and bracket, does that mean you can no longer use runState in that Haskell project and have to use runStateIORef instead?

No, that’s not the case. Whether it becomes a practical issue depends on trying out a few real-world programs. However, when I personally created a small automation program using Heftia with concurrency, transactional DB access, and runState (without IORef), it was not an issue. Here’s how to handle it.

First, perform effect handling involving higher-order effects and continuation operations. Once that handling is complete and continuation operations are no longer needed, you can handle UnliftIO and other higher-order and first-order effects last. Essentially, this means separating the stage of handling continuation operations from the stage of handling IO dependencies with UnliftIO:

main :: IO ()
main = runApp yourApplication

runApp :: Eff '[...] '[...] ~> IO
runApp = runUnliftIOStage . runContinuationStage

runUnliftIOStage :: Eff '[... , UnliftIO] '[... , IO] ~> IO
runContinuationStage :: Eff '[...] '[...] ~> Eff '[... , UnliftIO] '[... , IO]

In this example, the stages are completely separated into two functions, but in reality, they can be mixed together more. The degree of inconvenience due to the lack of modularity is unknown. However, in most practical cases, this approach should work well.

This post addresses the following question in Reddit: https://www.reddit.com/r/haskell/comments/1fbvmo8/comment/lm92t03/

Yeah. Have a look at https://discourse.haskell.org/t/the-issues-with-effect-systems/5630/19?u=arybczak, in particular:

Why we can’t support both nondeterminism and MonadUnliftIO? It is perhaps less well known that many IO functions accessible via MonadUnliftIO, mainly those that use fork and bracket, only work when the control flow is deterministic. This has not been a problem since the beginning since Haskell was a deterministic language - but not now! Future effect system users will probably face a choice between nondeterminism-capable libraries and MonadUnliftIO-capable libraries and they will need to choose based on their specific needs.

Your library claims to support both, which, according to my knowledge, can’t work. What happens when you combine NonDet (or Throw / Catch) with unlifted bracket or forkIO?

  1. As for the return value here, by adding the UnliftIO <<| eh constraint, it can return Either e a instead of just a. It’s simply that UnliftIO.Exception.try isn’t being used. ↩

  2. I haven’t tried it yet, but it’s likely possible to write continuation-based handlers like runThrow that can be used alongside higher-order effects by using Shift_ internally. In that case, the behavior when higher-order effects are involved would probably inherit from Shift_—meaning the context would be reset at the point where the higher-order effect’s scope ends. ↩