Has anyone managed to use parEval in the context of an effect system (e.g. effectful)?

[jerbaroo]

Pseudocode to show the intention, being able to use parEval in code where capabilities are determined by an effect system.

foo :: forall es. (StreamParE :> es)
  => Stream (Eff es) Int -> Stream (Eff es) Int
foo = parEval id . fmap (+1) . parEval id . fmap (+1)

Effect systems and streaming libraries are two great tools, would be great to have an example showing their full capabilities being able to be used together!

[adithyaov]

parEval imposes the following constraints on the monad:

MonadIO m
MonadBaseControl IO m
MonadThrow m

There is already a MonadThrow instance defined for the Eff monad.
MonadIO and MonadBaseControl are satisfied by IOE.

This defines the capabilities using the effect system:

foo :: forall es. (IOE :> es)
  => Stream (Eff es) Int -> Stream (Eff es) Int

[jerbaroo]

I am hoping not to introduce an IO constraint on my program, I would prefer the program not to have this strict dependency.

One option is being discussed here. In this case a new effect StreamParEvalE is introduced, and where IO is available then this effect may be implemented with streamly's parEval. While it does compile, it is producing a run-time error. Where IO is unavailable it is trivial to implement the effect with some identity function, that just returns the same Stream it received.

[adithyaov]

While it does compile, it is producing a run-time error

What is the runtime error you see?

[jerbaroo]

The following comment shows the code and run-time error:
haskell-effectful/effectful#333 (reply in thread)

[adithyaov]

Your question may have been answered there.