Adapter for effectful effects library

[istathar]

We have found ourselves in a situation where we want to use the Program monad from within a codebase that has been infested with the Eff effect system from the effectful package. This branch introduces a new ProgramE effect that allows lifting and unlifting from Eff es to Program.

[istathar]

Thanks again to @guaraqe's help yesterday, this is looking good. I'm going to add some Haddock documentation based on the example EffectSnippet.hs that we worked though and then this will be good!

[istathar]

One problem I can think of is that the Context that is current right before the inner Eff is run is possibly not the same as that that was stored when we fist invoked runProgramE. So we need a way to update (?) the Context that's stored in the effect before returning to the effect system.

[guaraqe]

Thanks for putting my name there!

Do you have an example of this... context changing between layers?

[istathar]

You're right, I'll need to make an example that shows the problem.

[istathar]

The best I can do is something like this:

01:06:18Z (00.002) Begin trace
01:06:18Z (00.002) trace = 1104168dfdd8f371513f286b3545024a
01:06:18Z (00.002) Enter program2
01:06:18Z (00.002) span = 9ae1768dfdd8f87c
01:06:18Z (00.002) parent = 
01:06:18Z (00.002) Enter retrieveProgramName
01:06:18Z (00.002) span = 3d2e768dfdd8c35d
01:06:18Z (00.002) parent = 9ae1768dfdd8f87c
01:06:18Z (00.002) In Program
01:06:18Z (00.002) Enter (inner effect invocation)
01:06:18Z (00.002) span = ae67868dfdd808fa
01:06:18Z (00.002) parent = 3d2e768dfdd8c35d
01:06:18Z (00.002) Running effects
01:06:18Z (00.002) Enter nested ProgramE
01:06:18Z (00.002) span = 3aa2968dfdd89240
01:06:18Z (00.002) parent = 9ae1768dfdd8f87c
01:06:18Z (00.002) Inside inner Program
01:06:18Z (00.002) Leave nested ProgramE
01:06:18Z (00.002) Finished effects
01:06:18Z (00.002) Leave (inner effect invocation)
01:06:18Z (00.000) Sent 1 event
01:06:18Z (00.000) Sent 1 event
01:06:18Z (00.002) path = ()
01:06:18Z (00.002) Leave retrieveProgramName
01:06:18Z (00.000) Sent 1 event
01:06:18Z (00.002) Leave program2
01:06:18Z (00.000) Sent 1 event

which doesn't really help, other than to note that the parent inside nested ProgramE is wrong (it says 9ae1768dfdd8f87c but should be ae67868dfdd808fa)

I have an idea of what to do about this; I'll have a go. If I get stuck I'll post the code that generated the above.

[istathar]

Sorted it out with putStaticRep:

withProgram action = do
    -- extract Context τ
    ProgramE context1 <- Effect.getStaticRep

    -- lift to IO, using the provided specialized function which gives an unlift
    -- function for later use.
    Effect.withEffToIO $ \runInIO ->
        -- now in IO. Lift to Program τ
        subProgram context1 $ do
            -- now in Program τ. We form the function that will run an effect
            -- in Program τ, and pass it to the supplied action.
            action $ \inner -> do
                context2 <- getContext
                liftIO $ do
                    -- now in IO
                    runInIO $ do
                        -- now in (IOE :> es, ProgramE :> es) => Eff es, but
                        -- we need to update the Context τ in the ProgramE τ
                        -- effect with the one that was present just before we
                        -- ran the unlifting function.
                        Effect.putStaticRep (ProgramE context2)

                        -- now we can proceed with running the nested effects.
                        inner

I verified all this by inspection of the --debug=internal output from a crazy example that had was hacked to have a Program > Eff > Program > Eff > Program nesting, but for completeness, here's a trace in Honeycomb.

[guaraqe]

Nice!