Weird casuality loop detection

[bglgwyng]

Here I made a reproducible.
https://github.com/bglgwyng/reflex-casuality-loop-report

The network description reads as follows.
You don't need to understand the following code. This is the result of the removal of domain-specific code from my project.

app = do
  eTick <- tickLossyFromPostBuildTime 0.1

  -- `mergeMapIncremental` cause the casuality loop
  e1 <- mergeMapIncremental <$> holdIncremental @_ @_ @(PatchMap Int (Event t ())) mempty never
  d1 <- holdDyn mempty e1

  dGate <- toggle False (eTick $> True)

  e2 <- mergeMapIncremental <$> holdIncremental @_ @_ @(PatchMap Int (Event t ())) mempty (eTick $> mempty)
  d2 <- holdDyn mempty e2

  let d3 = do
        d1
        gate <- dGate
        when gate (d2 $> ()) -- d2 is optionally evaluated by dGate
  performEvent_ $ updated d3 $> pure () -- subscribe `updated d3`
  pure never

I don't think there is any causality loop. The usage of mergeMapIncremental and toggling of evaluation of d2 in Dynamic monad is crucial to reproduce the causality loop.

Does anyone have an idea why this code is reported to include a loop?

[bglgwyng]

Here is the profiling result.

Main.main (app/Main.hs:11:1-25)
|
`- Reflex.Host.Headless.runHeadlessApp (src/Reflex/Host/Headless.hs:(61,1)-(147,12))
   |
   `- Reflex.Spider.Internal.withSpiderTimeline (src/Reflex/Spider/Internal.hs:(2749,1)-(2751,49))
      |
      `- Reflex.PerformEvent.Base.hostPerformEventT (src/Reflex/PerformEvent/Base.hs:(135,1)-(155,15))
         |
         `- Reflex.Spider.Internal.runFrame (src/Reflex/Spider/Internal.hs:(2347,1)-(2416,15))
            |
            +- Reflex.Spider.Internal.subscribe (src/Reflex/Spider/Internal.hs:416:1-44)
            |  |
            |  `- Reflex.Requester.Base.Internal.runRequesterT (src/Reflex/Requester/Base/Internal.hs:(315,1)-(318,122))
            |     |
            |     `- Reflex.TriggerEvent.Base.runTriggerEventT (src/Reflex/TriggerEvent/Base.hs:52:1-47)
            |        |
            |        `- Main.app (app/Main.hs:(24,1)-(41,12))
            |           |
            |           `- Reflex.Spider.Internal.newJoinDyn (src/Reflex/Spider/Internal.hs:(2567,1)-(2573,56))
            |              |
            |              +- Reflex.Spider.Internal.coincidence (src/Reflex/Spider/Internal.hs:(1286,1)-(1291,5))
            |              |  |
            |              |  `- Reflex.Spider.Internal.eventCoincidence (src/Reflex/Spider/Internal.hs:441:1-90)
            |              |     |
            |              |     `- Reflex.Spider.Internal.eventSubscribedCoincidence (src/Reflex/Spider/Internal.hs:(648,1)-(660,3))
            |              |
            |              `- Reflex.Class.mergeInt (src/Reflex/Class.hs:355:1-74)
            |                 |
            |                 `- Reflex.Spider.Internal.mergeInt (src/Reflex/Spider/Internal.hs:2173:1-37)
            |                    |
            |                    `- Reflex.Spider.Internal.mergeIntCheap (src/Reflex/Spider/Internal.hs:(2177,1)-(2282,59))
            |
            +- Reflex.Requester.Base.Internal.runRequesterT (src/Reflex/Requester/Base/Internal.hs:(315,1)-(318,122))
            |  |
            |  `- Reflex.TriggerEvent.Base.runTriggerEventT (src/Reflex/TriggerEvent/Base.hs:52:1-47)
            |     |
            |     `- Main.app (app/Main.hs:(24,1)-(41,12))
            |        |
            |        `- Reflex.Spider.Internal.newJoinDyn (src/Reflex/Spider/Internal.hs:(2567,1)-(2573,56))
            |           |
            |           +- Reflex.Class.mergeInt (src/Reflex/Class.hs:355:1-74)
            |           |  |
            |           |  `- Reflex.Spider.Internal.mergeInt (src/Reflex/Spider/Internal.hs:2173:1-37)
            |           |     |
            |           |     `- Reflex.Spider.Internal.cacheEvent (src/Reflex/Spider/Internal.hs:(335,1)-(382,72))
            |           |
            |           `- Reflex.Spider.Internal.cacheEvent (src/Reflex/Spider/Internal.hs:(335,1)-(382,72))
            |
            `- Reflex.Spider.Internal.subscribe (src/Reflex/Spider/Internal.hs:416:1-44)
               |
               `- Reflex.Requester.Base.Internal.runRequesterT (src/Reflex/Requester/Base/Internal.hs:(315,1)-(318,122))
                  |
                  `- Reflex.Class.mergeInt (src/Reflex/Class.hs:355:1-74)
                     |
                     `- Reflex.Spider.Internal.mergeInt (src/Reflex/Spider/Internal.hs:2173:1-37)
                        |
                        `- Reflex.Spider.Internal.mergeIntCheap (src/Reflex/Spider/Internal.hs:(2177,1)-(2282,59))

[bglgwyng]

I found that this issue is related to ApplicativeDo. When I disable the ApplicativeDo extension, the app works without the causality loop.

It seems that Applicative (Dynamic t) and Monad (Dynamic t) handle conditional reads differently. In the d3 definition, dGate makes the read of d2 conditional, and this conditional behavior appears to be treated differently under Applicative vs Monad operations when ApplicativeDo is enabled.