race condition for fast effects

[joelgrus]

I want an Action to do nothing to the State and kick off an effect (whose result does something to the state). The problem is that in update I don't see a way not to set the state, even if I want it not to change. And if the effect returns very quickly, its signal gets rendered before the do-nothing change to the state. Here is a really contrived minimal example (the actual way I found this was more complicated and involved random numbers):

data Action = Increment | EffIncrement | Receive Int

type State = Int

update :: Action -> State -> EffModel State Action (console :: CONSOLE)
update Increment count =
  trace ("Increment: " ++ show count) \_ -> { state: count + 1, effects: [] }
update (Receive c) count =
  trace ("Receive: " ++ show c) \_ -> { state: c, effects: [] }
update EffIncrement count =
  -- I don't actually want to change the state here, but what can I do?
  trace "EffIncrement" \_ -> { state: count, effects: [pure $ Receive (count + 1)]}

view :: State -> Html Action
view count =
  div # do
    button ! onClick (const Increment) # text "Increment"
    span # text (show count)
    button ! onClick (const EffIncrement) # text "EffIncrement"

If I click on EffIncrement, the Receive (count + 1) signal gets rendered first. So if I click EffIncrement, what happens is the Receive 1 redraw happens before the state: 0 redraw, so the state gets updated but the UI appears not to. Then if you (e.g.) click Increment it goes up to 3.

A hacky solution is to artificially add a small delay to my fast effects, is there a better way?

(gist with full code: https://gist.github.com/joelgrus/f65b4106cdf32f4bc1109f0e1a0e0497)

[alexmingoia]

You should not be incrementing the count in your effect. Instead, the Receive action should include the amount to increment not the new count, because that count might be stale.

[joelgrus]

I think what you're saying is change it to

  trace "EffIncrement" \_ -> { state: count, effects: [pure $ Increment]}

But that doesn't solve the problem. I think the state updates are happening in the right order, it's the renderings that get out of sequence? i.e., I think (modulo me not understanding purescript-signal that well) what's happening is something like

• set state to count
• send "render" signal with state = count (A)
• receive "effectful" Increment action
• increment state to count + 1
• send "render" signal with state = count + 1 (B)
• receive "render" signal with state = count + 1 (from B, out of order)
• receive "render" signal with state = count (from A)

Does that make sense?

[alexmingoia]

Ah, I see the issue. This looks like a bug. I'm working on it.

[alexmingoia]

This is fixed in v2.0.1