Include a helper function to move a MonadHold to the outside of a dynamic.

[michaelmesser]

The weird type signature is required because m1 must be PushM and m.

f :: forall t m a. (Reflex t, MonadHold t m) => (forall m1. (MonadHold t m1) => Dynamic t (m1 a)) -> m (Dynamic t a)
f d = do
    let events = updated d & pushAlways id
    _initial <- sample $ current d
    initial <- _initial
    holdDyn initial events

[cgibbard]

How is this being used? Do you find you need it very often?

The requirement of sampling the Dynamic immediately in order to execute the action initially is something that we usually try to avoid in widget code because it's stricter than most other operations, and can result in loops that are trickier than usual to decipher.

[michaelmesser]

@cgibbard I found a use for it when I had Dynamic t (Incremental t p) and I needed to transform the incremental in a way that required monad hold.

[ryantrinkle]

@2426021684 This is very similar to runWithReplace. Would that be helpful here?

One of the rules we try to abide by in all our widget code is not to be strict in any Behaviors (including the current of a Dynamic). It can lead to awkward code sometimes, but it also seems to make people far less likely to write code that loops, so we've considered it an acceptable tradeoff. On the other hand, there may be some other approach we could use to avoid loops. If you have any ideas, let me know!

[michaelmesser]

@ryantrinkle
This function appears to do the same thing as f

f2 :: (Adjustable t m, MonadHold t m) => Dynamic t (m a) -> m (Dynamic t a)
f2 d = uncurry holdDyn =<< runWithReplace (join (sample (current d))) (updated d)