Add useSteppingAff hook

[jship]

This PR adds a useSteppingAff variant of the useAff hook that preserves the previous async effect's result up until the next async effect's run completes. We use this variant at Lumi in cases where we'd like to avoid showing a page load indicator when refetching data from the backend, as the stepping behavior allows us to show the previous value right up until we've finished the API calls.

[pete-murphy]

What do you think about exporting useAff' to allow a user to define custom hooks built on top of it? Contrived example, but I could imagine maybe wanting to pass something other than identity or const Nothing as the initUpdater if wanting to track loading state along with the Aff's result (to preserve and render the previous result but disable a button while some request is in flight, e.g.)

useReloadingAff
  :: forall deps a
   . Eq deps
  => deps
  -> Aff a
  -> Hook
       (UseAff deps { isLoading :: Boolean, result :: a })
       (Maybe { isLoading :: Boolean, result :: a })
useReloadingAff deps aff =
  React.useAff' initUpdater deps aff'
  where
  initUpdater = (map <<< map) (_ { isLoading = true })
  aff' = map { isLoading: false, result: _ } aff

[maddie927]

I'd want to have a pretty convincing use case. It's easy enough to copy/paste for current edge cases, so I worry it's not worth exposing that complexity.

[maddie927]

But happy to defer to others since I'm not an active user right now.

Same goes for merging/publishing this.

[pete-murphy]

It's easy enough to copy/paste for current edge cases, so I worry it's not worth exposing that complexity.

Keeping the external API simple makes sense to me 👍 In cases where I've wanted anything other than the Maybe a state that useAff returns I'd just use a separate useState and use useAff for scheduling. So for the snippet I had above I think I'd probably just do

useReloadingAff deps aff = React.do
  state /\ setState <- useState Nothing
  useAff deps do
    liftEffect $ setState (map (_ { isLoading = true }))
    result <- aff
    liftEffect $ setState \_ -> Just { isLoading: false, result }
  pure state

which is not much more complicated than the version using useAff'.

I'm not really an active user either aside from side projects. Lumi's use case for useSteppingAff seems like enough motivation for merging this as-is, but I'll leave this open for feedback for a couple more days before merging.