What's the reason updateAction API doesn't allow modifiction?

[nanonaren]

Is there any particular reason for Control.AutoUpdate.updateAction not providing an API to update the existing value? Seems rather a useful thing to have to me.

[snoyberg]

Can you clarify what kind of API you're looking for, ideally with a type signature?

[nanonaren]

So, I was looking for something like the following

updateAction :: Maybe a -> IO a

That way, when run run the first time it is called with updateAction Nothing and at later times as updateAction (Just a) where the current value is supplied as argument.

[snoyberg]

I'm not actually seeing how such a function would work. Are you missing a parameter? Is this intended to be a field in a data type?

[MaxGabriel]

@nanonaren wants something like this I think:

foobar :: IO (IO Text)
foobar = mkAutoUpdate defaultUpdateSettings
  { updateAction = \maybePreviousValue -> foo -- Potentially use previous value to calculate the new value
  , updateFreq = 24 * 60 * 60 * 1000000 -- Update once per day
  }

@nanonaren It would probably help clarify things if you provided an example use case

[nanonaren]

The example by @MaxGabriel is what I'm looking for. I have a structure in memory that gets updated periodically. Specifically, it maintains a vector of computed values based on recent data in the DB. So it pulls in records after the last update time and then drops very old items. This is why I need to update the previous value rather than refresh it with a completely new one.

[snoyberg]

OK, that makes sense. To provide for maximum performance and not break backwards compatibility, I'd imagine we could add a new field:

updateActionModify :: Maybe (a -> IO a)

The idea would be:

• updateAction would always be used for the first action
• If provided, updateActionModify would be used for subsequent actions
• If not provided, current behavior continues

[MaxGabriel]

@nanonaren Are you interested in implementing this?

[nanonaren]

Sure, I'll do it. Please check if it makes sense...

[nanonaren]

^ anyone want to check?

[snoyberg]

LGTM. I didn't review this earlier since Github doesn't send notifications for commits pinging issues/PRs.