[Haskell-cafe] Reactive config file generation

Heinrich Apfelmus apfelmus at quantentunnel.de
Wed Aug 19 16:12:04 UTC 2015

Hello David,

author of the FRP library "reactive-banana"[1] here.

Well, you can certainly use FRP in this context, but your situation 
seems so simple that I'm not sure if it's worth it here. The way I see 
FRP is that it helps you deal with time-varying state in a way that does 
not lead to spaghetti code. The fundamental insight is a semantic one, 
but on the practical side, FRP simplifies change propagation and 
reasoning about the order of changes.

Your situation looks like you do not have to keep track of state, i.e. 
most of your operations seem to be pure or idempotent, so while FRP 
won't hurt it, it won't make your life significantly easier either.

On the question of choosing a library, I would, of course, recommend 
"reactive-banana", but I may be a little biased there. :) I can also 
recommend "sodium" [2], which is very similar. There are also two very 
recent offerings, "reflex" [3] and "frpnow" [4]. I don't know much about 
the former. The latter has a nice theory background, but seems to be 

Best regards,
Heinrich Apfelmus

   [1]: https://wiki.haskell.org/Reactive-banana
   [2]: https://hackage.haskell.org/package/sodium
   [3]: https://hackage.haskell.org/package/reflex
   [4]: https://hackage.haskell.org/package/frpnow


David Turner wrote:
> Hi all,
> I am using Apache Zookeeper as a service directory: I have a bunch of
> services which announce their presence by making nodes in Zookeeper, so
> that dependent services can update their configuration to make use of the
> available services, and stop trying to use services that have died.
> Zookeeper is a pretty nice fit for this because it supports watching a node
> for changes, so in theory there is no need to poll Zookeeper periodically.
> The services that I control work with this just fine, but there are some
> (e.g. an Nginx reverse-proxy) that are reconfigured using the rather more
> common approach of updating a file (or files) and then sending the process
> a signal. I am currently pondering how to make this work without polling,
> or manually triggering a refresh script, which is how it is currently done.
> I've never used FRP, but am at least vaguely aware of it and from my
> high-level understanding it seems like this could be a very good fit. The
> Zookeeper state is a time-varying value which I want to convert into a
> time-varying set of files, ideally as declaratively as possible.
> So, two questions to the FRP congnoscenti out there. Firstly, is it
> worthwhile to attempt this using FRP at all? I'm sure I could do it by hand
> too. Secondly, which of the many FRP libraries would you recommend for it?
> There are loads on the Wiki, seemingly in various states of repair and
> documentation. Any pointers on how to choose one that's featureful enough,
> performant enough, and being actively maintained?
> Thanks in advance,
> David
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