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I'm taking "reactive" here to mean the language tracks data dependencies, so expensive computations are automatically made incremental. Facebook had a research language attempting to do "reactive" programming called Skip (originally Reflex), which is now defunct. The runtime made the language like textual statically-typed Excel.

The use-case was to abstract away having to manually figure out where to put caches and how to invalidate them on front-end servers. Rather, have the runtime figure out how to cache content and page fragments when e.g. rendering the Facebook timeline. However, it was too difficult to bridge the gap to the existing Hack codebase, iirc in particular to the entity system used. There were also a lot of headaches trying to figure out how to power the underlying cache invalidation system.

https://www.youtube.com/watch?v=AGkSHE15BSs

https://web.archive.org/web/20200219222902/http://skiplang.c...

The author I think means something slightly different though, closer to prologue where you define facts and then ask the runtime to make an inference about those facts.

⬐ bruce343434

my take on a reactive language was a tiny AST manipulator language. Since `a=b+3` assigned the ast `b+3` to `a`, you would implicitly get `a==4` when `b=1`. There was also an "eval" operator for when you really wanted `a=b+3` to just assign the evaluation of `b+3` (a single number) to a.

⬐ jcelerier

There are a ton of reactive languages though ? QML is a mainstream one used in plenty of UIs and shipped as part of Qt (https://qmlonline.kde.org), there's ReactiveML, Céu...

From what I can tell, incremental computation is starting to be used in practice:

Jane St has an OCaml library called Incremental:

"Data Driven UIs, Incrementally" by Yaron Minsky at Strange Loop 2018: https://www.youtube.com/watch?v=R3xX37RGJKE

This talk has a pretty impressive live JS table demo! It looks faster and nicer than most JS UIs I've seen.

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And the Skip language was developed Facebook. Apparently they started to use it for some production stuff, but somehow they decided against it and spun it out:

http://skiplang.com/

https://news.ycombinator.com/item?id=18077612

Julien Verlaguet - Reflex: Reactive Programming at Facebook -- https://www.youtube.com/watch?v=AGkSHE15BSs

Reflex was an earlier name or earlier incarnation of Skip.

⬐ Blaisorblade0

Is there some website with some more info to read, for all those like me who dislike videos? I already googled "reflex facebook programming language" and found very little.

Quoting the talk abstract (http://www.curry-on.org/2017/sessions/reflex-reactive-progra...):

> Reflex is a new programming language for reactive programming developed at Facebook. It has many interesting characteristics, mixing OO and functional programming, it allows rapid development of applications with “spreadsheet semantics.

EDIT: to clarify: that's literally all I found. (I dislike videos so much I had missed that abstract is also on Youtube).

⬐ janvite6

I don't think you'll find more about Reflex. The language is still in the late design stages, it has not yet been released. That video is probably the only public information about Reflex, straight from its creator's lips.

⬐ pgz

Currently this is vaporware, but there are a lot of interesting ideas. Can't wait to get my hands on it.

The syntax for Lenses is particularly cool imho. Since it's bootstrapped on top of OCaml I doubt it has HKT though.