Using a Reactive Contexts to work with reactive values in first-order expressions without lifts or combinators

gistfile1.js

behave1 = ...//some FRP behaviour of Number
behave2 = ...//some FRP behaviour of Number

behave3 = ReaCtx.with(function(re) {
    if (re.val(behave1) < 50) {
      return re.val(behave2) + 1000;
    } else {
      return 3;
    }
  });

// behave3 will update when behave1 changes
// it won't even respond to changes in behave2 until behave1 is first seen to be < 50
// Crucially, there is no precompilation like FlapJax, and no manual lifting like Elm -- just JavaScript expressions

Also, I think this approach would work for lots of Applicative-style expressions. I think it would work for nicely for computing with Futures or over Option types, for example.

Neat! I assume re.val() operates by throwing?

A danger would be side-effecting functions – this would leak out of the abstraction. (Still want Javascript continuations.)

Yep - it would need to throw in the case of Options or Futures. You wouldn't have to for FRP contexts as behaviours can deliver a value whenever they exist.

Definitely not equivalent to real effect handlers for the reasons you point out -- effectful values from the real world won't be closed over but recomputed every update. I suppose, as always, practical use-cases are needed to see what kind of confusion this might cause. I am optimistic because this is operationally similar to how Clojure's STM transactions behave: it seems to work okay just asking devs to be a bit more careful with effects when you're in a transaction context.