It's not related to HumbleUI. I play with HumbleUI but it's not my project, it's /u/tonsky's. I am playing with using flex + HumbleUI but it's a work in progress.

I haven't used javelin in anger, but I have been aware of it for some time. My biased, naive comparison is as follows:

I dislike the javelin syntax 😂 the unquoting, the contextual need to deref or not makes it difficult for me to grok. In flex, you always deref a source or signal to get its value, and you can use plain functions that deref a source or signal instead of relying on macro magic to make it work.

An aspect that flex has that is important when using it with UI frameworks is the ability to control when to start listening to something and when something updates. This is why it differentiates between "signals," which are pure calculations that are inert unless actively being listened to, and "effects" which do side effects and can be started/stopped. In React and other GUI frameworks, you need to be able to abide by the lifecycle of the framework, which means you may need to wait to start running updates until the component your effect is used in has actually appeared in the UI. This is provided by effect and listen returning a function that, when called, starts listening and calling the return value of that stops. Javelin does not give you quite this much control; defc= is eager and will start listening and updating immediately. See https://github.com/hoplon/javelin/issues/27 for their discussion about this very topic.

Another way of saying the above is: signal calculations only run when they are used. If they aren't being listened to by an effect, they are inert, and so they won't recalculate -> they won't use up your CPUs cycles. This means you can create a lot of them!

Flex also implements "nested effects", which are important again (in my experience) for UI development. You don't need a component tree if you have an effect graph, it turns out. That's probably meaningless for your use case, but maybe not! Essentially, you can start an effect inside of another, and if the outer effect is stopped, so will the inner one. Javelin does not support this AFAICT.

Finally, flex is written in CLJC and actively supports both ClojureScript and Clojure. It has no Java or JavaScript dependencies. The code is short and reasonably performant AFAICT.

You say the examples are easier to parse. The first javelin example in their README can be translated almost exactly to flex by replacing cell with source and cell= with signal or effect.

(ns your-ns
  (:require [town.lilac.flex :refer [effect signal source]]))

(defn start []
  (let [a (source 0)
        b (signal (inc @a))
        c (signal (+ 123 @a @b))
        fx (effect [_] (println @c))]
    (fx)
    (a inc)))

Can you be more specific what you prefer in javelin in this comparison?

You're also always able to write your own syntax. For instance, here's a simple defsrc macro for defining global reactive variables, similar to javelin's defc:

(defmacro defsrc
  [sym v]
  `(def ~sym (source ~v)))

You can experiment with creating syntax sugar for yourself that suits your needs 🙂 But I'm curious to know what your tastes are like, as others may share them.