Transform-Signal-Executor framework for Reactive Streams
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Transform-Signal-Executor framework for Reactive Streams (RxJS only at the moment... 😞).

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In the era of the JavaScript fatigue, new JS frameworks pop up like mushrooms after the rain, each of them providing some new and revolutionary concepts. So overwhelming! That's why TSERS was created. It doesn't provide anything new. Instead, it combines some old and well-known techniques/concepts and packs them into single compact form suitable for the modern web application development.

Technically the closest relative to TSERS is Cycle.js, but conceptually the closest one is CALM^2. Roughly it could be said that TSERS tries to combine the excellent state consistency maintaining strategies from CALM^2 and explicit input/output gates from Cycle - the best from both worlds.

Hello TSERS!

The mandatory "Hello world" applicatin with TSERS:

import {Observable as O} from "rx"
import TSERS from "@tsers/core"
import ReactDOM from "@tsers/react"
import Model from "@tsers/model"

function main(signals) {
  const {DOM, model$: text$, mux} = signals
  const {h} = DOM

  const vdom$ = DOM.prepare(text$.map(text =>
    h("div", [
      h("h1", text),
      h("button", "Click me!")

  const click$ =$, "button", "click")
  const updateMod$ = text$.mod(
    click$.map(() => text => text + "!")

  return mux({
    DOM: vdom$,
    model$: updateMod$

TSERS(main, {
  DOM: ReactDOM("#app"),
  model$: Model("Tsers")

Core concepts

TSERS applications are built upon the three following concepts

  1. Signals flowing through the application
  2. Signal Transform functions transforming input signals into output signals
  3. Executors performing effects based on the output signals


Signals are the backbone of TSERS application. They are the only way to transfer inter-app information and information from main to interpreters and vice versa. In TSERS applications, signals are modeled as (RxJS) observables.

  • Observables are immutable so the defined control flow is always explicit and declarative
  • Observables are first-class objects so they can be transformed into other observables easily by using higher-order functions

TSERS relies entirely on (RxJS) observables and reactive programming so if those concepts are not familiar, you should take a look at some online resources or books before exploring TSERS. One good online tutorial to RxJS can be found here.

TODO: muxing and demuxing

Signal transforms

Assuming you are somehow familiar with RxJS (or some other reactive library like Kefir or Bacon.js), you've definitely familiar with signal transform functions.

The signature of signal transform function f is:

f :: (Observable A, ...params) => Observable B

So basically it's just a pure function that transforms an observable into another observable. So all observable's higher order functions like map, filter, scan (just to name a few) are also signal transformers.

Let's take another example:

function titlesWithPrefix(item$, prefix) {
  return item$
    .map(it => it.title)
    .filter(title => title.indexOf(prefix) === 0)

titlesWithPrefix is also a signal transform function: it takes an observable of items and the prefix that must match the item title and returns an observable of item titles having the given prefix.

titlesWithPrefix :: (Observable Item, String) => Observable String

And as you can see, titlesWithPrefix used internally two other signal transform functions: map and filter. Because signal transform functions are pure, it's trivial to compose and reuse them in order to create the desired control flow from input signals to output signals.

If the signals are the backbone of TSERS applications, signal transformers are the muscles around it and moving it.


After flowing through the pure signal transformers, the transformed output signals arrive to the executors. In TSERS, executors are also functions. But not pure. They are functions that do nasty things: cause side-effects and change state. That is, executors' signature looks like this:

executor :: Observable A => Effects

Let's write an executor for our titles:

function alertNewTitles(title$) {
  title$.subscribe(title => {
    alert(`Got new title! ${title}`)

And what this makes executors by using the previous analogy... signals flowing through the backbone down and down and finally to the... anus 💩. Yeah, unfortunately the reality is that somewhere in the application you must do the crappy part: render DOM to the browser window, modify the global state etc. In TSERS applications, this part falls down to executors.

But the good news is that these crappy things are (usually) not application specific and easily generalizable! That's why TSERS has the interpreter abstraction.

Application structure

As told before, every application inevitably contains good parts and bad parts. And that's why TSERS tries to create an explicit border between those parts: the interpreter abstraction.

The good (pure) parts are inside the signal transform function main, and the bad parts are encoded into interpreters.

Conceptually the full application structure looks like:

function main(input$) {
  // ... app logic ...
  return output$
interpreters = makeInterpreters()
output$ = main(interpreters.signals())


main function is the place where you should put the application logic in TSERS application. It describes the user interactions and as a result of those interactions, provides an observable of output signals that are passed to the interpreters' executor functions.

That is, main is just another signal transform function that receives some core transform functions (explained later) plus input signals and other transform functions from interpreters. By using those signals and transforms, main is able to produce the output signals that are consumed by the interpreter executors.


Interpreters are not a new concept: they come from the Free Monad Pattern. In common language (and rounding some edges) interpreters are an API that separates the representation from the actual computation. If you are interested in Free Monads, I recommend to read this article.

In TSERS, interpreters consist of two parts:

  1. Input signals and/or signal transforms
  2. Executor function

Input signals and signal transforms are given to the main. They are a way for interpreter to encapsulate the computation from the representation. For example HTTP interpreter provides the request transform. It takes an observable of request params and returns an observable of request observables (request :: Observable params => Observable (Observable respose)).

Now the main can use that transform:

function main({HTTP}) {
  const click$ = ....
  const users$ = HTTP.request(click$.map(() => ({url: "/users", method: "get"})).switch()
  // ...

Note that main doesn't need to know the actual details what happens inside request - it might create the request by using vanilla JavaScript, superagent or any other JS library. It may not even make a HTTP request every time when the click happens but returns a cached result instead! It's not main's business to know such things.

Some interactions may produce output signals that are not interesting in main. That's why interpreters have also possibility to define an executor function which receives those output signals and interprets them, (usually) causing some (side-)effects.

Let's take the DOM interpreter as an example. main may produce virtual dom elements as output signals but it's not interested in how (or where) those virtual dom elements are rendered.

function main({DOM}) {
  const {h} = DOM 
  return DOM.prepare(Observable.just(h("h1", "Tsers!")))

What to put into main and what into interpreters?

In a rule of thumb, you should use interpreter if you need to produce some effects. Usually this reduces into three main cases:

  1. You need to use Observable's .subscribe - you should never need to use that inside the main
  2. You need to communicate with the external world somehow
  3. You need to change some global state

Encoding side-effects into signal transforms or output signals?

In a rule of thumb, you should encode the side-effects into signal transform functions if the input signal and the side effect result signal have a direct causation, for example request => response.

You should encode the side-effects into output signals and interpret them with the executor when the input there is no input => output causation (only effects), for example VNode => ().

Why the separation of main and interpreters?

You may think that the separation of main and interpreters is just waste. What benefit you get by doing that? The answer is that separating those significantly improves testability, extensibility and the separation of concerns of the application.

Imagine that you need to implement universal server rendering to your application - just change the DOM interpreter to server DOM interpreter that produces HTML strings instead of rendering the virtual dom to the actual DOM. How about if you need to test your application? Just replace the interpreters with test interpreters so that they produce signals your test case needs and assert the output signals your application produces. How about if you need to implement undo/redo? Just change the application state interpreter to keep state revisions in memory. How about if you API version changes? Just modify you API interpreter to convert the new version data to the current one.

From theory to practice

Now that you're familiar with TSER's core concepts and the application structure, let's see how to build TSERS application in practice. This section is just a quick introduction. For more detailed tutorial, please take a look at the TSERS tutorial in the examples repository.

Starting the application

First you need to install @tsers/core and some interpreters. We're gonna use two basic interpreters: React DOM interpreter for rendering and Model interpreter for our application state managing.

npm i --save @tsers/core @tsers/react @tsers/model

Now we can create and start our application. @tsers/core provides a function that takes the main and the interpreters that are attached to the application. The official interpreter packages provide always a factory function that can be used to initialize the actual interpreter.

import TSERS from "@tsers/core"
import ReactDOM from "@tsers/react"
import Model from "@tsers/model"

function main(signals) {
  // your app logic comes here!

// start the application with model$ and DOM interpreters
TSERS(main, {
  DOM: ReactDOM("#app"),     // render to #app element
  model$: Model(0)           // create application state model by using initial value: 0

Adding some app logic inside the main

Now we can use the signals and transforms provided by those interpreters, as well as TSERS's core transform functions (see API reference below). Interpreters' signals and transform functions are always accessible by their keys. Also main's output signals match those keys:

function main(signals) {
  // All core transforms (like "mux") are also accessible 
  // via "signals" input parameter
  const {DOM, model$, mux} = signals
  const {h} = DOM

  // model$ is an instance of @tsers/model - it provides the application
  // state as an observable, so you can use model$ like any other observable
  // (map, filter, combineLatest, ...).

  // let's use the model$ observable to get its value and render a virtual-dom
  // based on the value. DOM.prepare is needed so that we can derive user event
  // streams from the virtual dom stream
  const vdom$ = DOM.prepare(model$.map(counter =>
    h("div", [
      h("h1", `Counter value is ${counter}`),
      h("", "++"),
      h("button.dec", "--")

  // model$ enables you to change the state by emitting "modify functions"
  // as out output signals. The modify functions have always signature
  // (curState => nextState) - they receive the current state of the model
  // as input and must provide the next state based on the current state

  // Let's make modify functions for the counter: when increment button is
  // clicked, increment the counter state by +1. When decrement button is clicked,
  // decrement the state by -1
  const incMod$ =$, ".inc", "click").map(() => state => state + 1)
  const decMod$ =$, ".dec", "click").map(() => state => state - 1)

  // And because the mods are just observables, we can merge them
  const mod$ = O.merge(incMod$, decMod$)

  // Finally we must produce the output signals. Because JavaScript functions
  // can return only one value (observable), we must multiplex ("mux") DOM
  // and model$ signals into single observable by using "mux" core transform
  return mux({
    DOM: vdom$,
    model$: model$.mod(mod$)

Again: more detailed tutorial can be found from the TSERS examples repository.

What's different compared to Cycle?

If you read through this documentation, you might wonder that TSERS resembles Cycle very much. Technically that's true. Then why not to use Cycle?

Although the technical implementations of TSERS and Cycle are very similar, their ideologies are not. Cycle is strongly driven by the classification of read-effects and write-effects which means that drivers are not "allowed" to provide signal transforms encoding side-effects. Instead, all side effects must go to sinks and their results must be read from the sources, regardless of the causation of the side-effect and it's input.

Cycle's drivers are also meant for external world communications only, hence e.g. maintaining the global application state with drivers is not "allowed" in Cycle (although maintaining it with e.g. Relay driver is!!).

In practice, those features in Cycle result in some unnecessary symptoms like the existence of isolation, usage of IMV instead of MVI (which works pretty well btw, until your intents start to depend on the model), proxy subjects usage, performance issues and unnecessary complexity whe sharing the state between parent and child components.

And those are the reasons for the existence of TSERS.

Core transforms API Reference


JavaScript allows function to return only one value. That means that main can return only one observable of signals. However, applications usually produce multiple types of signals (DOM, WebSocket messages, model state changes...).

That's why TSERS uses multiplexing to "combine" multiple types of signals into single observable. Multiplexing is way of combining multiple signal streams into one stream of signals so that different type of signals are identifiable from other signals.

The signature of mux is:

mux :: ({signalKey: signal$}, otherMuxed$ = Observable.empty()) => muxedSignal$

mux takes the multiplexed streams as an object so that object's keys represent the type of the multiplexed signals. mux takes also second (optional) parameter, that is a stream of already muxed other signals (coming usually from the child components) and merges it to output.

Usually you want to use mux in the end of main to combine all application signals into single observable of signals:

function main({DOM, model$}) {
  // ....
  return mux({
    DOM: vdom$,
    model$: mod$


De-muxing (or de-multiplexing) is the reverse operation for muxing: it takes an observable of the muxed signals, extracts the given signal types by their keys and returns also the rest of the signals that were not multiplexed

demux :: (muxedSignal$, ...keys) => [{signalKey: signal$}, otherMuxed$]

Usually you want to use this when you call child component from the parent component and want to post-process child's specific output signals (e.g. DOM) in the parent component:

const childOut$ = Counter({...signals, model$: childModel$})
const [{DOM: childDOM$}, rest$] = demux(childOut$, "DOM")


loop is a transform that allows "looping" signals from downstream back to upstream. It takes input signals and a transform function producing output$ and loop$ signals array - output$ signals are passed through as they are, but loop$ signals are merged back to the transform function as input signals.

const initialText$ = O.just("Tsers").shareReplay(1)
const vdom$ = loop(initialText$, text$ => {
  const vdom$ = DOM.prepare(text$.map(...))
  const click$ =$, "button", "click")
  const updatedText$ = click$.withLatestFrom(text$, (_, text) => text + "!")
  // vdom$ signals are passed out, updatedText$ signals are looped back to text$ signals
  return [vdom$, updatedText$]


Takes a list observable (whose items have id property) and iterator function, applies the iterator function to each list item and returns a list observable by using the return values from the iterator function (conceptually same as list$.map(items =>

  • Item ids must be unique within the list.
  • Iterator function receives two arguments: iterated item id and an observable containing the item and it's state changes

ATTENTION: iterator function is applied only once per item (by id), although the list observable emits multiple values. This enables some heavy performance optimizations to the list processing like duplicate detection, cold->hot observable conversion and caching.

TODO: example...


Same as mapListById but allows user to define custom identity function instead of using id property. Actually the mapListById is just a shorthand for this transform:

const mapListById = mapListBy(item =>


demuxCombined has the same API contract as demux but instead of bare output signals, demuxCombined handles a list of output signals. The name already implies the extraction strategy: after the output signals are extracted by using the given keys, their latest values are combined by using Observable.combineLatest, thus resulting an observable that produces a list of latest values from the extracted output signals. Rest of the signals are flattened and merged by using Observable.merge so the return value of demuxCombined is identical with demux (hence can be used in the same way when muxing child signals to parent's output signals).

TODO: example...

Interpreter API reference

TODO: ...



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