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JavaScript embedded effects compiler

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JavaScript embedded effects compiler

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This is a tool to build JavaScript to JavaScript transpilers(babel plugins) along with a few predefined ones. It extends JavaScript language with various computation effects by means of runtime libraries but without any syntax extension.

There are such libraries for:

Not yet implemented:

  • algebraic effects
  • probabilistic programming
  • parallel and distributed programming
  • persistent continuations
  • adaptive computations
  • reactive programming (with RxJS) - Old version

They are typically small libraries, some of them are just tiny wrappers around well-known interfaces, such as Promises or Observables.

The compiler converts ES2018 to ES2018 and doesn't need any syntax extensions. So it may be applied to results of other compilers targeting JS such as CoffeeScript, TypeScript etc.


EffectfulJS is a tool to build transpilers. There are many packaging options and usages, but typically the follow next pattern. Transformations usually contain a babel plugin (and/or macro) along with some runtime. They can be in a single package, with Effectful to be a peer dependency (along with a dev dependency). This way the transpiler can be installed with:

$ npm install --save 
$ npm install --save-dev @effectful/core

If the babel plugin is in

can be:
   "plugins": [/transform]

The package can also contain

file for zero-configuration using babel-plugin-macros. This works also for Create Reat App) where
is disabled.

Check the libraries documentation for exact details:

  • @effectful/js compiles to the most abstract code, single or double level depending on parameters
  • @effectful/es for two-levels syntax either concrete ECMAScript compatible effects or abstract for async, generator and async generators semantics overloading.
  • @effectful/es-inline A shortcut for @effectful/es with concrete ECMAScript effects implementations.

All the plugins and macros are just shortcuts to @effectful/core setting needed lower level options described in config.js thus generated code uses different runtime interfaces.


These examples are not the full list of features. The project is abstract, with a lot of different applications.

Persistent state

Save all your application state in DB or file. The state includes the point where it executes now along with all variables values implementing any serialization interface.

Restore the saved state after, maybe in a different process or even on a different hardware.

For the application, the state transfer will be transparent.

This feature greatly simplifies defining long-running workflows. If the program requires some not immediate action from a human, e.g. email-confirmation, or it should await some exact date. No needs to bother implementing all these details, just write a very simple script describing the business logic and nothing else.

Here's some shop business logic example.

function order(customer, item) {
  customer.balance -= item.price;
  item.vendor.balance += item.price;
  scheduleShipment(customer, item);


function may require some other people involved. They can reject the transaction and so the balances values must be reset to original levels, even if a few days already passed. We don't care about implementation details. Such as where and how to store the state, how to reset it, etc. Everything is done by some underlying generic framework. The framework is not a subject of EffectfulJS though.

There are a lot of other applications for a persistent state. It can be used also for debugging purposes. This makes it very simple to implement hot reloading and time traveling, as this just saving and restoring the application state at some particular execution points.

More details are in @effectful/es project.

Delimited continuations

This is the most generic computational effect. Any other can be implemented using it. It looks very similar to Coroutines (async functions and generators). But unlike Coroutines, it allows resuming computation from the same point more than once.

Unlimited continuation like

is a special case. They allow capturing all and only all execution context. While with delimited continuations it is possible to specify only some region. For example, for the persistent state, this allows saving not the whole application state, which may be big, but only some interesting part.

Check @effectful/cc for more details.

Implicit parallelism

Async functions considerably simplify asynchronous source code. But it serializes operation execution. Next operation executes only after the current is finished. If we want to run the operations in parallel we can use


Say, we have two async operations (probably some remote server requests) -

async function a() {
  const a = await A;
  const b = await B;

If these operations are independent they can be executed in parallel:

async function a() {
  const [a, b] = await Promise.all([A, B]);

This doesn't look more complex, but what if we make

dependent on
and add another operation
async function parDemo() {
  const a = await A;
  const b = await B(a);
  const c = await C;

Now it is impossible to start

is finished, but we can start
in parallel with
async function parDemo() {
  const [a, c] = await Promise.all([A, C]);
  const b = await B(a);

This version is not effective, operation

may take much longer than
and this will prevent
from starting execution. The correct version is:
async function parDemo() {
  let a;
  const [b, c] = await Promise.all([
    (async () => {
      a = await A;
      b = await B(a);

This is obviously significantly more complex, error-prone, harder to read and maintain. It will become even much worse with some next small changes. They may require almost full function rewrite.

Fortunately, EffectfulJS can do the transformation automatically, just specify which block you want to parallelize. The transpiler will automatically compute variables dependencies and structure the block parts in


Here is how the last example looks with EffectfulJS:

async function parDemo() {
  const a = await A;
  const b = await B(a);
  const c = await C;

Switching between parallel and sequential code is just a matter of adding "seq"/"par" directives.

This works for loops too. With the help of a persistent state some jobs may be delegated to WebWorker or some cloud server, making simple but effective distributed programs.

Reactive programming (WIP)

Reactive programs operate with values which change with time. While in the source code it looks like a single value variable:

function drawBox(root) {
  const down = event(root, "mousedown");
  if (down && down.button === 0) {
    const move = event(root, "mousemove");
    const up = event(root, "up");
    const cur = move || up;
    const box = (
style= {{ left: Math.min(down.x, cur.x) + pageXOffset, top: Math.min(down.y, cur.y) + pageYOffset, width: Math.abs(down.x - cur.x), height: Math.abs(down.y - cur.y) }} >
); const del = event(box, "contextMenu"); if (!del) root.appendChild(box); if (!move) return; } }

Here all the variables may change when some mouse event is received, while the program looks like it is executed only once. The framework is responsible for recalculating dependant parts of the program. Such programs are much easier to write, test, debug and maintain.

Logical programming (WIP)

This is a library for adding logical programming languages features to JavaScript. Most Prolog books start from classical bi-directional list append function. Depending on input arguments it may either append two lists or split some list in two. Here is an almost literal translation of its implementation in JavaScript:

function append(a, b, r) {
  const [h, t, rt] = newRefs();
  unify(a, cons(h, t));
  unify(r, cons(h, rt));
  append(t, b, rt);
  unify(a, nil());
  unify(r, b);

For front-end, this, for example, can be useful to specify some control's values constraints. In the next example, there are three cyclically dependent fields, namely payment amount without discount, with discount and the discount value. The end-user may enter two of them while the third will be calculated automatically.

function PaymentForm() {
  const [cost, discount, finalCost] = newRefs();
  constraint(finalCost === (cost * discount) / 100);
  return (
); }

The same can be done with the new React hooks, which is an implementation of computational effect relying on runtime only. Transpiler like EffectfulJS doesn't have runtime-only limitations, namely, React Hooks Rules. They significantly reduce the number of useful use cases. Some complex program logic may be simplified with loops, conditions and other common JavaScript constructs.

How it works

The compiler stratifies input JavaScript code into two levels. The first level (pure) contains JS constructs (expressions, statements) kept as is in generated code. The second level (effectful) contains effectful constructs. The compiler converts them into abstract function calls.

I'm abusing term pure here. The statements or expressions on this level can have any side effect already implemented in JavaScript. Namely IO, references, exceptions, loops, breaks, returns, etc.

The levels may be either explicit or implicit in syntax (two-level or single-level syntax resp.).

Async functions or generators syntax can be used for marking expressions of explicit effectful level.

async function a() {
  console.log("x:", await getX());

Unlike standard ECMAScript async function this will be converted to abstract API calls. The API is

The overloading may be either compile time - implementing some syntax transforms, or runtime - providing libraries with abstract interfaces implementations.

This is an example of one of a few possible outputs:

function a() {
  return M.chain(getX(), _1);
  function _1(a) {
    console.log("x:", a);


methods are provided by some runtime library. For example, its concrete implementation for Promises calls
function for
and rejects the promise in

The names and interfaces of the functions in the generated code are configurable.

There is a dozen of such functions required to be implemented in concrete effects implementation library but, usually, most of them are trivial.

The interface is based on Monads. However understanding Monads is required only to implement some new effects library. No needs to know anything about them to use the libraries.

With the implicit single-level mode the same code may be even more succinct:

function() {

There are more examples of input/output in the test folder.

It is arguable if explicit or implicit levels separation is better. This likely depends on what kind of effects is used. The succinct and more readable code is good, but if effects are heavy making them explicit may be better. So EffectfulJS compiler supports both options.


Distributed under the terms of The MIT License (MIT).

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