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The two-level syntax for effects based on ECMAScript async, generator and async generator functions syntax overloading.

The project is a babel plugin for integrating new custom computational effects into JavaScript.

The first level is plain JavaScript constructs, and the second level is async, generators and async generators function syntax. By means of the plugin default implementations for the effectful(second) level syntax can be overloaded.

As a proof of concept, there are default implementations of corresponding ECMAScript standard effects.

The project is a part of EffectfulJS tool chain.


Here is a list of things not available with native or other transpiler implementations:

  • Abstract API - can be re-implemented or amended for your purposes, next items are consequences of this one
  • Custome Concrete API for better performance
  • Persistent state
  • Implicit parallelism - Docs SOON
  • Deriving program's graph for static analysis - SOON


Abstract interface

$ npm install --save-dev @effectful/es
$ npm install --save @effectful/es-rt

For example:

  "plugins": ["@effectful/es/transform"]


$ babel --plugins @effectful/es/transform index.js

By default it injects imports for @effectful/es-rt.

It is an implementation of the abstract interface for EcmaScript async, generators and async functions. The interface doesn't yet have documentation. The default implementation is pretty small and can be used as a reference for now. There is importRT option to pass some overloaded concrete implementations libraries.

  "plugins": [["@effectful/es/transform",{"importRT":"my-custom-effects"}]]

Zero-config transformation

Zero-configuration using babel-plugin-macros, or any other tool where it is enabled by default (such as Create Reat App since v2).

Just import "@effectful/es/macro" module in the module you want to transpile.

Inlined concrete implementation

$ npm install --save-dev @effectful/es
$ npm install --save @effectful/es-inline-rt

This .babelrc

  "plugins": [["@effectful/es/transform",{"inline":true}]]

It is a much faster implementation, but there is no way to overload anything.

Other options are:

  • loose - like inline but it also drops a few ECMAScript compatibility features to make the generated code even more smaller and faster
  • defunct - if true state is encoded as number and a there is a single function to move between states; otherwise, the state is a state transition function callback
  • topLevel - if true moves state transition functions outside original function's body
  • par - enables implicit parallelism. It doesn't yet work with "inline", "loose" and "topLevel"

There are quite a few of lower level options described in config.js.

Their values can be provided using all, file, pure, effectful, generators, async, asyncGenerators fields in the plugin options. For example, to use CommonJS modules instead of defaulting ES for runtime injection use:

  "plugins": [["@effectful/es/transform",{"file":{"modules":"commonjs"}}]]

Implicit parallelism

If enabled the compiler will locate "par" and "seq" in the beginning of the block to switch between parallel and sequential mode resp.

For example:

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

ESLint complains about not used expressions, so as an alternative to the directives there is profile function which expects a single string literal parameter ("par" or "seq"):

import * as M from "@effectful/es"

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


By default, the runtime also adds M.cancelSymbol property to each returned Promise. It is a function and it tries to stop the execution of the corresponding async function. The cancelation is propagated to await expressions if they have this method, but it isn't propagated to children.

There is also a helper function M.cancel which simply calls promise[M.cancelSymbol]() if it exists.


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

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