1.1.0 • Public • Published


Create truly pure functional runtime envs

The goals

  • Separate pure functions from impure functions in a clear, composable manner, using ES2015.
  • Create easily testable logics, even impure ones
  • Easy inter-op with "regular" JS


npm i -S feffect


// index.js
const {
  effect, // the most basic atom for `feffect` 
= require('feffect')();
const world = require('./world');
// all this function does is to create an effect-intention.
// nothing actually gets executed here, this is truly pure.
const request = options => effect('write:net', { options });
const log = (...args) => effect('write:log', { args });
const main = impure(function* (url) {
  // yielding from an impure function
  // ACTUALLY executes the world's interpretation of the 'write:net' effect, see below
  const body = yield request(url);
  yield log(body);
// calling an impure function simply returns another effect, though
const mainEffect = main('http://example.com');
// the `run` function converts an effect (intention) into a Promise (action),
// according to the world's interpretation of the effect's type
run(mainEffect, world)
  .then(() => console.log('DONE'))
  .catch(() => console.error('Boo!'));
// world.js
const request = require('request');
// A world object must handle every type of effect the program uses.
// the handler receives 3 parameters:
// - the object passed as the effect's second parameter
// - a resolver function that marks the effect as successful with an optional value
// - a rejecter function that marks the effect as failed with an optional value
// You might notice that we don't explicitly handle any effect type which starts with
// the `impure:` prefix. These are reserved for internal usage.
module.exports = {
  'write:net': ({ options }, resolve, reject) => request(options, (err, resp, body) => {
    if (err) return reject(err);
    if (resp.statusCode >= 400) return reject(resp);
    return resolve(body);
  'write:log': ({ args }, resolve) => resolve(console.log(...args)),



Requiring feffect returns a function that, when called, creates an entirely new functional environment, with the API below.

env.effect(effectType, [effectParameters])

This is the most basic part of feffect. Every effect has a type, and an optional parameters object. Effects are immutable, with no way to gain direct access to their properties (not even a get access), except via ensure (below). Every effect type should be handled explicitly as part of a world object (again, below).

env.ensure(effect, [type, [params]])

When provided a single parameter, this function simply ensures it is an effect belonging to this env. When given a type, it also checks for type equality. When also given params, it checks that for every key in the params object, its value equals (===) the effect's.


const eft = effect('read:file', { path: './foo' })
ensure(eft) // true
ensure(eft, 'read:file') // true
ensure(eft, 'read:file', { path: './foo' }) // true
ensure(eft, 'write:file') // false
ensure(eft, 'read:file', { filePath: './foo' }) // false
ensure(eft, 'read:file', { path: './_foo' }) // false


This function accepts a generator function that can yield effect objects, and get back their resolved values. It returns a function, that when called, does nothing but return an effect object of type impure:call.

env.run(effect, world)

This function converts an effect (which is a symbol for an intent) into a Promise (which is a symbol for an action), via the world parameter's interpretation of the effect. Should the effect type not be handled by the world, this function rejects immediately.


This function returns an effect of the impure:concurrent type, which has a default interpretation of interpreting all effects in its effects parameter, according to the same world that interprets itself, and resolves with an array of the return values in the same order as their respective effects. Basically, concurrent is to effect objects, as Promise.all is to Promise objects.

The world object

The world object passed to the run function is not magic, but a plain JS object. For every type of effect your env uses, you must include it as a property of the world, with a value that looks like so: (effectParams, resolve, reject) => effectParams.shouldWork ? resolve(10) : reject(new Error('Meow')) Basically, use resolve to mark (with a possible value) a successful side-effect, and reject to mark a failed one - just like with a Promise.

Usage in testing

It is highly advised to test your entire program's logic with different world objects to simulate as many possible scenarios as you feel appropriate, while testing your actual world object separately, using Promises.

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