1.0.0 • Public • Published


Monadic Middleware

npm install --save monastic fluture momi

Middleware - specifically in the case of Connect, Express and Koa - is a mechanism which encodes several effects:

  • A build-up of state through mutation of the req parameter
  • An eventual response through mutation of the res parameter
  • Inversion of control over the continuation by means of calling the next parameter
  • Possible error branch by means of calling the next parameter with a value

If we would want to encode all of these effects into a data-structure, we could use a StateT(Future) -> StateT(Future) structure:

  • A build-up of state through the State monad
  • An eventual response through the right-sided value of Future
  • Inversion of control by passing the whole structure into a function
  • Possible error branch through the left-sided value of Future

In other words, the StateT(Future)-structure might be considered the Middleware monad. This packages exposes the Middleware monad, comprised of State from monastic and Future from Fluture. Besides the monad itself, it also exposes some utility functions and structures for practically applying Middleware. One such utility is the App class, which allows composition of functions over Middleware to be written more like what you are used to from middleware as it comes with Express or Koa.



$ npm install --save momi

On Node 12 and up, this module can be loaded directly with import or require. On Node versions below 12, require or the esm-loader can be used.

Deno and Modern Browsers

You can load the EcmaScript module from various content delivery networks:

Old Browsers and Code Pens

There's a UMD file included in the NPM package, also available via jsDelivr:

This file adds momi to the global scope, or use CommonJS/AMD when available.

Usage Example

import Z from 'sanctuary-type-classes';
import qs from 'querystring';
import http from 'http';

import {compose, constant} from 'monastic';
import {go, mount, get, put} from 'momi';

const queryParseMiddleware = go (function* (next) {
  const req = yield get;
  const query = qs.parse (req.url.split ('?')[1]);
  yield put (Object.assign ({query}, req));
  return yield next;

const echoMiddleware = (req => ({
  status: 200,
  headers: {'X-Powered-By': 'momi'},
  body: req.query.echo,
}), get);

const app = compose (
  constant (echoMiddleware)

mount (http, app, 3000);


  • Readme the code from Usage, ready to run.
  • Express shows how to embed Momi within Express.
  • Bootstrap an example showing application structure.
  • Real World how momi is being used in real life.

Package Sidebar


npm i momi

Weekly Downloads






Unpacked Size

15.9 kB

Total Files


Last publish


  • avaq
  • dicearr
  • davidchambers