Effects.js

Algebraic effects in javascript with scoped handlers, multishot delimited continuations and do notation

How to start?

You can try it out at codesandbox, or install it in npm:

$ npm install effkit

Documentation

You can read the docs here.

What are Algebraic Effects?

Algebraic effects are based on two primary concepts: effects and handlers. Effects are just a representation of an action that can be performed. Handlers will catch the performed effects and can choose to resume the program (continuation) with a result (like a promise), resume the continuation multiple times (like a forEach), or not resume at all and cancel the computation (like an exception). Handlers can also transform the result of the computation (into a promise, an array, etc).

To learn more about algebraic effects, see here.

Algebraic effects bring a multitude of advantages:

• dependency injection
• programming in direct-style (like async await - but for any data structure: promises, arrays, generators, etc)
• maintaining code pure (referentially transparent) while working with effects
• many control flow constructs can be expressed with only algebraic effects: async/await, coroutines/fibers, generators, exceptions, backtracking, react hooks & suspense, and more
• combining monads (almost any monad can be represented as an effect, which allows them to be used/chained together)

It's easier to understand what it allows by seeing it in action:

  // create your effects
  const getAuth = effect()
  const getUser = effect()
  const sendNotification = effect()
  
  // write your program in direct style using the generator do notation
  const onUserClick = eff(function* () {
     // get the current request from express
     const auth = yield getAuth() 

     // await for async call
     const user = yield getUser(auth.user.id) 
     
     // throw recoverable exception
     const token = user.token || yield raise(new MissingTokenError())
     
     // for each subscriber in the users list of subscribers
     const subscriber = yield forEach(user.subscribers) 
     
     // await for async call
     const result = yield sendNotification(subscriber, 'clicked', { details: mouseEvent, user, token }) 

     return { user, subscriber, result }
  }) 

  // handle your effects   
  express.post('actions/user-clicked', async (req, res) => {
   const withDependencies = handler({
     getAuth: () => resume(req.auth),
     getUser: (id) => getFirebaseUser(...),
     sendNotification: (subscriber, type, data) => sendFirebaseNotification(...),
   })
   const result = await run(pipe(
     onUserClick,
     handleError((e) => e instanceof MissingTokenError && cachedUser ? resume(cachedUser, k) : raise(e)) // recover with CachedUser if possible
     withDependencies,
     withForEach,
   ))
   // the final value depends on how you order the handlers 
   console.log(result) // [{ user, subscriber1, result1 }, { user, subscriber2, result2 }, ...], 
   
   res.send(result)
  })

All of the effects (request, getUser, sendNotification, etc) are highly testable, and can be replaced with testing/production/alternative versions.

This library

This library brings a algebraic effects implementation to Javascript using an Action monad, which means you can use the monadic API (map, chain, of), or use generator functions as a "do notation" to make the code look more natural. It is based on the languages Koka and Eff, and tries to bring all the algebraic effects features they have.

Performance

See benchmarks, it is expected to perform better than using native Promises (although they can't really be compared, because Algebraic Effects completely encapsulates Promises and is infinitely more extensible). Still, just like async/await code (or javascript code in general), it should not be used for cpu-heavy computations, only for non-blocking IO.

Stack-safety

It's 100% stack-safe!

Assistance is welcome

Feel free to create PRs or issues about bugs, suggestions, code review, questions, similar projects, improvements, etc. You can also get in contact with me, don't be shy to send a message!

Inspirations

koka

Eff

fx-ts

forgefx

Acknowledgments

Thanks so much to the people who helped me with this library! Thanks to Ohad Kammar for answering all my questions on algebraic effects, and Michael Arnaldi for showing me how to implement custom interpreters in order to achieve stack-safety

Roadmap:

• Get rid of scope and resume limitations
• Create monadic API
• Add docs
• Make it 100% stack safe
• Benchmarks
• Descriptive errors on dev mode
• Make a do notation babel plugin to compile the generator into chains
• Make a typescript version
• Expose API functions that work only with generators, and API functions that work with raw monads and continuations
• Create normal handlers and control handlers (like in koka)