Building Expressive Monads in Javascript: Introduction

Rob Porter on June 07, 2019

The monad is a powerful design pattern that, when used correctly, can completely change how you think about handling values in Javascript (JS). Thi... [Read Full]
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Happy to see your interest in Monad. However, there are numerous errors throughout this tutorial beginning from a wrong definition of Monad.

I hope you continue to study what a Monad is. In the interim, for the sake of other learners, this tutorial should be taken offline until it can be corrected.

Give me a shout and I'll provide what information and answers I can to help you fix this tutorial.


If you can find code not working correctly, please post it here. I would love to know what you have found wrong in the code if so.

It was stated at the beginning of the article that is more of an expressive, practical approach in JS, and not a lesson in category theory or mathematics.

I'm more or less following the principal of not overloading the reader with information that isn't required in order to make use of the pattern in day to day programming. This approach is used often in many forms of education, where certain things are glossed over in order to serve the broader goal of transferring knowledge. One doesn't learn how a car's engine works when learning to drive, just how to operate it, as an example. But if you want to be a car mechanic: yes, you learn more than how to accelerate and break to get to know a lot more and know that everything is not so simple.


Explaining concepts clearly and simply is a laudable task. However, several parts, while perhaps being digestible, are factually wrong.

An example: emit is not part of monads. Also, emit and join are not the same thing.

If the goal is just to present some technique for writing JS programs then I have no qualms. If the technique is ascribed to monads, however, then I have to object.

emit as I described, is a made up term. I borrowed it from CouchDB because I do find it far more expressive. join was also made-up when monads were adapted to Haskell, as a term to remove an abstraction layer. This is why I mention about expressivity in the article, to someone unfamiliar the term join (and Haskell, etc) doesn't make a lot of sense and it's just another weird thing to learn that I feel could be made easier. In my opinion as long as the functionality of a join exists, the name doesn't matter. But I do know others do not share that opinion and that is fine.

Like I say above, naming things is hard. Expressivity helps people immensely overcome barriers to using new techniques. Some monad libraries for JS use value for a join, but to me value is not being used as a verb, so I prefer emit. When teaching folks how to use it, many pause and ask for why it's called join, which interrupts the flow in learning just to satisfy some historical reason that has nothing to do with Javascript.

When I do write a monad I always make sure that join is there, and emit is just added as an alias. Same with map and fmap and so on. I always make sure the Monad Laws pass (I didn't even mention those here because that would be a whole other explanation). But for an "introduction" that can get someone doing something that is so different and almost counter to how JS works, I think a lot of the historical names and advanced conventions can be postponed for another day.

I have no problems with the names you have chosen.

When I say emit is not the same as join, what I mean is that emit (as you have defined it) is not equivalent to join (as Haskell defines it — if you wish to use that as a point of reference).

Emit is in fact equivalent to extract, which is part of Comonad but not part of Monad. There is no way to do what emit does with a Monad.

You can define join from Monad, or join can be used in the definition of Monad — there are different equivalent ways to formulate Monad.

This is not a naming problem. This is a problem with the definition of Monad.

Well, at this point I'd have to say you'll need to also take it up with various other existing monad implementations that have been created in Javascript. These are all from folks who know way more than me and could answer better your concerns.

It's done in Professor Frisby's guide here and in his videos:

And in the Composing Software book (this is a part of it):

(He also points out rightly that one can just omit join altogether and use a chained identity for this kind of thing. That might actually be easier from an educational perspective, I'm not sure.)

As does the monet.js monad library which aliases join to be like .chain(a => a) which might as well be () => x in the end, it's just a semantic difference.

And this last one on Maybe by James Sinclair is another, just using a class-style monad.

For Monad to make sense, types cannot be ignored. That is why join is being misunderstood.

I have not watched Professor Frisby's explanation so I have no comment there.

Eric Elliot misunderstands join in his article you linked. I'll try and get in touch.

monet.js does it right. Read carefully what the type of join is.

James Sinclair describes join accurately and then, following that, explains why emit cannot be defined for Maybe. I recommend rereading his work carefully.


TS Identity interfaces

interface IdentityMonad<A> {
   * method that just returns the value contained within.
  emit: () => A

   * method which is intended to chain various monads together
  chain: <B>(f: (x: A) => B) => B
   * `map` it is the chain function with a built-in rewrapping of the resulting value into a
   * new Identity, which itself can be subject to map, chain, and emit on and on for as many
   * functions you'd like to apply to it.
  map: <B>(f: (x: A) => B) => IdentityMonad<B>

  inspect: () => string

interface IdentityFactory {
  <A>(x: A): IdentityMonad<A>


Great explanations. I liked how you started with Identity and slowly build up to Maybe.


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