Someone asked about this on reddit today and I wanted to think about this a bit.
- This is written (or thought) from the perspective of a JS developer who discovered functional programming as recently as 2019 and has used monads.
- This is not a tutorial on monads, by the way.
Propriety
I still do not profess to know monads ... but I consider that I do understand core idea of functional monads. FWIW, I've implemented monads from scratch in Purescript through this fascinating resource.
Has monads changed the way I write JS code?
Absolutely.
It's quite normal for JS developers to be okay with doing possibleObject.someProperty when possibleObject is not guaranteed to be an object in the first place. That is, it could be "null" or "undefined". Learning monads and using them (the Maybe monad in this case) has helped me be aware of and mitigate this risk (of property access in vanilla JS). Now, I use get from Lodash or the fancier S.prop from Sanctuary depending on the project.
Functions that could throw errors (known and unknown) will need to be handled with try catch blocks. The Either monad has helped me not only be more conscious about such functions when I use them but also model my data structures and pipeline functions better to handle this easily. Think of this: instead of try catch all over the place, I wrap functions in a way that they will return a tuple of [error, data] and I just need to check for either to be null to know if the function worked or failed. No try catch blocks all over the place.
Monad's bind and map have helped me understand how useful such functions are when dealing with pipelines and data transformation (which is about 80% of my work). Maybe they are only useful when you write code in a functional-style but the utility is enormous nevertheless.
Learning monads has also helped me quickly identify certain patterns (for example, think of running an array of promises parallelly and then processing the result. In JS, this is [Promise<value1>, Promise<value2>, Promise<value3>] but using this data as-is is a messy business. But if you convert this into a Promise<[value1, value2, value3]>, suddenly, you only have to unwrap a single promise (via await) to get all the values and have fun using simple, synchronous array functions. This idea is called sequence and within the FP-world, most libraries provide this function).
Learning monads
While this is a completely subjective feeling, I think the best way to learn about monads would be to actually build them from scratch.
The monad challenges were hard. I spent days solving some of them and had to rope in some help from the FP community.
But in the end, while I came out of it battered, some of the core ideas of monads (and why we have them in the first place) got ingrained in me. And they've made writing and approaching problems (to be solved through code) much easier.
Also: