Sure thing, but what is a monad anyway?
this post was submitted on 27 Mar 2025
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It's a container with certain behaviors and guarantees making them easy and reliable to manipulate and compose. A practical example is a generic List, that behaves like:
List[1, 2, 3], i.e. ("new", "unit", "wrap") to create, containing obj(s)map(func)to transform objs inside, List[A] -> List[B]first(), i.e. ("unwrap", "value") to get back the objflat_map(func), i.e. ("bind") to un-nest one level whenfunc(a)itself produces another List, e.g.[3, 4].flat_map(get_divisors) == flatten_once([[1, 3], [1, 2, 4]]) == [1, 3, 1, 2, 4]
Consider the code to do these things using for loops -- the "business logic" func() would be embedded and interlaced with flow control.
The same is true of Maybe, a monad to represent something or nothing, i.e. a "list" of at most one, i.e. a way to avoid "null".
Consider how quickly things get messy when there are multiple functions and multiple edge cases like empty lists or "null"s to deal with. In those cases, monads like List and Maybe really help clean things up.
IMO the composability really can't be understated. "Composing" ten for loops via interlacing and if checks and nesting sounds like a nightmare, whereas a few LazyList and Maybe monads will be much cleaner.
Also, the distinction monads make with what's "inside" and what's "outside" make it useful to represent and compartmentalize scope and lifetimes, which makes it useful for monads like IO and Async.
It's a monoid in the category of endofunctors. Obviously.
In practical terms, it's most commonly a code pattern where any function that interacts with something outside your code (database, filesystem, external API) is "given permission" so all the external interactions are accounted for. You have to pass around something like a permission to allow a function to interact with anything external. Kind of like dependency injection on steroids.
This allows the compiler to enhance the code in ways it otherwise couldn't. It also prevents many kinds of bugs. However, it's quite a bit of extra hassle, so it's frustrating if you're not used to it. The way you pass around the "permission" is unusual, so it gives a lot of people a headache at first.
This is also used for internal permissions like grabbing the first element of an array. You only get permission if the array has at least one thing inside. If it's empty, you can't get permission. As such there's a lot of code around checking for permission. Languages like Haskell or Unison have a lot of tricks that make it much easier than you'd think, but you still have to account for it. That's where you see all the weird functions in Haskell like fmap and >>=. It's helpers to make it easier to pass around those "permissions".
What's the point you ask? There's all kinds of powerful performance optimizations when you know a certain block of code never touches the outside world. You can split execution between different CPU cores, etc. This is still in it's infancy, but new languages like Unison are breaking incredible ground here. As this is developed further it will be much easier to build software that uses up multiple cores or even multiple machines in distributed swarms without having to build microservice hell. It'll all just be one program, but it runs across as many machines as needed. Monads are just one of the first features that needed to exist to allow these later features.
There's a whole math background to it, but I'm much more a "get things done" engineer than a "show me the original math that inspired this language feature" engineer, so I think if it more practically. Same way I explain functions as a way to group a bunch of related actions, and not as an implementation of a lambda calculus. I think people who start talking about burritos and endofunctors are just hazing.
I don’t know if this is correct, but if it is, this is best answer to this question I’ve ever seen.
I'm sure someone will be like "um akchuly" to my explanation. But for me it's good enough to think if it that way.
I've worked in Haskell and F# for a decade, and added some of the original code to the Unison compiler, so I'm at least passingly familiar with the subject. Enough that I've had to explain it to new hires a bunch of times to get them to to speed. I find it easier to learn something when I'm given a practical use for it and how it solves that problem.
Lovely response! Very cool to see Unison mentioned. Haskell and Purescript are my daily drivers but I have a huge crush on it even though it intimidates me.
Ps. Unison doesn’t have monads. They are replaced by “abilities”.
It is, although I'm not sure it's complete. A list is one kind of monad, despite working like non-mutable linked lists would in any other language. They just happen to behave monadically, providing an obvious and legal interpretation of the monad functions. Going off of OP you might think monads are all Maybe.
I will say that the concept is overhyped in at this point, at least in Haskell, and there's a lot of monads available that do what plain functional code could but worse.
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It's a month in Sweden
No. That's a månad.
Close enough
Idk. I tried a string comparison and rustc said equality was false. 😝
It's just a monoid object in a category of endofunctors, no biggie
Whatever Haskell programmers decide to call a monad today. It's wandered pretty far away from whatever mathematical definition, despite insistences to the contrary.
(Technically, the requirement is to implement a few functions)
Only monad I know is xmonad. My favourite x11 window manager.
Monads are like a burrito.
A reproductive organ
Not Mossad.
It's a burrito
Can't spell "functional" without "fun"!
N is for No Surviiiivors, here in the deep blue sea!
This is why I make sure that nothing I code functions in any way at all
functional programmers when they look at their code 2 years later
~~functional~~ programmers when they look at their code 2 years later
FTFY
Yeah, no side-effects seems like it could only improve readability.
Functional programmers still pretending side effects snd reals world applications don’t exist.
As a senior engineer writing Haskell professionally, this just isn't really true. We just push side effects to the boundaries of the system and do as much logic and computation in pure functions.
It's basically just about minimizing external touch points and making your code easier to test and reason about. Which, incidentally, is also good design in non-FP languages. FP programmers are just generally more principled about it.
Reasoning about memory use for example is difficult with FP.
Using pure functions is a good idea for non FP languages as well.
I've never had the chance to use a functional language in my work, but I have tried to use principles like these.
Once I had a particularly badly written Python codebase. It had all kinds of duplicated logic and data all over the place. I was asked to add an algorithm to it. So I just found the point where my algorithm had to go, figured out what input data I needed and what output data I had to return, and then wrote all the algorithm's logic in one clean, side effect-free module. All the complicated processing and logic was performed internally without side effects, and it did not have to interact at all with the larger codebase as a whole. It made understanding what I had to do much easier and relieved the burden of having to know what was going on outside.
These are the things functional languages teach you to do: to define boundaries, and do sane things inside those boundaries. Everything else that's going on outside is someone else's problem.
I'm not saying that functional programming is the only way you can learn something like this, but what made it click for me is understanding how Haskell provides the IO monad, but recommends that you keep that functionality at as high of a level as possible while keeping the lower level internals pure and functional.
It heavily depends on the application, right? Haskell is life for algorithmically generating or analysing data, but I'm not really convinced by the ways available in it to do interaction with users or outside systems. It pretty much feels like you're doing imperative code again just in the form of monads, after a while. Which is actually worse from a locality of ~~reference~~ behavior perspective.
Not really, it's just good practice. You write your application in layers, and the outer layer/boundary is where you want your side effects and that outer layer takes the crazy effectful world and turns it sane with nice data types and type classes and whatnot and then your inner layers operate on that. Data goes down the layers then back up, at least in my experience with functional projects in OCaml, F#, Clojure, and Haskell.
The real sauce is immutability by default/hard-to-do mutation. I love refs in OCaml and Clojure, so much better than mutation. Most of the benefits of FP are that and algebraic data types, in that order imo.
I'm not sure what you mean by "locality of reference". I assume you mean something other than the traditional meaning regarding how processors access memory?
Anyway, it's often been said (half-jokingly) that Haskell is a nicer imperative language than imperative languages. Haskell gives you control over what executing an "imperative" program actually means in a way that imperative languages don't.
To give a concrete example: we have a custom monad type at work that I'm simply going to call Transaction (it has a different name in reality). What it does is allow you to execute database calls inside of the same transaction (and can be arbitrarily composed with other code blocks of type Transaction while still being guaranteed to be inside of the same transaction), and any other side effects you write inside the Transaction code block are actually collected and deferred until after the transaction successfully commits, and are otherwise discarded. Very useful, and not something that's very easy to implement in imperative languages. In Haskell, it's maybe a dozen lines of code and a few small helper functions.
It also has a type system that is far, far more powerful than what mainstream imperative programming languages are capable of. For example, our API specifications are described entirely using types (using the servant library), which allows us to do things like statically generate API docs, type-check our API implementation against the specification (so our API handlers are statically guaranteed to return the response types they say they do), automatically generate type-safe API clients, and more.
We have about half a million lines of Haskell in production serving as a web API backend powering our entire platform, including a mobile app, web app, and integrations with many third parties. It's served us very well.
I’m not sure what you mean by “locality of reference”. I assume you mean something other than the traditional meaning regarding how processors access memory?
Shit! Sorry, got my wires crossed, I actually meant locality of behavior. Basically, if you're passing a monad around a bunch without sugar you can't easily tell what's in it after a while. Or at least I assume so, I've never written anything big in Haskell, just tons of little things.
To give a concrete example:
Yeah, that makes tons of sense. It sounds like Transaction is doing what a string might in another language, but just way more elegantly, which fits into the data generation kind of application. I have no idea how you'd code a game or embedded real-time system in a non-ugly way, though.
It also has a type system that is far, far more powerful than what mainstream imperative programming languages are capable of.
Absolutely. Usually the type system is just kind of what the person who wrote the language came up with. The Haskell system by contrast feels maximally precise and clear; it's probably getting close to the best way to do it.
Shit! Sorry, got my wires crossed, I actually meant locality of behavior. Basically, if you're passing a monad around a bunch without sugar you can't easily tell what's in it after a while. Or at least I assume so, I've never written anything big in Haskell, just tons of little things.
I'm not sure if I entirely follow, but in general you actually have much better locality of behavior in Haskell (and FP languages in general) than imperative/OOP languages, because so much more is explicitly passed around and immutable. Monads aren't an exception to this. Most monadic functions are returning values rather than mutating some distant state somewhere. Statefulness (or perhaps more precisely, mutable aliasing) is the antithesis of locality of behavior, and Haskell gives you many tools to avoid it (even though you can still do it if you truly need it).
I'm not really sure what you mean by "don't really know what's in it after a while". It might be helpful to remember that lists are monads. If I'm passing around a list, there's not really any confusion as to what it is, no? The same concept applies to any monadic value you pass around.
Yeah, that makes tons of sense. It sounds like Transaction is doing what a string might in another language, but just way more elegantly,
I think you might have misunderstood what I was describing. The code we write doesn't actually change, but the behavior of the code changes due to the particular monad's semantics. So for example, let's say I write a query that updates some rows in a table, returning a count of the rows affected. In this Transaction code block, let's say I execute this query and then send the returned number of rows to an external service. In code, it looks like the API call immediately follows the database call. To give some Haskell pseudocode:
example :: Transaction ()
example = do
affectedRows <- doUpdateQuery
doApiCall affectedRows
return ()
But because of how Transaction is defined, the actual order of operations when example is run becomes this:
- Send
BEGIN;to DB - Call
doUpdateQuery - Send
COMMIT;to DB - If transaction was successfully committed, execute
doApiCall affectedRows. Otherwise, do nothing
In essence, the idea is to allow you to write code where you can colocate your side-effectful code with your database code, without worrying about accidentally holding a transaction open unnecessarily (which can be costly) or firing off an API call mistakenly. In fact, you don't actually have to worry about managing the transaction at all, it's all done for you.
which fits into the data generation kind of application. I have no idea how you'd code a game or embedded real-time system in a non-ugly way, though.
I mean, you're not going to be using an SQL database most likely for either of those applications (I realize I assumed that was obvious when talking about transactions, but perhaps that was a mistake to assume), so it's not really applicable.
I also generally get the impression that you have a notion that Haskell has some special, amorphous data-processing niche and doesn't really get used in the way other languages do, and if that's the case, I'd certainly like to dispel that notion. As I mentioned above, we have a pretty sizeable backend codebase written in Haskell, serving up HTTP JSON APIs for a SaaS product in production. Our APIs drive all (well, most) user interaction with the app. It's a very good choice for the typical database-driven web and mobile applications of businesses.
Ironically, I actually probably wouldn't use Haskell for heavy data processing tasks, namely because Python has such an immense ecosystem for it (whether or not it should is another matter, but it is what it is).. What Haskell is great at is stuff like domain modeling, application code (particularly web applications where correctness matters a lot, like fintech, healthcare, cybersecurity, etc.), compilers/parsers/DSLs, CLI tools, and so on.*
Taking a wild guess at the source of the confusion, I should be clear that I love Haskell. It's great for a lot of what I personally end up coding, namely math things that are non-heavy by computer standards but way too heavy to solve by hand. This isn't naysaying.
I mean, you’re not going to be using an SQL database most likely for either of those applications (I realize I assumed that was obvious when talking about transactions, but perhaps that was a mistake to assume), so it’s not really applicable.
To be clear, I was introducing two new examples where I think this problem would come up. It could be that I'm missing something, but I've had this exchange a few times and been unimpressed by the solutions offered. The IO in those cases could get pretty spaghetti-ish. At that point, why not just use a state?
Like, using a list, which is a monad, you could code a Turing machine, and it could have a tape specifying literally anything. I can't imagine that one would ever come up, though.
Ironically, I actually probably wouldn’t use Haskell for heavy data processing tasks, namely because Python has such an immense ecosystem for it (whether or not it should is another matter
It certainly is, haha. If it's heavy Python is just calling Fortran, C or Rust anyway.
Excellent write-up. People who complain about Haskell and purely functional languages just don't understand it, I think. Take me for example. I tried learning Haskell many years ago, and while I learned so many new and incredibly useful concepts from my short adventure, that I use everyday in my career, I just couldn't wrap my head around the more abstract concepts, like monads e.g. And the feeling I got was that Haskell is a difficult language, but probably it's the terminology and abstract mathematical concepts which are the real issue for me here. Because the syntax isn't really that complicated. Especially the way space is used to call functions. I'm really sick of all the parentheses in other languages.
But, if you understand all about functional programming, for those that do, it seems to really enrich the way they write and maintain code from what I've seen. People who dog on it just don't understand (including me). Of course it's hard to maintain something you don't understand. But if you do understand it, it's easy to maintain. 🤷♂️ Seems logical.
What next, where is the line drawn for what kind of code we can write? Why introduce more useful concepts in programming if we risk losing maintainability because some devs won't learn the new concepts?
Life means change. Adapt. Learn new things. Expand the mind. Learn how to do things in a good way, and then do the things in that good way. Why stagnate just because we don't understand something. Better to learn a new thing to understand the better way, than to dumb it down to a worse state just so we understand it.
Bah.
I'd love to work on a codebase like that
my_balls |> ligma() |> gotem(laugh=TRUE)
Somebody who worked here before tried to do functional in C# by passing delegates into methods instead of injecting interfaces into constructors, across hundreds of repositories. This is why clever people should not be allowed to write code.
Okay but partial application of curried functions is a really cool way of doing dependency injection and you haven't experienced bliss until you create a perfect module of functions that are exactly that
Also languages with macros and custom operators (where operators are just functions with special syntactic sugar) are so much cooler than those without (Clojure and elixir my beloved)
Additionally a system where illegal states are made impossible is soooo nice to work in. It's like a cheat code
Do curried functions come with grated coconut and a lime wedge?
Thankfully never got sucked into that void. I had a coworker who really evangelized functional programming. I wonder what he's up to now.
We have a principal engineer on our team that is pushing this sort of style, hard.
It's essentially obfuscation, no one else on the team can really review, nevermind understand and maintain what they write. It's all just functional abstractions on top of abstractions, every little thing is a function, even property/field access is extracted out to a function instead of just.... Using dot notation like a normal person.
even property/field access is extracted out to a function
Java, the most functional programming language there is.
Well, this is in JS to be clear
Instead of
const name = user.name
It's
const userToName(user) => user.name;
const name = userToName(user);
Ad nauseum.
I was afraid you’d say that. That’s stupid.
Do they give a reason for why that’s ‘necessary’?
(Also it should be const userToName = (user) => user.name;)
That was the impression I got about functional programming, from what little I read about it like 15 years ago. Sounds like somebody found a pretty hammer and everything became a nail.
I dabbled in some Haskell a few years ago but quit trying when I got to the hard parts like monads and functors and stuff. All those mathematical concepts were a little too abstract for me.
But what I did bring with me from the experience changed my way of programming forever. Especially function composition and tacit (point-free) style programming. It makes writing code so much faster and simpler and it's easier to read and maintain.
You can utilize some functional programming concepts without being too hardcore with it and get the best of both worlds in the process. 👍
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