HN Reader

A list is not a monad. A list is a data structure; a monad is more like a "trait" or "interface." So you can define a List type that "implements" the monad interface, but this is not an inherent property of lists themselves. That's the sense in which a list "is a" monad: the OOP sense.

Haskell's List monad provides a model for nondeterminism. But that certainly isn't the only way List could satisfy the monad interface! It was a deliberate choice -- a good choice, possibly the best choice, but a choice nonetheless.

However, I personally think that monad tutorials tend to give people the wrong impression and leave them more confused than they were before, because they focus on the wrong thing.

A monad is not a complex concept, at all. IMO a more useful way to present the topic would be with one separate lesson for every common monad instance. Start with Maybe, then IO, then maybe State and List, and so on... because ultimately, every instance of a Monad works very differently. That's why the pattern is so useful in the first place, because it applies to so many places. (Note: this is a criticism of monad tutorials in general, not this one in particular, which seems to do a decent job on this front).

In my experience, people new to Haskell focus way too much on getting the "a-ha" moment for monads in general, when really you want a bunch of separate "a-ha" moments as you realize how each instance of a monad takes advantage of the pattern differently.

I also tend to think that monads are best demonstrated in Haskell rather than in other languages, if only because the notation is so much less clunky. That may just be me though. (EDIT: well, also because almost no other languages have typeclasses, so you have to approximate it with interfaces/traits/etc)

Also FYI: in part 2, the code examples have extra newlines in between every line, which makes it hard to read (I'm on firefox, if that matters).

A list is not a monad. List is a monad. A list is an algebra for the List monad.

All a way of saying that, yep, you always have `map` when you have a Monad, but you don't need a Monad to have `map`.

If you want an example we can compare a regular list and a Ziplist. A regular list's Applicative instance does a cross product, while a Ziplist's applicative instance does a dot product.

    (*) <$> [2,3] <*> [5,7, 11]
    --> [10,14,22,15,21,33] 

    getZipList $ fmap show $ (*) <$> ZipList [2,3] <*> ZipList [5,7, 11]
    --> ["10","21"]  

For quirky reasons in Haskell, `fmap` the function implemented for every Functor instance. This is because `map` was already taken by lists. Weird, I know.

But its not a very robust one: its never true of fast programs on realistic hardware for example (not for a long time now). And all the rule bending (-fstrict-alias, bunch of stuff) exists in this tension between the grade school natural language paradigm and the reality of computers. I say grade school not to be pejorative, but rather because it is roughly the boundary where written natural languages begin to have interesting tensions around past and future and simultaneous, changing and not changing.

Functors and applicatives and monads and other type classes like these are the source of endless analogies because there isn't an accepted, broadly-understood terminology for this "well its roughly what would happen if you had a piece of paper and wrote things on it at every statement boundary and scratched off the old ones" (though Turing and von Neumann did formalize this in useful ways, they just don't generalize well to realistic computers anymore).

Monads are the mathematical object that is forced on you if you want a rigorous way to describe the semantics of program execution in the vicinity of this "common sense" notion. That's really what everyone is dancing around: your program is only well defined with either:

- a big rulebook full of exceptions and edge cases

- a compositional rule strict enough to give some useful predictability but lax enough to admit most useful programs.

It is this rigor/laxity tension as concerns text on a page and gates on a semiconductor that gives monads a privileged place in the towers of categories. When I worked on Sigma we were among the earlier adoptors of ApplicativeDo, for example, because we wanted a slightly different rigor/laxity tradeoff for performance reasons.

Monads are what happens when you do shift the giant pile of "back of the book" compiler details that describe program execution semantics into a much simpler set of rules, but at the cost of increasing the barrier to entry because you need to know the rules before you can print "hello world".

    for x in list
        doThings(x)

    newlist=
        for x in list1
            y in list2 if y > 3
            z in list3
            doThings(x, y, z)

Without comprehensions, you can still implement monadic functions in any old language (probably in C...?), and they're handy in their own right, but you don't get the flattening-of-callback-hell magic.

https://hackage.haskell.org/package/Agda-2.6.4.2/docs/Agda-S...

IMO if you already have it, this will be a lovely comparison full of insight, but if you haven't then it's full of confusing statements.

IMO what they are is utterly unimportant, except to mathematicians, and what you can do with them is more to the point.

The fact that explanations are so often in Haskell just makes them more unintelligible because you really need to know what problem they solve.

The monad interface only requires ways to construct object using callbacks. The ‘bind’ operation takes a callback as an argument, but says nothing about when it’s actually called; it could be immediately, deferred, multiple times, or even never. It’s up to the implementation of the monad, as well as the language, if it’s a lazy language.

This is basically a framework. Like with other frameworks, the principle is “don’t call us; we’ll call you.” Arbitrary computation can happen between callbacks. The framework can do whatever control flow it wants, and this is what often makes frameworks opaque. Hiding control flow is what frameworks do, for better or worse.

So far, none of this is specific to a Monad. The Monad part comes from the type signature of the callback function passed in to flatmap(), which allows ‘bind’ operations to be nested.

Once you know what kind of thing you’re dealing with (frameworks) then you can go into why some frameworks qualify as a monad.

Monads in software are just a standard API for any given type. That’s it. Theres no magic here. Just implement the standard and you have a monad.

It grinds my gears seeing monad tutorial after tutorial using the wrong metaphors or misleading explanations

Meaning that every collection is a set of possible inputs to the computation that is provided as the argument to a `flatMap` operation. Each `flatMap`, by definition, returns a new collection of possible outputs for each of the inputs, and each of those collections gets concatenated. Every item in the final output collection represents the result of following some path through the computations, selecting a single item at each step. Importantly, the type of the output of each `flatMap` operation can differ from the input.

You can imagine extending this by assigning probabilities, or making the domain continuous (I think...). These extensions would still be monads, just without being simple collections.

It's kind of like how multiplication over whole numbers is repeated addition, but that metaphor becomes less useful for other domains of numbers.

The statement as-is breaks pretty much immediately because, while there is a canonical list monad, there isn't a list monad, there are in fact several[1].

There are several more correct ways of phrasing the idea among:

"List can be given a monad instance"

"List forms a monad with pure and bind as defined"

"List is the underlying functor of a monad"

The point is that picking any old list implementation is likely not a monad without the supporting structure.

Will any of these help you learn what a monad is? Likely not. Monadology is a Mary's Room[2] problem; there is a qualia, a subjective sensation, when one understands monads having experienced them first hand. Subsequently monad tutorials are the best case against physicalism[3] yet devised.

1. https://hackage.haskell.org/package/exotic-list-monads-1.1.0...

2. https://en.wikipedia.org/wiki/Knowledge_argument

3. https://en.wikipedia.org/wiki/Physicalism

[Content Warning: Lisp]

We define a small OOP framework for monads, plus macros which then can succinctly define different kinds of monads, including generating a comprehension macro for each one.

When I was first learning Haskell a million years ago, I was completely confused by the concept of a monad; I could, after enough fighting with the compiler, usually get something working, but it was a stochastic guess-and-check process trying to figure out what `IO` actually means. Even the `Maybe` was confusing to me, because I couldn't really figure out how the hell you relate something like "checking for null" with "writing to the disk".

I can't remember where I saw it, probably on the Haskell wiki somewhere, but when they pointed out the List is a monad, and after seeing an example of how it worked, I suddenly got it: in a hand-wavey way, a monad is basically just a value with a wrapper context [1], and from a practical perspective that's all it is. In the case of a List its wrapper context is that there might be 0 or many of those things in there, in the case of a Maybe its wrapper context is that it might exist or it might not, in the case of IO its wrapper context is that it's interfacing with the outside world, and once you abstract away the entire idea of context, you can suddenly open up an entire world of reusability.

This is a good tutorial, I will probably be linking it to people if they ever make the mistake of asking about monads.

[1] I don't need a lecture on the minutia of this, I know that there's a lot more to it in the theory world, I went to graduate school specifically to study functional language verification. I'm keeping it simple.

Context: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

Usually articles that describe them in a very Math-y way go above my head. But the definition above was immediately clear (I saw it on HN).

I think this article is a bit more approachable than others I've read, but it still gets very confusing near the end.

Let's start with function composition. We know that for any two types A and B we can consider functions from A to B, written A -> B. We can also compose them, the heart of sequentiality. If f: A -> B and g: B -> C then we might write (f;g) or (g . f) as two different, equivalent syntaxes for doing one thing and then the other, f and then g.

I'll posit this is an extremely fundamental idea of "sequence". Sure something like [a, b, c] is also a sequence, but (f;g) really shows us the idea of piping, of one operation following the first. This is because of how composition is only defined for things with compatible input and output types. It's a little implicit promise that we're feeding the output of f into g, not just putting them side-by-side on the shelf to admire.

Anyway, we characterize composition in two ways. First, we want to be clear that composition only cares about the order that the pipes are plugged together, not how you assemble them. Specifically, for three functions, f: A->B, g: B->C, h: C->D, (f;g);h = f;(g;h). The parentheses don't matter.

Second, we know that for any type A there's the "do nothing" identity function id_A: A->A. This doesn't have to exist, but it does and it's useful. It helps us characterize composition again by saying that f;id = id;f = f. If you're playing along by metaphor to lists, id is the empty list.

Together, composition and identity and the rules of associativity (parentheses don't matter) and how we can omit identity really serve to show what the idea of "sequences of pipes" mean. This is a super popular structure (technically, a category) and whenever you see it you can get a large intuition that some kind of sequencing might be happening.

Now, let's consider a slightly different sort of function. Given any type types, what about the functions A -> F B for some fixed other type F. F here exists to somehow "modulate" B, annotate it with additional meaning. Having a value of F B is kind of like having a value of type B, but maybe seen through some kind of lens.

Presumably, we care about that particular sort of lens and you can go look up dozens of useful choices of F later, but for now we can just focus on how functions A -> F B sort of still look like little machines that we might want to pipe together. Maybe we'd like there to be composition and identity here as well.

It should be obvious that we can't use identity or composition from normal function spaces. They don't type-check (id_A: A -> A, not A -> F A) and they don't semantically make sense (we don't offhand have a way to get Bs out of an F B, which would be the obvious way to "pipe" the result onward in composition).

But let's say that for some type constructors F, they did make sense. We'd have for any type A a function pure_A: A -> F A as well as a kind of composition such that f: A -> F B and g: B -> F C become f >=> g : A -> F C. These operations might only exist for some kinds of F, but whenever they do exist we'd again capture this very primal form of sequencing that we had with functions above.

We'd again capture the idea of little A -> F B machines which can be plugged into one another as long as their input and output types align and built into larger and larger sequences of piped machines. It's a very pleasant kind of structure, easy to work with.

And those F which support these operations (and follow the associativity and identity rules) are exactly the things we call monads. They're type constructors which allow for sequential piping very similar to how we can compose normal functions.

1. For any class X, there is a canonical method

  F: X -> T<X>

  G: T<T<X>> -> T<X>.

  f: X -> Y, 

  “T<f>”: T<X> -> T<Y>.

Here “any type” means any type that is compatible with the template.

And then there’s some additional rules which make all these methods compatible, based on the different ways of stacking nested T’s and the “canonical” maps you get. Admittedly there is some confusing accounting here, but I also think most natural ways of constructing the above three requirements are going to satisfy them anyway. For List and Maybe it’s fairly obvious what the above methods are.

I dunno, maybe I have it wrong and someone can correct my understanding.

List can be an instance of a monad, i.e. a monadic type.

I think the trick to understanding monads is to see what benefits monad interface gives to the types that implement it.

I mean really. Look at posts like this[0]. What does this give you? Nothing, in practical reality. Nothing.

[0] https://news.ycombinator.com/item?id=44446472