Types of types: Haskell's Kind system
thunk.blog60 points by Thom2503 3 days ago
60 points by Thom2503 3 days ago
My main problem with Haskell is that it never fully embraced dependent types. Instead, it kept adding one type system extension after another, making the language unnecessarily complicated.
I used to be a big Haskell fan, until I discovered languages like Lean, Agda, and Idris 2 and realized how much cleaner and more coherent the whole thing could be with dependent types built in from the start.
I highly recommend Haskell fans explore Lean/Agda/Idris. They make Haskell feel like a pile of spaghetti by comparison.
The more of Haskell's ideas that bleed into mainstream languages, the happier the world will become. It is just so beautiful how these higher abstractions can become so useful and powerful for tying all kinds of things together.
I've been vibe coding my own language and it's compiler is in Haskell, it's been great for the agents to be inspired by Haskell and I'm having fun just adding random language features, currently implementing prophecy ghost variables. Not really a public project but can suss it out for now at http://lsalpha.com
This is super cool! You should put a clear example implementation of a feed forward network, transformer block, loss function, or something more ML specific on the homepage, assuming you're primarily targeting that domain.
EDIT: also the links to GitHub on https://lsalpha.com/systems are 404ing for me, so I exited your site without an example of what a real implementation might look like.
I've long suspected that one of the things that makes monads notoriously difficult to explain to people outside of the Haskell ecosystem is that very few other languages even have a kind system that you are allowed to manipulate in code (and in some, like C++, it's so esoteric that you basically never see it). First explaining that you can even abstract over "type->type" kinded things gets you a lot of the way towards monads without actually describing monads.
The problem is not monads. Any programmer understands the need to represent "the world". It is the need to keep that representation modular that leads to monad transformers, which makes regular people (like me) simply check out and go back to C++
That may be true for functions which operate on a generic Monad typeclass, but typeclasses (Higher kinded types) aren't required to implement concrete Functor / Applicative / Monad instances for a particular type, e.g. Maybe:
fmap f Nothing = Nothing
fmap f (Just x) = Just (f x)
pure x = Just x
Nothing >>= f = Nothing
Just x >>= f = f xWhen people say they don’t understand Monad, my first assumption is that they are wearing their ignorance as a badge of honor.
Otherwise memorize a little bit and dispose of the mystery. It’s not that far out.
> things that makes monads notoriously difficult to explain to people outside of the Haskell ecosystem
Huh, I thought a monad is just a monoid in the category of endofunctors, what's the problem?
And here I was convinced that they’re burritos!
I will say that explaining functors to my coworkers was always much simpler, and then naturally led to monads
I suspect all branches of tech have their rite of passage.
For haskellers, it is writing a monad tutorial
For javascripters, it is writing a framework
For make-stuff-with-LLM people, it is producing a diagram explaining your code generation factory
... and so on
Some OO languages have a parallel concept in "Metaclass" (https://en.wikipedia.org/wiki/Metaclass) i.e. the class of a class which is structurally similar to "Kind" though of course the mechanics are quite different.
I still have an old book Putting Metaclasses to Work - A New Dimension in Object-Oriented Programming by Forman et.al somewhere.
There is also the classic The Art of the Metaobject Protocol by Kiczales et.al which is quie relevant here - https://en.wikipedia.org/wiki/The_Art_of_the_Metaobject_Prot...
The key difference is that the Type guys do it at compile time (correctness) while the OO guys do it at runtime (flexibility).
The thing that makes monads difficult is laziness and the anti math circle jerk.
You start from a functor which is a data type that admits a map operation.
Then you add of, or pure, to make an applicative functor, and then you add flatmap to make a monad.
It's odd, the stereotype of software developers is a know-it-all attitude, but start talking about types and you get excuses that boil down to 'I'm incapable of learning to understand the thing you learned to understand'.
> The thing that makes monads difficult is laziness and the anti math circle jerk.
That's quite a hateful way to describe lack of knowledge.
There are some prerequisites that make it easier to understand monads, and a lot of programmers don't even know those. That can make learning monads difficult right off the bat.
No there's no complexity behind monads except laziness and anti math circle jerk.
It's really basic concepts, much easier than the analysis you do in high school.
But people would rather sit through monad as boxes or burritos cringe tutorials.
Your math explanation starts with a math concept that the vast, vast, vast majority of people will have never even heard of, even in math-adjacent fields. It's not anti-math reasoning. It's a lack of self-awareness that your background is very math heavy, which makes that particular explanation make sense to you. I've gone my entire schooling, as a student who was always one of the few who were "good at math", all the the way through my CS education, without ever having heard of a functor until I came across the Haskell monad explanation meme.
Your next step could be to blame the school systems if you want, but that doesn't help the situation at hand.
>> You start from a functor which is a data type that admits a map operation. Then you add of, or pure, to make an applicative functor, and then you add flatmap to make a monad.
> Your math explanation starts with a math concept that the vast, vast, vast majority of people will have never even heard of, even in math-adjacent fields
Let me take this apart.
1. a data type that admits a map operation
Present in most languages including JavaScript. Not a mathematical issue.
2. add of, or pure, to make an applicative functor
I had to look this up. It was new content for me, maybe 5 or 10 minutes believing I would get it.
3. add flatmap
This didn’t scare me but maybe it scares others. Not a mathematical issue here, though. Being in C++ doesn’t make this concept mainstream, I guess?
FWIW I dropped out of high school to pursue programming more directly. I regret it now because of the job market crash, but it's also too late for me to go back.
Lack of knowledge is not the problem at all. Anti-intellectualism is.
You can try to blame lack of knowledge on anything you want, but at the end of the day Hacker News is for intellectual curiosity and even HN readers don't all know what monads are.
One thing I've noticed since I've become an engineer who gets to design the architecture:
No matter what I do, eventually algebraic structures like semi-groups and monads along with categorical structure like lenses and traversals keep popping up.
It seems like the sort of thing that system designers could benefit from understanding, but the ordinary users of a system shouldn't need to know about it.
In my experience such categories pop up when dealing with immutable data, which is common in many domains.
Missed opportunity to say "Kinds of kindness". ;-)
I really like the concept of dependent types, all type systems that I use rust or haskell have a very limited set of dependent types, but I do like the things that it let's you do
> These are the constructs we manipulate in our programs, things like 1, 2, True, "hello", etc… In functional programming contexts, they're also called "terms."
This is not correct. Terms are syntactic expressions in a program. For example, 1+2 is a term, but it’s not a value. At runtime, it evaluates to 3. As such, some terms correspond directly to values - like 1 - but other terms represent computations that will be performed at runtime.
tl;dr: terms are a syntactic concept, values are a runtime concept. Terms evaluate to values.
Following on from the above, this statement is also incomplete at best:
> Types are groups of related values.
This is an operational perspective which misses one of the most important aspects of types: in a statically typed language, types are a way to classify terms, i.e. they're not just about values.
As an example of why this distinction matters, consider this expression: 1 + "hello". In most statically typed languages, this will be rejected at compile time. It doesn't have to be evaluated (i.e. at runtime) to be rejected - it's invalid because the types of the terms - 1 and "hello" - aren't compatible.
This compile-time rejection is only possibly because types apply to terms, and terms aren't values. This is critical to the usefulness of statically-typed languages - without this, they'd have to run the program to detect type errors.
Benjamin Pierce's book "Types and Programming Languages" has some great coverage of this topic.
Do you mean that if types had only to do with values, you would have to evaluate the term to determine type?
Also, 1 + "hello" is "1hello", obviously. Have you never read JavaScript? :)
I gather it goes through an intermediate coercion step where it is "1" + "hello".
What trips us up is having existing expectations for what an operator called + should do.
> This is not correct.
Why do you think so? It's a completely standard usage. For example, we can ask MathWorld:
> In logic, a term is a variable ['a', 'f'], constant ['1', '"hello"'], or the result of acting on variables and constants by function symbols ['1+2', 'f(a)'].
(I've added the bracketed material, which provides examples of the senses that MathWorld gives. https://mathworld.wolfram.com/Term.html )
In particular, note that by this wording, if you're looking at '1+2', the "term" is not the string '"1+2"' but the result of adding '1' to '2', which is '3'.
You can talk about 'terms' in the context of how a compiler views source code. Or you can talk about 'terms' in the context of how type theory applies to some computation. They'll be called 'terms' either way; you seem to have staked out the position that the compiler context is obligatory and the type theory context is somehow in error.