Futhark is really such a great idea. I'm not convinced that dependent types are worth the cognitive overhead in general, but it's definitely worth it to include the length as part of the type information for dynamic arrays, e.g.:
concat(Vec<T, n>, Vec<T, m>) -> Vec<T, n+m>
matmul(Mat<T, n, m>, Mat<T, m, l>) -> Mat<T, n, l>
head(Vec<T, n+1>) -> (T, Vec<T, n>)
This would have saved me so much headache debugging CUDA kernels and numpy!! I wish it were a first-class feature in those frameworks, and even general-purpose languages, but alas.
VorpalWay•May 16, 2026
You can do this with templates in C++ and generics in Rust I'm pretty sure. I think the Eigen C++ library supports this. (I have yet to do a linear algebra heavy Rust project, so I can't speak to the options that exist there.)
otabdeveloper4•May 16, 2026
Yeah, C++ arrays are literally that.
alpinisme•May 16, 2026
Arrays are not dynamically sized though (handling runtime sizes) and don’t have efficient append/concat. The point of the dependent types is that you can have the type system track that concat creates an M+N length vector, sort preserves length (and adds a sorted guarantee that slice preserves), etc.
Sure you can do a lot with templates, but that’s advanced templates not just “C++ arrays” in a throwaway “literally that” way.
ethanlipson•May 16, 2026
I'm talking about cases where the array size is not known at compile time. For example, say the user passes in a list of numbers as command line arguments. Then we have
argv: Vec<String, argc>
If I want to map these to ints, then I'd like a compile-time guarantee that the resulting array
nums: Vec<Int, argc>
is the same length as argv. Lean and Idris can do this, but AFAIK no commonly used languages can. But unlike general dependent types, these are not hard to wrap one's head around and would save a lot of frustration, in my experience.
itishappy•May 16, 2026
Here they are in Futhark:
val concat [n] [m] 't : (xs: [n]t) -> (ys: [m]t) -> *[n + m]t
val matmul [n] [m] [l] 't : (xs: [n][m]t) -> (ys: [m][l]t) -> *[n][l]t
val head [n] 't : (x: [n]t) -> t
And here's the pathological case (length cannot be determined at compile time):
val filter [n] 'a : (p: a -> bool) -> (as: [n]a) -> *[]a
Other pathological cases include conditionals and loops.
ainch•May 16, 2026
The Pyrefly type checker is starting to work on this kind of shape hinting - so far it only works on Torch but I believe the plan is for it to work with other array packages (eg. JAX, NumPy)
I didn't know that, thanks for sharing. It makes sense, but then it also makes me wonder why none of the deep learning libraries (Torch, Jax/NNX, Eigen etc...) make this information available. Instead, ML people all have their own schemes for tracking shape information, like commenting '# (b, n, t)' on every line, or suffixing shapes to variable names - and in my experience it's a common source of bugs.
mathisfun123•May 17, 2026
> Torch, Jax
Both of these "make it available". Just because people don't know how to use/find them doesn't mean they're not "available".
> Eigen
This is not an ML anything, it's a linear algebra library.
> like commenting '# (b, n, t)' on every line, or suffixing shapes to variable names
There's a difference between tracking shapes in the compiler and specifying shapes in the model.
nestorD•May 17, 2026
See also jaxtyping which, contrary to what its name might imply, covers JAX/PyTorch/NumPy/MLX/TensorFlow arrays and tensors.
Futhark is pretty great! And I have to say that the maintainer is insanely quick. It has happened on more than one occasion that I reported a bug and it's solved within the day. I have been using Futhark in prod for two years now and never had serious problems.
Reefersleep•May 16, 2026
What is your use case?
keyle•May 16, 2026
Interesting, what do use it for if you can share?
rowanG077•May 16, 2026
Optimization algorithms. The build in automatic differentiation is great!.
Ferret7446•May 16, 2026
It would be nice to not name your language after another language. (Yes I know it's a script, that doesn't change my point). I came here expecting something else.
Yeah, when naming your language, it's important to keep mind the expectations of people seeing headlines about articles about your language on blog aggregation sites :^)
Now I'm thinking about "Smalltalk by Example" and "Slang by Example"
fulafel•May 16, 2026
Futhark is a glimmer of light in the wasteland of C/C++ styled low level GPU languages.
pjmlp•May 16, 2026
We also need to have StarLisp back, it would be quite fitting.
guessmyname•May 16, 2026
Couldn’t have chosen a more difficult (and ambiguous) name to pronounce, could you? It almost sounds like a curse that I often hear people say out in the bad streets of New York City.
antran22•May 16, 2026
Elder [0] and Younger [1] Futhark (or Fuþark) are the name of two runic writing systems used by Germanic and Scandinavian Vikings. The name Futhark is a combination of the first 6 runes /f/, /u/, /ð/, /ɑ/, /r/, and /k/. Similar to how you get the name "alphabet" from the first two letters of Greek's writing system.
Same. Thought I had a copy of Uthark [1][2] on my shelf too but alas I seem to have only retained Svartkonst [3] during the tragic downsizing of my library.
That's like calling your programming language Latin?! The title could use some disambiguation...
WJW•May 16, 2026
Eh. Words get overloaded all the time and this is a website focusing on programming after all. Would you consider "Python by example" or "C by example" to need disambiguation because they're not about snakes or about the third letter of the alphabet?
Also the very first line on that page is "The Futhark Programming Language" so if you were still confused after that I think it's on you.
hnarn•May 17, 2026
> That's like calling your programming language Latin?!
More accurately it would be like calling it Alphabet, since that takes its name from Alpha Beta (AB) just like the Futhark takes its name from the first letters in it.
pseudohadamard•May 17, 2026
It is a pretty confusing name. Personally I'm waiting for someone to name a programming language Womble. Underground, overground, you're free to use it as long as you work as a team and are tidy and clean.
7 Comments
https://pyrefly.org/en/docs/tensor-shapes/#how-it-works
https://mlir.llvm.org/docs/Dialects/ShapeDialect/
Both of these "make it available". Just because people don't know how to use/find them doesn't mean they're not "available".
> Eigen
This is not an ML anything, it's a linear algebra library.
> like commenting '# (b, n, t)' on every line, or suffixing shapes to variable names
There's a difference between tracking shapes in the compiler and specifying shapes in the model.
https://docs.kidger.site/jaxtyping/
https://en.wikipedia.org/wiki/Runes#Runic_alphabets
For those that don't know, Futhark is comes from the first 6 letters of the runic alphabet (F, U, Þ, A, R, K)
https://www.youtube.com/@JacksonCrawford
Now I'm thinking about "Smalltalk by Example" and "Slang by Example"
[0]: https://en.wikipedia.org/wiki/Elder_Futhark
[1]: https://en.wikipedia.org/wiki/Younger_Futhark
Futhark appears in the first paragraph! i don't think i ever read the term before
https://en.wikipedia.org/wiki/Elder_Futhark
[1] https://archive.org/details/karlsson-thomas-uthark-nightside...
[2] https://www.84cxrarebooks.com/pages/books/090763/t-ketola-th...
[3] https://www.miskatonicbooks.com/product/thursakyngi-iv-svart...
Turns out, Futhark != https://en.wikipedia.org/wiki/Futhark (runes, old germanic alphabet)
That's like calling your programming language Latin?! The title could use some disambiguation...
Also the very first line on that page is "The Futhark Programming Language" so if you were still confused after that I think it's on you.
More accurately it would be like calling it Alphabet, since that takes its name from Alpha Beta (AB) just like the Futhark takes its name from the first letters in it.