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What might be the best example of an open source Haskell project that sees meaningful commercial use?

I realize I’m not sure I’ve ever seen what that looks like, having only been exposed to personal/small projects.

pandoc!
I’ve said for years that Pandoc is pretty much the only Haskell program that’s used outside people who want the novelty of using a Haskell program.

It’s also a program I use all the time, I haven’t touched a WYSIWYG word processor in years.

That may be true — it may be the only Haskell program that people regularly use — but I'm not sure how that counts against Haskell. I'm no fan of Haskell (I used to be a huge pure functional programming fan, but over time the shine has worn off and I've realized it's just another dogmatic paradigm) but having even one widely used and liked tool is actually exemplary for a highly idiosyncratic non-mainstream language like Haskell, to be honest. I think it's doing pretty well for itself.
I didn't say it was a bad thing, just pointing out that Haskell is not a terribly popular language and as such most of the apps that I've seen written in it are only used by Haskell enthusiasts (Darcs comes to mind).

I'm kind of a functional programming fanatic, evidently.

The best answer to this question is, in the compilers of other programming languages lol.

Elm, for example, is in use in production at a variety of companies.

It sees quite a lot of use in the traditional finance industry in europe from what I've been told but basically all of that is closed source.

However probably the "biggest" open source project with direct commercial use would be the Cardano blockchain (IOG, one of the main developers for Cardano, is one of the top sponsors for the Glasgow Haskell Compiler as well as a major contributor nowadays).

The cardano node implementation is linked here: https://github.com/IntersectMBO/cardano-node

However a lot of tooling for the ecosystem is spread between that org, the IOG org, and the CF org, as well as others.

https://github.com/IntersectMBO/

https://github.com/input-output-hk/

https://github.com/cardano-foundation/

By no means is this the best example but it is a decent example of an extremely large, primarily haskell based project that sees regular use.

Supabase is built on top of PostgREST, which is written in Haskell.
The Co-Star astrology web app has its backend coded in Haskell, which I've always found fascinating. I love the overlap between esoteric communities.
> "Stand out to employers"

Sure...

99% of employers: it's impressive that you master Haskell and passed this test. Still, you need to solve this leetcode problem (and preferably not using Haskell)

You may be being generous. 99.9% of employers: "What is Haskell"?
I really believe employers in this industry now essentially assume everything on your resume is a bunch of lies, and the interview process is to check you can string a sentence together, and know the two pointers algorithm.
I am reasonably certain I could pass this exam, I signed up for the waiting list, but I am doubtful that would in any way translate to any kind of job opportunity.

Even certs in more commercially popular languages aren’t taken terribly seriously most of the time; I have a few and I eventually took them off my resume to save space.

As someone who has worked on haskell professionally, it's probably the easiest language to interview in/for. It's really easy to see how deep a dev has gone down the haskell rabbit hole. Forget a certificate, just talk for 5 minutes.

Any dev who has fully gone down the haskell rabbit hole can definitely grok any other paradigm. The challenge isn't technical skills with devs like this.

It's probably easy to get certified and still be an incompetent programmer (in the same way it's easy to get a university degree and still be an incompetent programmer)
Finally, a commercial application of Haskell.
It's stupidly difficult to get a Haskell role. There are long time, knowledgeable forum members on the Haskell discourse who have related on there that they've never been able to secure a full time Haskell role.
Really? IOHK are almost always hiring - apply there. They’ll know the people hiring if they’re that familiar surely?
Is that a blockchain company? Every potential employer that has ever mentioned Haskell to me seems to be one.
It is. They are one of the main developers for Cardano (I linked some of their stuff in another comment talking about examples of commercial open source haskell because I keep up with the project).

I'm actually interested in what other projects in the cryptocurrency space use haskell? I thought Cardano was the only project that did. Most everyone else seems to use Rust or some mishmash of web oriented languages.

I went back to check my email and it turns out that I received several Haskell enquiries from the same recruiter over a period of a few years. He only ever described the employer as a “Global Blockchain Fin-Tech Firm” without being more specific. It seemed pretty fishy to me.
Yeahhhh that sounds about right for a shitty recruiter.
It was probably IOHK, or maybe Algorand.
Yes, but it’s got some cool Haskell projects (like a new language). Also, it has some big Haskell names there. When I worked there I debugged a compiler performance issue with one of the original creators, and there’s people like Lamport or Wolfram or Cerf brought in for talks.

It’s not building products but the core platform.

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Cool, a cryptocurrency company trying to improve Haskell's reputation.
How is a cryptocurrency company involved? I don't see that anywhere, this is the Haskell foundation's site. Maybe I missed it?
What they don't tell you is haskell integration is still poor on interview platforms so you will be solving them in python.