PCG VS haxl

The American Museum of Natural History uses Haskell to study the phylogenetic graphs reflecting the evolutionary relationships between different species. The code of this project is also available on GitHub.

Sounds a bit like Haxl. From this page:

+1. The JS event loop auto-monad-izing Promises into Haxl [1]-esqe trees of implicitly-batched loads has been a big win for us building on JavaScript/TypeScript.

If I had to move to another language, I'd really want to find a "powered by the event loop / dataloader" framework, i.e. Vert.x for Java.

Also, per dataloader, a shameless plug for our ORM that has dataloader de-N+1-ing built natively into all object graph traversals:

https://joist-orm.io/docs/goals/avoiding-n-plus-1s

[1]: https://github.com/facebook/Haxl

and Haxl

Haxl, a framework for efficient and concise data fetching.

You mean, Haxl? [1] But AFAIK only a few devs are involved for the development.

[1] https://github.com/facebook/Haxl

Probably, the most googled and well-known example of Haskell application in major companies is the Facebook Meta spam filters. Messages containing spam, links to potential malware and phishing attacks are filtered by Sigma, the system that was rewritten in Haskell in 2015. The choice of language was based, among other things, on performance requirements, the need for pure functions and static typing, as well as the possibility of interactive development. The Haxl framework is used under the bonnet, which the company has open-sourced. You can find more details about the development process and technical aspects in the company’s blog post.

Their GitHub still seems pretty active https://github.com/facebook/Haxl

Since you mention Any real-world application uses a database or seventeen in the backend, it's probably worth checking out https://github.com/facebook/Haxl as its goal is to allow you to write high-level queries in Haskell and have the library call out to different datasources as efficiently as possible.