You write a standard solution, just like on LeetCode, and run it through the CLI. It identifies the problem by ID or title, executes your code against local test cases, and shows the result.
It currently supports around 500 problems and multiple languages, including Python, C++, Rust, Java, Go, TypeScript, Swift, and others.
The project is still an MVP. System design, SQL, and concurrency problems are not supported yet, but support for more problem types is planned.
GitHub: GitHub - therepanic/openleetcode: we have democratized the LeetCode tests · GitHub
As for Haskell, I wanted to hone my skills in the language when I first started the project. Before that, I had learned the basics from Learn You a Haskell and subsequently written a simple C compiler in it. When the idea for this project came to me, I decided without hesitation to write it in Haskell.