I would like to show off my own work on the Ninety-nine Haskell problems:
Why do I think this is worth showing off?
It really has 99 problems.
Its Haddock documentation was written in a way such that it could serve as a list of standalone problems. One can work on the problems without having to refer back to the original list.
Tests and benchmarks for trying out and comparing solutions are included. They are written in a way where you can try more than one implementation which solves a problem.
I had a lot of fun and learned a lot implementing solutions, writing tests, and coming up with new problems. For anyone looking for problems to practice coding in Haskell, consider cloning this package from GitHub and using it to test out your solutions. I hope you have as much fun as I did!