I think Brogue is quite nice to work with and well structured: https://github.com/tsadok/brogue https://github.com/tsadok/brogue Kudos also to Postgres, SQLite

I think Brogue is quite nice to work with and well structured: https://github.com/tsadok/brogue https://github.com/tsadok/brogue Kudos also to Postgres, SQLite, Lean (theorem prover), and the containers library for Haskell.