Lean 4
Lean 4 programming language and theorem prover
...One of its defining characteristics is its emphasis on extensibility, since Lean 4 is built to allow users to develop custom automation, metaprogramming tools, and domain-specific extensions instead of being limited to a fixed proving workflow. The broader Lean ecosystem also includes official tutorials, language references, examples, and installation and build tools, which reflects that the project is not just a core compiler repository but the center of a mature development platform.