Lean mathlib was originally a type checker proof assistant, but now leanprover-community is implementing like all math as proofs in Lean in the mathlib project.

Lean (proof assistant) https://en.wikipedia.org/wiki/Lean_(proof_assistant)

"Lean mathlib overview": https://leanprover-community.github.io/mathlib-overview.html

"Where to start learning Lean": https://github.com/leanprover-community/mathlib/wiki/Where-t...

leanprover-community/mathlib: https://github.com/leanprover-community/mathlib