The Haskell GHC compiler is written mostly in Haskell. Rust's compiler is also written in Rust, these days. TypeScript's compiler is in TypeScript. It's a pr

The Haskell GHC compiler is written mostly in Haskell. Rust's compiler is also written in Rust, these days. TypeScript's compiler is in TypeScript. It's a pretty common state of affairs, actually. Often arises out of the second or third implementation of the compiler being much better than the first attempts, probably coupled with the momentum of people using the language who can contribute to the tooling because it's in the same language.