The power of these is that a handful of implementations can act as enormous levers to create useful abstractions for everyone else. As an example, Rust's type s

The power of these is that a handful of implementations can act as enormous levers to create useful abstractions for everyone else. As an example, Rust's type system is difficult to implement and hard to prove correct - there was a problem with `std::thread::scoped`'s soundness for example - but once done, the work can be leveraged into higher (and often simpler) abstractions! Session types in Rust are exciting to me because we might see implementations of consensus algorithms leverage the type system for correctness. Everyone else then gets to benefit from a simple API on those (complicated) types.