From Haxl paper: "We present an Applicative abstraction that allows implicit concurrency to be extracted from computations written with a combination of Monad

From Haxl paper: "We present an Applicative abstraction that allows implicit concurrency to be extracted from computations written with a combination of Monad and Applicative." The more your code is Applicative the more concurrency you can extract. It's a matter of good abstractions rather than the language. I'm sure Rust can handle Applicative and Monad just fine.