Is Rust functional?

> Finally, it’s a bit unfair for me to compare Rust’s facilities here against Haskell. Haskell’s handling of lambdas, closures, first class and higher-order functions is best-in-class, since it’s explicitly the goal of the language. If you compare Rust to most other languages out there, like Javascript, Ruby, or Python, Rust compares even more favorably.

And yet creating a closure in those languages is natural and trivial. None of the higher-order examples he provided in Rust were closures.

Closures are at best clunky in a language without automatic GC and heap allocation (auto-boxing?). But if you can't readily create and use closures (i.e. functions that use their lexically bound environment outside their original execution scope, such as invoking it after returning it from the defining function) then how could you claim strong support for first-class functions? You don't have first-class functions; you have closures and non-closures, each with different treatment by the language. Otherwise we could just as well claim that GCC's flavor of C is a strongly functional language because it supports nested functions.

The defining characteristic, IMO, of functional style programming is using closures to encapsulate and reify state; not the ability to anonymously define functions or functions which can access their lexical environment from within the same execution scope. If the language doesn't make that natural then it can't be fairly described as friendly to a functional style. In languages like Javascript people intuitively do this without even realizing it, which is about as natural as you can get.