I think the OP makes it clear that this nuance is actually quite tricky in a native systems language like Rust. A good example in C++ would be: - A function (

I think the OP makes it clear that this nuance is actually quite tricky in a native systems language like Rust. A good example in C++ would be: - A function (non capturing/lambda) is created with auto f = [] { ... }; - A function (capturing/closure) is created with auto s = "..."; auto f = [&] { s; ... }; In source code both of those look similar and you can even define them with the same type: std::function<void()> However, the resulting assembly for both couldn't be more different. The Lambda case is as raw as any normal C function type, which the closure case creates a C++ class that closes over the outer method's state. If you're not a seasoned C++ engineer, this nuance will be lost on you. It's fair to say that everyone should understand Lambda Calculus rules, but that makes the language less accessible to new-comers. In the case for non-systems languages, you can blur the lines with ease, but something Rust and C++ just cannot afford to do. That makes understanding the nuance important to be effective.