[nasretdinov]

Well, yes, that's the thing: you don't get any special syntax for generators (like "yield" keyword), which makes them look quite weird compared to other languages that have native support for them. You need to have very clunky and verbose syntax (at least I view it as such) which consists of having to define an extra nested closure and use a function pointer that was passed to you. Having a new keyword would allow for a much nicer looking generator functions, but that would break all existing tooling that doesn't yet support that keyword (and potentially break existing programs that use yield as a variable name or something like that).

[foldr]

Seems like a total non-issue to me. It’s conceptually very easy to grasp and follows the normal Go syntax for defining functions. And what percentage of your code base is really going to consist of custom iterators? A syntactic shortcut might save you two lines of code for every 5000 you write. A lot of Go programmers will probably never write a custom iterator from scratch. The important thing is to make the custom iterators easy to use, which I think has been achieved. I’m sure they’ll consider adding some syntax sugar in time, but it would be a trivial convenience.

The benefit of Go’s generator implementation is a huge simplificación of the language semantics compared to other approaches. The generator function has no special semantics at all, and when used with ‘range’, all that occurs is a very simple conversion to an explicit loop repeatedly calling the function. Other popular approaches require either special runtime support for coroutines of some form, or a much more elaborate translation step within the compiler.