Since we're talking about implementing it in usercode, as opposed to adding it to the language, we fortunately have the option to rework it (or have a couple completing implementations to see what works in practice).

If you're looking for those particular features, I'd recommend adopting go's `, ok` idiom that appears in several places in the language. A type like:

    type Iterator[T any] interface {
        Next() (T, bool)
    }

Lets you write code like this, if that's more to your liking:

    for current, ok := i.Next(); ok; current, ok = i.Next() {
        // do things here
    }

Which, really seems like a more go-like way to do this in the language as well (ranging over a function then becomes syntactic sugar for the above).