I had the same question. More generally, it's a "design smell" for me to see something like this:

        // define a new timely dataflow computation.
        timely::execute_from_args(std::env::args(), move |worker| {

Why would something as fundamental as creating a new dataflow need a comment? In other words: why is that not self-evident from the syntax?

Not to denigrate the technical accomplishments. And elegance/readability is a very personal thing. But it does seem jarring when the language constructs don't obviously map cleanly to the design intent.