pub fn doubler_closure_factory() -> Box<dyn Fn(i32) -> i32>
Creates a closure which doubles an integer.
Compare with the function above: this is basically identical to
doubler_factory, the only difference being that in this case, we return a
closure rather than a standalone function. That has substantial
ramifications for using closures and named functions, of course; refer to
the discussion in e004 for details