> The result of `longer_of` is a projection of the longer argument, so the mutation of `z` by `emphasize` occurs directly on the value of `y`. The value is neither copied, nor moved, and yet it is not being passed by reference to `emphasize`. The body of `emphasize` owns `z` in exactly the same way as it owns `strength`, which is passed by value: `z` is an independent value that can only be touched by `emphasize`.
This evasive phrasing, which continuoes after this excerpt too, has me highly skeptical of their good intentions… Any good reason they are not more explicit?
As someone who has deliberately avoided C++ for the last 20 years (in favour of functional programming) I find the discussions of "reference semantics" and "value semantics" (let alone "mutable value semantics") to be quite opaque. It is as if the C++ community has become an enclave of folks who put up with the extreme complexity of C++ and speak a correspondingly tortuous theoretical language.
What the seem to be saying here is that the "subscripting" operation returns a view into its argument, not entirely unlike the concept of a lens. The only thing that view can be used for is directly accessing the the part of the value that is in focus—the view is not itself a first class value, which means that so-called "reference semantics" don't come into the picture.
I don't think they're being evasive or promoting their idea in bad faith. They are just operating in a characteristically arcane way for C++ language design people.
To me, this seems like a proliferation of distinctions and enthusiastic theorizing. Finding solutions which actually simplify the task of programming and/or clarify matters seems a long way from this attitude.
This evasive phrasing, which continuoes after this excerpt too, has me highly skeptical of their good intentions… Any good reason they are not more explicit?