Hi Javier,
On Mon, 11 Jul 2016 07:47:01 -0600, Javier Bezos
A related point is that luatex is explicitly designed for extensibility of, e.g, primitives via lua, so there should also be a straightforward way for experts to implement the desired low-level behavior. For example, something along the lines of \pardirHR \pardirLR could be reimplemented in lua. That *might* be a better approach than the mirroring Javier requested and which Hans has agreed to do; especially if there is additional residue out there of more primitives that need this kind of treatment.
Actually, \parshape can be mirrored even in pure TeX. But I still believe this must be done at the primitive level, and not with some hack. I think luatex is about making even better paragraphs from good paragraphs, and not as a hack to fix some directional properties.
lua extension =! hack In MkIV Hans built a paragraph optimizer in lua; he did not do it at the primitive level where one might have expected. A proper extension of functionality is just that, an extension in functionality. If \pagedir TRT etc. is no longer there, a properly designed lua extension to provide the needed functionality is not a "hack". luatex is about paragraphs, but it is equally about taking advantage of lua to extend functionality as needed by various applications, *without having to recode the primitives*. Indeed, that kind of extensibility was arguably its most pressing raison d'etre. Put another way: In luatex we are not given the binary opposition of primitive support vs TeX-"hack". We have a third choice, and that is genuine extensibility. Best wishes Idris -- Idris Samawi Hamid Professor of Philosophy Colorado State University Fort Collins, CO 80523