[XeTeX] [luatex] Info on direction primitives/implementation

[Vafa Khalighi]

>
> That suggests that TeX--XeT is not providing the tools required to do a
> decent job in XeTeX: is that a fair reading? (I'd guess that there were
> reasons for the Omega/Aleph/LuaTeX move from TeX--XeT to other
> approaches, but as a non-expert was not sure how to read it.)
 

Yes, the limitations of TeX--XeT are:

    Only four primitives \beginR \endR, \beginL \endL are provided which makes typesetting RTL documents very hard and complicated.
    The primitives above only work in horizontal mode.
    No way to typeset RTL tabular, the only approach is to put tabular inside an RTL box which itself introduces lots of problems.
    \special do not work properly in RTL mode.
    There is no way to change the direction of boxes and even if you do by trick, the order of TOC or anything that has to do with \write at shipout time gets wrong.
    left/right skips do not get reversed in RTL, so you have to replace them with each other and this is not always the case, e.g. \vbox inside \hbox
    \parshape is not reversed in RTL mode so you have to do some macro programming and this is not always the case, e.g, \vbox inside \hbox
    No tool for controlling equation number; it only provides \predisplaydirection which is buggy in RTL.
    ....



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