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diff --git a/doc/context/sources/general/manuals/followingup/followingup-whatsits.tex b/doc/context/sources/general/manuals/followingup/followingup-whatsits.tex new file mode 100644 index 000000000..64c817573 --- /dev/null +++ b/doc/context/sources/general/manuals/followingup/followingup-whatsits.tex @@ -0,0 +1,78 @@ +% language=us + +\startcomponent followingup-whatsits + +\environment followingup-style + +\startchapter[title={Whatsits}] + +% \startsection[title={...}] + +Whatsits provide the natural extension mechanism for \TEX. In \PDFTEX\ there are +plenty such whatsits, for instance for \PDF\ annotations. In \LUATEX\ this +mechanism was reorganized so that the code was better isolated. In the first +versions of \LUAMETATEX\ only a handful was left. Stepwise some were removed and +in the end we could stick to only one general whatsit because one can implement +the few needed to be compatible with \TEX. + +We started out with this set of whatsits: + +\starttabulate[|B|p|] +\NC open \NC open a file for writing (delayed) \NC \NR +\NC write \NC write to an open file (or terminal otherwise) \NC \NR +\NC close \NC close an opened file \NC \NR +\NC special \NC write some literal \PDF\ code to the output file \NC \NR +\NC user \NC store and retrieve data in a node \NC \NR +\NC latelua \NC execute code delayed (in the backend) \NC \NR +\NC literal \NC write some literal \PDF\ code to the output file, controlled by a mode \NC \NR +\NC save \NC push the transformation state \NC \NR +\NC restore \NC pop the transformation state \NC \NR +\NC matrix \NC apply a transformation (\type {rx sx sy ry}) \NC \NR +\NC savepos \NC register a position to be queried afterwards (\type {x y}) \NC \NR +\stoptabulate + +The \type {\openout}, \type {\write} and \type {\closeout} primitives relate to +the first three but they can be prefixed with \type {\immediate} in which case +they don't end up as whatsits but are applied directly. The \type {special} is +actually meant for \DVI\ while the \type {\(pdf)literal} is for \PDF\ output. The +first four are available in regular \TEX. + +The last four are dealt with exclusively in the backend and by removing the +backend they basically became no|-|ops. I kept them for a while but in the end +decided to kick them out. Instead a generic whatsit was introduced that could be +used as signal with the same function. That simple whatsit only has a subtype +(and of course optionally attributes). And, as \CONTEXT\ has its own backend, we +can intercept them as we like. The saving in code is not spectacular but keeping +it around (basically doing nothing) neither. The impact on \CONTEXT\ was not that +large because for instance saving positions is done differently and +transformations are encapsulated in a few helpers that could easily be adapted. + +From there it was a small step to also remove the literal whatsit, so then we had +five whatsits left, plus the generic one. I then entered sentimental mode: should +we keep the first four or not. Of course we want to be \TEX\ compatible but we +can remove the code and provide a compatible replacement using macros and our own +simple whatsit nodes. That keeps all the housekeeping at the \LUA\ end, +simplifies the \CCODE, and we're still \TEX. + +Of course, once we remove these and only have the delayed \LUA\ whatsit and user +whatsits left, we can as well replace these too. In \LUATEX\ user nodes are +actually not dealt with in the backend. One can create them at the \LUA\ end and +query them in callbacks. The \TEX\ machinery just ignores them, like any whatsit. +In retrospect they could have been first class nodes, but making them whatsits +was wise because that way they can be ignored consistently when needed. + +So, in the end all we need is a simple whatsit. As I removed the subtypes +stepwise there was an intermediate mix of code to recognize simple whatsits from +core whatsits but that distinction went away. Doing this kind of refactoring is +best done stepwise because that way I can compile some large documents and see if +things break. As a consequence again some code could be simplified as we +basically no longer have extensions. Of course at the \CONTEXT\ end the removed +primitives had to be added but that didn't take much effort. The binary shrunk +some 30K but (a small amount of) \LUA\ code was added to provide a compatible +functionality (not that we use it). + +% \stopsection + +\stopchapter + +\stopcomponent |