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-rw-r--r-- | doc/context/documents/general/manuals/lowlevel-macros.pdf | bin | 82764 -> 88216 bytes | |||
-rw-r--r-- | doc/context/sources/general/manuals/lowlevel/lowlevel-macros.tex | 75 |
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diff --git a/doc/context/documents/general/manuals/lowlevel-macros.pdf b/doc/context/documents/general/manuals/lowlevel-macros.pdf Binary files differindex 5f89d204f..5b3bcae14 100644 --- a/doc/context/documents/general/manuals/lowlevel-macros.pdf +++ b/doc/context/documents/general/manuals/lowlevel-macros.pdf diff --git a/doc/context/sources/general/manuals/lowlevel/lowlevel-macros.tex b/doc/context/sources/general/manuals/lowlevel/lowlevel-macros.tex index 200cd8650..00a438781 100644 --- a/doc/context/sources/general/manuals/lowlevel/lowlevel-macros.tex +++ b/doc/context/sources/general/manuals/lowlevel/lowlevel-macros.tex @@ -905,6 +905,81 @@ the values 1 and 3 give warnings and the values 2 and 4 trigger an error. \stopsectionlevel +\startsectionlevel[title=Arguments] + +The number of arguments that a macro takes is traditionally limited to nine (or +ten if one takes the trailing \type {#} into account). That this is enough for +most cases is demonstrated by the fact that \CONTEXT\ has only a handful of +macros that use \type {#9}. The reason for this limitation is in part a side +effect of the way the macro preamble and arguments are parsed. However, because +in \LUAMETATEX\ we use a different implementation, it was not that hard to permit +a few more arguments, which is why we support upto 15 arguments, as in: + +\starttyping[option=TEX] +\def\foo#1#2#3#4#5#6#7#8#9#A#B#C#D#E#F{...} +\stoptyping + +We can support the whole alphabet without much trouble but somehow sticking to +the hexadecimal numbers makes sense. It is unlikely that the core of \CONTEXT\ +will use this option but sometimes at the user level it can be handy. The penalty +in terms of performance can be neglected. + +\stopsectionlevel + +\startsectionlevel[title=Constants] + +The \LUAMETATEX\ engine has lots of efficiency tricks in the macro parsing and +expansion code that makes it not only fast but also let is use less memory. +However, every time that the body of a macro is to be injected the expansion +machinery kicks in. This often means that a copy is made (pushed in the input and +used afterwards). There are however cases where the body is just a list of +character tokens (with category letter or other) and no expansion run over the +list is needed. + +It is tempting to introduce a string data type that just stores strings and +although that might happen at some point it has the disadvantage that one need to +tokenize that string in order to be able to use it, which then defeats the gain. +An alternative has been found in constant macros, that is: a macro without +parameters and a body that is considered to be expanded and never freed by +redefinition. There are two variants: + +\starttyping[option=TEX] +\cdef \foo {whatever} +\cdefcsname foo\endcsname{whatever} +\stoptyping + +These are actually just equivalents to + +\starttyping[option=TEX] +\edef \foo {whatever} +\edefcsname foo\endcsname{whatever} +\stoptyping + +just to make sure that the body gets expanded at definition time but they are +also marked as being constant which in some cases might give some gain, for +instance when used in csname construction. The gain is less then one expects +although there are a few cases in \CONTEXT\ where extreme usage of parameters +benefits from it. Users are unlikely to use these two primitives. + +Another example of a constant usage is this: + +\starttyping[option=TEX] +\lettonothing\foo +\stoptyping + +which gives \type {\foo} an empty body. That one is used in the core, if only because +it gives a bit smaller code. Performance is no that different from + +\starttyping[option=TEX] +\let\foo\empty +\stoptyping + +but it saves one token (8 bytes) when used in a macro. The assignment itself is +not that different because \type {\foo} is made an alias to \type {\empty} which +in turn only needs incrementing a reference counter. + +\stopsectionlevel + \stopdocument % freezing pitfalls: |