% language=us runpath=texruns:manuals/math

\environment math-layout

% goes to oddities

\startcomponent math-notdone

\startchapter[title=Why we don't do this]

The \TEX\ engine has two math modes: inline and display. What exactly happens
before we enter math depends on what mode we are in.

\startitemize
    \startitem
        When we are in horizontal mode, single dollar signs makes us go into
        inline math mode. Double dollars on the other hand trigger display math
        and one of the first things that does is flush the current vertical list
        contributions. As we are in horizontal mode it is likely that something
        shows up.
    \stopitem
    \startitem
        When we are in vertical mode the engine will trigger a new paragraph in
        the hard way: the initial par state node gets added as well as the
        indentation and everything that \type {\everypar} likes to inject. Inline
        math then stays inline but display math starts out with flushing the
        current paragraph and that happens to be the state, indentation and
        everything else injected at the beginning. Often nothing is seen, just
        an empty line sits there.
    \stopitem
    \startitem
        In math mode the engine will check if we ended the right way and then
        render and inject the formula.
    \stopitem
\stopitemize

Especially the second case is tricky. The \quote {empty} line is placed with
proper \typ {\parskip}, \typ {\parindent} and whatever. That can end up with a
lot of supposedly whitespace of which is not really clear (for the user) what
comes from where: interline space, whitespace, line height, display spacing, etc.
To that you can add that empty lines are special themselves: in \MKII\ we relax
\type {\par}, in \MKIV\ we setup {\LUATEX} to accept \type {\par} equivalents and
\LUAMETATEX\ has no checks at all. \footnote {The same can be told about \type
{\outer}.}

Before we show some examples of this we have to make clear that \CONTEXT\ users
don't have to worry about all this. We don't follow the (Plain and
\LATEX) \AMS\ policy of \quotation {no empty lines before display math} and
\quotation {no empty lines in display math}. We don't even use the display skip
parameters and we do compensate for these empty line issues. The basics haven't
changed between \MKII, \MKIV\ and \LMTX\ but it has become leaner and meaner deep
down. We could actually remove display mode (not to be confused with \type
{\displaystyle}) from the engine but then we could not show the examples below.

\startbuffer[zero]
\dorecurse{10}{test #1 }
\startformula
    x = \the\mathdisplayskipmode + \the\mathdisplaymode
\stopformula
\dorecurse{10}{test #1 }
\stopbuffer

\startbuffer[one]
\dorecurse{10}{test #1 }       \par
$$ x = \the\mathdisplayskipmode + \the\mathdisplaymode $$ \par
\dorecurse{10}{test #1 }       \par
\stopbuffer

\startbuffer[two]
\dorecurse{10}{test #1 }
$$ x = \the\mathdisplayskipmode + \the\mathdisplaymode $$
\dorecurse{10}{test #1 }
\stopbuffer

We use this as example:

\typebuffer[one]

as well as this:

\typebuffer[two]

In \CONTEXT\ one does this:

\typebuffer[zero]

and get:

{\showmakeup[line] \getbuffer[zero]}

% \predisplaysize

For the examples with dollars we use this setup:

\startbuffer[setup]
\everypar              {!}
\mathdisplaymode       0
\normalizelinemode     0
\parindent             30pt
\parskip               40pt
\abovedisplayskip      20pt
\belowdisplayskip      \abovedisplayskip
\abovedisplayshortskip \abovedisplayskip
\belowdisplayshortskip \abovedisplayshortskip
\stopbuffer

\typebuffer[setup]

We make sure that we get something typeset when we start a paragraph, just in
order that we don't necessarily get empty lines: there can be many. The two modes
make sure that we operate in regular \TEX\ mode, the way other macro packages do.

First we show what we get with \typ {\mathdisplayskipmode 1}. This makes the
engine behave like other engines. We don't show what we get with \typ
{\mathdisplayskipmode 2} where zero skips are ignored but we don't have these.
Last we show what we get with \typ {\mathdisplayskipmode 3}. Here the math
specific skips are completely ignored.

In both shown cases you will notice that the \type {\parskip} and \type
{\baselineskip} interfere which in turn demonstrates why in \CONTEXT\ we always
did things differently. It is possible to enhance the engine to deal with this
but because in \CONTEXT\ we don't need it and because it is unlikely that other
macro packages will use it no time will be wasted on this.

\startbuffer
\page
\start
\showmakeup[line]
\getbuffer[setup,one,two]
\stop
\page
\stopbuffer

{\mathdisplayskipmode 1 \getbuffer}
{\mathdisplayskipmode 3 \getbuffer}


% \mathdisplaymode 0
% before \par $$             \vadjust {\bf post 1} \vadjust {\bf post 2} x_2 \vadjust pre {\bf pre} $$ \par after \blank[3*line]
% before \par $$             \vadjust {\bf post 1} \vadjust {\bf post 2} x_2 \vadjust pre {\bf pre} $$ \par after \blank[3*line]
% before \par $              \vadjust {\bf post 1} \vadjust {\bf post 2} x_3 \vadjust pre {\bf pre} $  \par after \blank[3*line]
% before \par $\displaystyle \vadjust {\bf post 1} \vadjust {\bf post 2} x_3 \vadjust pre {\bf pre} $  \par after \blank[3*line]

\stopchapter

\stopcomponent