Using box dimensions in fp expressions

[blefloch]

Box dimensions can be used directly in dimension expressions but not floating point expressions.

\hbox_set:Nn \l_tmpa_box { abc }
\dim_show:n { \box_wd:N \l_tmpa_box }
\fp_show:n { \box_wd:N \l_tmpa_box }

[blefloch]

In particular, this issue forbids the \Height, \Depth, \Width, and \TotalHeight design-level shorthands in xcoffins to possibly work in a floating point expression. The reason l3fp doesn't just apply \tex_the:D blindly is that 2.5 \c_zero_dim should not be unpacked to 2.50pt but to 2.5*0pt.

This means that we should only apply \tex_the:D when l3fp starts reading a number (perhaps also just after e). If we decide to do that, 2 \c_two 3 would give 2 * 23, which is a bit odd, but unavoidable.

[josephwright]

Fix at the coffin level

[josephwright]

@blefloch Seems to me there are two slightly different issues here. If you look at xcoffins we have

\cs_new_protected:Npn \@@_design_names:N #1
  {
    \dim_set:Nn \l_@@_height_dim { \coffin_ht:N #1 }
    \dim_set:Nn \l_@@_depth_dim  { \coffin_dp:N #1 }
    \dim_set:Nn \l_@@_width_dim  { \coffin_wd:N #1 }
    \dim_set:Nn \l_@@_totalheight_dim
      { \l_@@_height_dim + \l_@@_depth_dim }
    \cs_set_eq:NN \Height \l_@@_height_dim
    \cs_set_eq:NN \Depth \l_@@_depth_dim
    \cs_set_eq:NN \Width \l_@@_width_dim
    \cs_set_eq:NN \TotalHeight \l_@@_totalheight_dim
  }

which means \Height, etc. is not the same as \coffin_ht:N <name>. So the first question is is it that right (I'd say yes: this works nicely) and the second is do we see a need to alter \box_wd:N (#227).

As an artificial example that \Height etc. do work

\documentclass{article}
\usepackage{expl3}
\usepackage{xcoffins}
\NewCoffin\CoffinA
\begin{document}

\SetHorizontalCoffin\CoffinA{\rule{10pt}{10pt}}
\ExplSyntaxOn
\SetHorizontalPole\CoffinA{a}
  {\fp_to_dim:n{0.666\Width}}
\ShowCoffinStructure\CoffinA

\end{document}

gives

Size of coffin \CoffinA:
> ht = 10.0pt
> dp = 0.0pt
> wd = 10.0pt
Poles of coffin \CoffinA:
>  l  =>  {0pt}{0pt}{0pt}{1000pt}
>  hc  =>  {5.0pt}{0pt}{0pt}{1000pt}
>  r  =>  {10.0pt}{0pt}{0pt}{1000pt}
>  b  =>  {0pt}{0.0pt}{1000pt}{0pt}
>  vc  =>  {0pt}{5.0pt}{1000pt}{0pt}
>  t  =>  {0pt}{10.0pt}{1000pt}{0pt}
>  B  =>  {0pt}{0pt}{1000pt}{0pt}
>  H  =>  {0pt}{0pt}{1000pt}{0pt}
>  T  =>  {0pt}{0pt}{1000pt}{0pt}
>  a  =>  {0pt}{6.66pt}{1000pt}{0pt}.
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