[R-pkg-devel] How to write a 'cases' environment in .Rd files?

[Ivan Krylov]

On Thu, 11 Aug 2022 18:43:44 +0000
Marius Hofert <marius.hofert using uwaterloo.ca> wrote:

> Since recently, r-devel-linux-x86_64-debian-clang and
> r-devel-linux-x86_64-debian-gcc trigger the note 'Undefined control
> sequence: \cases in F(x) = \cases{...', possibly due to KaTeX
> rendering help pages. I looked around quite a bit but couldn't find a
> way to implement 'cases' environments. For example,
> https://katex.org/docs/supported.html#environments mentions
> \begin{cases} \end{cases}, so I tried
> 
> \deqn{F(x) = \begin{cases}
> 1,&\text{if}\ x > 0,\\
> 0,&\text{if} x = 0.}}{...}
> 
> but that triggers:
> 
> ...
> * checking PDF version of manual ... WARNING
> LaTeX errors when creating PDF version.
> This typically indicates Rd problems.
> LaTeX errors found:
> ! Misplaced alignment tab character &

There is a possible workaround, but it's the opposite of beautiful. You
can use conditional text to compile one version of the formula for HTML
help and another one for PDF help:

\ifelse{html}{\eqn{\begin{cases}...}}{\eqn{\cases{...}}{ascii}}

It's as tedious as it looks. For extra style(?) points, you can put a
three-argument user-defined macro in man/macros/whatever.Rd that
forwards its arguments to one of the \ifelse branches. As far as I
remember, \newcommand{}{} has to fit on one line, but in R ≥ 3.6.0,
macros themselves can take multi-line arguments.

If you're targeting an earlier version of R, you can make multi-line
arguments work by forcing R to cache the parse tree of the man page
while building a source tarball with a newer R version using
\Sexpr[stage=build]{}, costing you some time during R CMD build and a
slightly larger source tarball. (It's an implementation detail that one
normally shouldn't depend upon. Thankfully, as of 2022, there won't be
any newer versions of R ≤ 3.5.x to break it.) Support for
man/macros/*.Rd only appeared in R-3.2.0, so I haven't researched
compatibility tricks for even older versions of R.

-- 
Best regards,
Ivan