[Rd] R CMD check for the R code from vignettes

[Yihui Xie]

Hi,

Recently I saw a couple of cases in which the package vignettes were
somewhat complicated so that Stangle() (or knitr::purl() or other
tangling functions) can fail to produce the exact R code that is
executed by the weaving function Sweave() (or knitr::knit(), ...). For
example, this is a valid document that can pass the weaving process
but cannot generate a valid R script to be source()d:

\documentclass{article}
\begin{document}
Assign 1 to x: \Sexpr{x <- 1}
<<>>=
x + 1
@
\end{document}

That is because the inline R code is not written to the R script
during the tangling process. When an R package vignette contains
inline R code expressions that have significant side effects, R CMD
check can fail because the tangled output is not correct. What I
showed here is only a trivial example, and I have seen two packages
that have more complicated scenarios than this. Anyway, the key thing
that I want to discuss here is, since the R code in the vignette has
been executed once during the weaving process, does it make much sense
to execute the code generated from the tangle function? In other
words, if the weaving process has succeeded, is it necessary to
source() the R script again?

The two options here are:

1. Do not check the R code from vignettes;
2. Or fix the tangle function so that it produces exactly what was
executed in the weaving process. If this is done, I'm back to my
previous question: does it make sense to run the code twice?

To push this a little further, personally I do not quite appreciate
literate programming in R as two separate steps, namely weave and
tangle. In particular, I do not see the value of tangle, considering
Sweave() (or knitr::knit()) as the new "source()". Therefore
eventually I tend to just drop tangle, but perhaps I missed something
here, and I'd like to hear what other people think about it.

Regards,
Yihui
--
Yihui Xie <xieyihui at gmail.com>
Web: http://yihui.name