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

[Kevin Coombes]

Hi,

Unless someone is planning to change Stangle to include inline 
expressions (which I am *not* advocating), I think that relying on 
side-effects within an \Sexpr construction is a bad idea. So, my own 
coding style is to restrict my use of \Sexpr to calls of the form 
\Sexpr{show.the.value.of.this.variable}. As a result, I more-or-less 
believe that having R CMD check use Stangle and report an error is 
probably a good thing.

There is a completely separate questions about the relationship between 
Sweave/Stangle or knit/purl and literate programming that is linked to 
your question about whether to use Stangle on vignettes. The underlying 
model(s) in R have drifted away from Knuth's original conception, for 
some good reasons.

The original goal of literate programming was to be able to explain the 
algorithms and data structures in the code to humans.  For that purpose, 
it was important to have named code chunks that you could move around, 
which would allow you to describe the algorithm starting from a high 
level overview and then drilling down into the details. From this 
perspective, "tangle" was critical to being able to reconstruct a 
program that would compile and run correctly.

The vast majority of applications of Sweave/Stangle or knit/purl in 
modern R have a completely different goal: to produce some sort of 
document that describes the results of an analysis to a non-programmer 
or non-statistician.  For this goal, "weave" is much more important than 
"tangle", because the most important aspect is the ability to integrate 
the results (figures, tables, etc) of running the code into the document 
that get passed off to the person for whom the analysis was prepared. As 
a result, the number of times in my daily work that I need to explicitly 
invoke Stangle (or purl) explicitly is many orders of magnitude smaller 
than  the number of times that I invoke Sweave (or knitr).

   -- Kevin


On 5/30/2014 1:04 AM, Yihui Xie wrote:
> 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
>
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