[Rd] Sweave driver extension

Kevin R. Coombes kevin.r.coombes at gmail.com
Mon Jan 30 19:07:59 CET 2012

I prefer the code chunks myself.

Function calls have overhead. In a bioinformatics world with large 
datasets and an R default that uses call-by-value rather than 
call-by-reference, the function calls may have a _lot_ of overhead.  
Writing the functions to make sure they use call-by-reference for the 
large objects instead has a different kind of overhead in the stress it 
puts on the writers and maintainers of code.

But then, I'm old enough to have looked at some of Knuth's source code 
for TeX and read his book on Literate Programming, where the ideas of 
"weave" and "tangle" were created for exactly the kind of application 
that Terry asked about.  Knuth's fundamental idea here is that the 
documentation (mainly the stuff processed through "weave") is created 
for humans, while the executable code (in Knuth's view, the stuff 
created by "tangle") is intended for computers.  If you want people to 
understand the code, then you often want to use a top-down approach that 
outlines the structure -- code chunks with forward references work 
perfectly for this purpose.

One of the difficulties in mapping Knuth's idea over to R and Sweave is 
that the operations of weave and tangle have gotten, well, tangled.  
Sweave does not just prepare the documentation; it also executes the 
code in order to put the results of the computation into the 
documentation.  In order to get the forward references to work with 
Sweave, you would have to makes two passes through the file: one to make 
sure you know where each named chunk is and build a cross-reference 
table, and one to actually execute the code in the correct order.  That 
would presumably also require a major rewrite of Sweave.

The solution I use is to cheat and hide the chunks initially and reveal 
them later to get the output that want. This comes down to combining 
eval, echo, keep.source, and expand in the right combinations. Something 

% set up a prologue that contains the code chunks. Do not evaluate or 
display them.
# do something sensible. If multiple steps, define them above here
# using the same idea.
% also define the other code chunks here

\section{Start the First Section}

The \texttt{coxme} function is defined as follows:
coxme <- function(formula, data, subset, blah blah  ){

Argument checking is important:
% Describe the other chunks here



On 1/24/2012 10:24 PM, Yihui Xie wrote:
> Maybe this is a my personal taste: I do not like pseudo R code in the
> form<<coxme-build>>  inside a chunk, and I'm curious about why you do
> not use real R functions to do the job.
> coxme<- function(formula, data, subset, blah blah  ){
>    coxme_check_arguments(...)
>    coxme_build(...)
>    coxme_compute(...)
>    coxme_finish(...)
> }
> You can define these coxme_xxx functions later in the parent
> environment. It is also easy for one function to call another, so the
> recursion is natural. Compared to text-processing tricks, I prefer
> well-defined functions.
> Your idea of using a named list to store R code is what I used in the
> knitr package (http://yihui.github.com/knitr/demo/reference/), e.g.
> % empty here
> <<chunk1, echo=TRUE>>=
> @
> % real code is defined here
> <<chunk1, echo=FALSE>>=
> rnorm(10)
> @
> The second chunk appears later, but when you weave the document, the
> code rnorm(10) will also go to the first chunk since the label
> 'chunk1' will index the code from the second chunk.
> Regards,
> Yihui
> --
> Yihui Xie<xieyihui at gmail.com>
> Phone: 515-294-2465 Web: http://yihui.name
> Department of Statistics, Iowa State University
> 2215 Snedecor Hall, Ames, IA
> On Tue, Jan 24, 2012 at 1:50 PM, Terry Therneau<therneau at mayo.edu>  wrote:
>> Almost all of the coxme package and an increasing amount of the survival
>> package are now written in noweb, i.e., .Rnw files.  It would be nice to
>> process these using the Sweave function + a special driver, which I can
>> do using a modified version of Sweave.  The primary change is to allow
>> the following type of construction
>> <<coxme>>
>> coxme<- function(formula, data, subset, blah blah  ){
>>    <<coxme-check-arguments>>
>>    <<coxme-build>>
>>    <<coxme-compute>>
>>    <<coxme-finish>>
>> }
>> @
>> where the parts referred to come later, and will themselves be made up
>> of other parts.  Since the point of this file is to document source
>> code, the order in which chunks are defined is driven by "create a
>> textbook" thoughts and won't match the final code order for R.
>> The standard noweb driver only allows one level of recursion, and no
>> references to things defined further down in the file.
>>   The primary change to the function simply breaks the main loop into
>> two parts: first read through the all the lines and create a list of
>> code chunks (some with names), then go through the list of chunks and
>> call driver routines.  There are a couple of other minor details, e.g. a
>> precheck for infinite recursions, but no change to what is passed to the
>> driver routines, nor to anything but the Sweave function itself.
>> Primary question: who on the core team should I be holding this
>> conversation with?
>> Secondary: Testing level?  I have a few vignettes but not many.
>>     I'll need a "noweb" package anyway to contain the drivers -- should
>> we just duplicate the modified Sweave under another name?
>>     Call the package "noweb", "Rnoweb", ...?
>> And before someone asks: Roxygen is a completely different animal and
>> doesn't address what I need.  I have latex equations just above the code
>> that impliments them, an annotated graph of the call tree next to the
>> section parsing a formula, etc. This is stuff that doesn't fit in
>> comment lines. The text/code ratio is>1.  On the other hand I've
>> thought very little about integration of manual pages and description
>> files with the code, issues which Roxygen addresses.
>> Terry Therneau
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