Caching child files in knitr

Question

I have a problem with child files in knitr. The caching works fine, but the dependencies do not work. My sandbox example looks like that:

\documentclass{article}

\begin{document}

<<setup, cache=FALSE>>=
opts_chunk$set(cache=TRUE, autodep=TRUE)
dep_auto() # figure out dependencies automatically
@

<<a>>=
x <- 14
@

<<b>>=
print(x)
@

<<child, child='child.Rnw', eval=TRUE>>=
@

\end{document}

With the 'child.Rnw' looking like this:

<<child>>=
print(x)
@

When I now compile the code, then change x in chunk a and then compile it again: chunk b reacts properly, but the child does not. Am I doing something obviously wrong?

Thanks for the help!

Answer 1

I thought for a while about this issue, and I find it difficult to fix at the moment. The problem is that the parent document does not really know what is in the child document, and dep_auto() does not take the child documents into account when setting the dependency structure. There are two ways to solve this problem. The first one is hackish:

knitr:::dep_list$set(a = c('child', 'b'))

As you probably know, ::: means "danger zone" in R. In knitr, dep_list is the internal object that controls the dependency structure. Both dep_auto() and dep_prev() rely on this object (similarly that is how the chunk option dependson works).

The second way is to write your object into chunk options, e.g.

<<child, whatever=x>>=
print(x)
@

Read the third section in the knitr cache page for details.