a hook to print highlighted source of a function

printfun.Rnw

\documentclass{article}

<<hook-printfun, echo=FALSE>>=
knit_hooks$set(printfun = function(before, options, envir) {
  if (before) return()
  txt = capture.output(dump(options$printfun, ''))
  ## reformat if tidy=TRUE
  if (options$tidy) txt = formatR::tidy.source(text = txt, output = FALSE)$text.tidy
  con = textConnection(txt)
  on.exit(close(con))
  out = capture.output({
    highlight::highlight(con, renderer = highlight::renderer_latex(document= FALSE), 
                         showPrompts = options$prompt, size = options$size)
  })
  paste(out, collapse = '\n')
})
@

\begin{document}

<<ordinary-print>>=
fivenum
@

<<test-fivenum, printfun='fivenum', echo=FALSE>>=
# source code of fivenum
@

<<my-fun, echo=FALSE>>=
foo = function(a = 1L) {
  # here is a comment
  1+2
}
@
<<print-my-fun, printfun='foo', prompt=TRUE>>=
# here is the foo function, printed with messy prompts
@

% or not formatted
<<print-my-fun, printfun='foo', tidy=FALSE>>=
@
\end{document}