rmarkdown read code from file and display with highlight - r

Question

I have two RMarkdown files. main.Rmd which is the main file which is rendered as well as example.Rmd which holds a longer example and is used elsewhere (hence it lives in its own document).
I want to include example.Rmd in the main.Rmd file with code highlighting of its RMarkdown code but the code of example.Rmd does not need to be executed, as if I set eval=FALSE and copied all code into the chunk by hand.
An example MWE is
main.Rmd
---
title: This is main.rmd
output: html_document
---
```{r}
# attempt that doesnt work
cat(readLines("example.Rmd"), sep = "\n")
```
and in example.Rmd
---
title: This is example.rmd
output: html_document
---
```{r}
# code that is not executed but shown in main.Rmd
data <- ...
```

Answer 1

Set eval=FALSE in the example.Rmd file and then include it in main.Rmd using child chunk option.
example.Rmd
---
title: This is example.Rmd
---
```{r setup, include=FALSE}
knitr::opts_chunk$set(eval = FALSE)
```
```{r}
# This is from example.Rmd
x <- rnorm(10)
y <- rnorm(10)
lm(y ~ x)
```
```{r}
# some comments
print("This is from example.Rmd")
```
main.Rmd
---
title: This is main.Rmd
output:
html_document:
highlight: haddock
---
```{r example, child="example.Rmd"}
```
Edit
To show full source-code of the Rmarkdown file, one possible option could be reading that Rmd file and then cat it with chunk option comment="".
Now about the syntax highlighting; there's a chunk option class.output with which it is possible to specify a language name for which pandoc supports syntax highlighting.
You can get the list of language names for which pandoc has syntax highlighting support by running the following,
pandoc --list-highlight-languages
(Note, if you don't have pandoc installed separately, you can also use the pandoc installed with Rstudio itself. Run rmarkdown::pandoc_exec() to get the pandoc executable path)
Now, the file we are trying to include actually contains not just R code, but also markdown and yaml syntaxes. So it's a kind of mixed thing and pandoc has no syntax highlighting support out of the box for this. Still I have chosen c as syntax highlighting language just to show the possibility. (Also tried r, but syntax-highlighting is not so distinctive)
---
title: This is main.Rmd
output:
html_document:
highlight: tango
---
## Rmarkdown
```{r example, echo=FALSE, class.output="c", comment=""}
cat(readLines("example.Rmd"), sep = "\n")
```
But still if you want a specific syntax-highlighting for Rmarkdown, you can actually create one. See here from the pandoc documentation itself and also this answer on SO about this.