Can rmarkdown return a value to a target - drake-r-package

Question

I find myself using rmarkdown/rnotebooks quite a bit to do exploratory analysis since I can combine code, prose and graphs. Many a times, I'll write my entire predictive modeling approach and the model itself within markdown.
However, then I end up with forecast models embedded within rmarkdown, unlinked to a target within my drake_plan. Today, I save these to disk first, then read them back in to my plan using file_in or other similar approach.
My question is - can I have a markdown document return an object directly to a drake target?
Conceptually:
plan = drake_plan(
dat = read_data(),
model = analyze_data(dat)
)
analyse_data = function(dat){
result = render(....)
return(result)
}
This way - I can get my model directly into my drake target, but if I need to investigate the model, I can open up my markdown/HTML.

Answer 1

I recommend you include those models as targets in the plan, but what you describe is possible. R Markdown and knitr automatically run code chunks in the calling environment, so the variable assignments you make in the report are available.
library(drake)
library(tibble)
simulate <- function(n){
tibble(x = rnorm(n), y = rnorm(n))
}
render_and_return <- function(input, output) {
rmarkdown::render(input, output_file = output, quiet = TRUE)
return_value # Assigned in the report.
}
lines <- c(
"---",
"output: html_document",
"---",
"",
"```{r show_data}",
"return_value <- head(readd(large))", # return_value gets assigned here.
"```"
)
writeLines(lines, "report.Rmd")
plan <- drake_plan(
large = simulate(1000),
subset = render_and_return(knitr_in("report.Rmd"), file_out("report.html")),
)
make(plan)
#> target large
#> target subset
readd(subset)
#> # A tibble: 6 x 2
#> x y
#> <dbl> <dbl>
#> 1 1.30 -0.912
#> 2 -0.327 0.0622
#> 3 1.29 1.18
#> 4 -1.52 1.06
#> 5 -1.18 0.0295
#> 6 -0.985 -0.0475
Created on 2019-10-10 by the reprex package (v0.3.0)