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Dean Attali wrote this nice post a few years ago describing knitr’s spin function. This function allows a regular R file, with comments written with the roxygen2-style comment tag #' to be rendered as an HTML document with the comments rendered as text and the results of the R code rendered in place, much as a RMarkdown document would. The basic rules for this are (from Dean’s post):

• Any line beginning with #' is treated as a markdown directive (#' # title will be a header, #' some **bold** text results in some bold text)
• Any line beginning with #+ is parsed as code chunk options
• Run knitr::spin on the file

In effect, this “spinnable” R script is the complement of a RMarkdown document with respect to format, since the RMarkdown document is primarily a text (Markdown) document with code chunks, and the R script is primarily a code document with text chunks.

RMarkdown is nice when you want to create a final document to report your analysis, but can be burdensome when you’re developing code. The R script is much nicer for developing code, but it would be nice to get a nice document at the end of development without too much hassle. It turns out we can flip back and forth between the two complementary formats through functions in knitr, where fname denotes the respecive source file:

• RMarkdown to R : knitr::purl(fname, documentation = 2)
• R to RMarkdown : knitr::spin(fname, knit = FALSE, format = "Rmd")

Why would you want to convert R to RMarkdown when it renders to HTML or Word or PDF automatically (within RStudio), you ask? Because spin uses knit2html/knit2pdf/knit for rendering the documents, rather than rmarkdown::render, which uses the power of Pandoc and hence has richer options in terms of formatting. You could create a pipeline that starts with your R script, creates the intermediate Rmd file using spin, and then uses rmarkdown::render to create the final product.