Use multi-line string to support multiple statements in R functions

This PR proposes to use literal style multi-line string in YAML to support seamless formatting of R functions defined in the knit field and avoid possible confusions from indentation details.

Specifically, the current code example for customizing the knit button behavior works but does not work for any non-trivial R functions containing multiple statements. For example, let's add a print(input) call before the rmarkdown::render call. Using the current approach:

knit: (function(input, ...) {
    print(input)
    rmarkdown::render(
      input,
      output_file = paste0(
      xfun::sans_ext(input), '-', Sys.Date(), '.html'
    ),
    envir = globalenv()
    )
  })

The function will be parsed as:

[1] "(function(input, ...) { print(input) rmarkdown::render( input, output_file = paste0( xfun::sans_ext(input), '-', Sys.Date(), '.html' ), envir = globalenv() ) })"

This does not execute:

Error: unexpected symbol in "(function(input, ...) { invisible(system(paste0('(function(input, ...) { print(input) rmarkdown::render( input, output_file = paste0( xfun::sans_ext(input), '-', Sys.Date(), '.html"
Execution halted

A workaround is adding ; after the first statement, which is non-idiomatic.

Instead, if we write

knit: |
  (function(input, ...) {
    print(input)
    rmarkdown::render(
      input,
      output_file = paste0(
      xfun::sans_ext(input), '-', Sys.Date(), '.html'
    ),
    envir = globalenv()
    )
  })

This will be parsed as:

[1] "(function(input, ...) {\n  print(input)\n  rmarkdown::render(\n    input,\n    output_file = paste0(\n    xfun::sans_ext(input), '-', Sys.Date(), '.html'\n  ),\n  envir = globalenv()\n  )\n})"

which can be correctly executed.

In reality, users can write the R function separately, copy and paste it here, and simply add two spaces before each line.

rstudio / rmarkdown-cookbook

Use multi-line string to support multiple statements in R functions #384