Convert Rmd document (knitr::knit) to an R script (knitr::spin) #RMarkdown #knitr #knit #spin

backstitch.R

warning("You probably don't want to use this `backstitch()` function.",
        "\n  It's hacky and there's a much better option in knitr called `purl()`.",
        "\n  More info at: `?knitr::purl`")

#' Backstitch an Rmd file to an R script
#'
#' Takes an Rmd file -- that would be converted with knitr::knit() -- and
#' "backstitches" it into an R script suitable for knitr::purl(). The output
#' file is the just the backstitched R script when `output_type = 'script'`, or
#' just the code chunks when `output_type = 'code'` (note that all inline code
#' is dropped in this case). Or finally, output both with 
#' `output_type = 'both'`.
#'
#' You can load this function by calling:
#' devtools::source_gist('284671997992aefe295bed34bb53fde6', filename = 'backstitch.R')
#' 
#' @param infile Input file name
#' @param outfile Output file name (`.R` extension added if not included)
#' @param ouput_type One of `script`, `code` or `both`. If `both`, two files are
#'   created, with `_code` appended to the file name of the code chunks.
#' @param chunk_header Chunk header style, valid options are `"#-"`, `"#+"`, and `"# ----"`
backstitch <- function(
  infile, 
  outfile = NULL, 
  output_type = c('both'),
  chunk_header = "# ----"
) {
  requireNamespace('knitr', quietly = TRUE)
  requireNamespace('stringr', quietly = TRUE)
  stopifnot(output_type %in% c('script', 'code', 'both'))
  
  if (is.null(outfile) && output_type == 'both') 
    stop("Please choose output_type of 'script' or 'code' when not outputting to a file.")
  
  knitr::knit_patterns$set(knitr::all_patterns[['md']])
  
  x <- readLines(infile)
  if (inherits(infile, 'connection')) close(infile)
  
  empty_lines <- which(stringr::str_detect(x, "^\\s?+$"))
  last_non_empty_line <- max(setdiff(seq_along(x), empty_lines))
  x <- x[1:last_non_empty_line]
  
  x_type <- rep('text', length(x))
  
  # Find YAML section
  yaml_markers <- which(stringr::str_detect(x, "^[-.]{3}\\s*$"))
  if (length(yaml_markers) > 2) {
    message("Input file may have multiple YAML chunks, only considering lines",
            paste(yaml_markers[1:2], collapse='-'), 'as YAML header.')
  }
  if (length(yaml_markers) > 0) {
    i.yaml <- yaml_markers[1]:yaml_markers[2]
    x_type[i.yaml] <- 'yaml'
  }
  
  # Mark code chunk.begin, chunk.end and regular chunk codelines
  i.chunk.begin <- which(stringr::str_detect(x, knitr::knit_patterns$get('chunk.begin')))
  i.chunk.end   <- which(stringr::str_detect(x, knitr::knit_patterns$get('chunk.end')))
  x_type[i.chunk.end] <- 'chunk.end'
  for (j in i.chunk.begin) {
    j.chunk.end <- min(i.chunk.end[i.chunk.end > j])-1
    x_type[j:j.chunk.end] <- 'chunk'
  }
  x_type[i.chunk.begin] <- 'chunk.begin'
  
  # Check for inline code
  i.inline <- which(stringr::str_detect(x, knitr::knit_patterns$get('inline.code')))
  i.inline <- intersect(i.inline, which(x_type == 'text'))
  x_type[i.inline] <- 'inline'
  
  # Check empty lines
  i.empty <- which(stringr::str_detect(x, "^\\s*$"))
  i.empty <- intersect(i.empty, which(x_type == 'text'))
  x_type[i.empty] <- 'empty'
  
  really_empty <- function(x_type, j, n = -1) {
    if (grepl('(chunk|yaml)', x_type[j + n])) {
      return('empty')
    } else if (n < 0) {
      return(really_empty(x_type, j, 1))
    } else if (x_type[j + n] %in% c('text', 'inline')) {
      return('text')
    } else {
      return(really_empty(x_type, j, n+1))
    }
  }
  
  for (j in i.empty) {
    x_type[j] <- really_empty(x_type, j)
  }
  
  # Rewrite lines helper functions
  comment <- function(x) paste("#'", x) 
  make_chunk_header <- function(x, chunk_header) {
    stringr::str_replace(stringr::str_replace(x, knitr::knit_patterns$get('chunk.begin'), "\\1"),
                         "^r[, ]?", paste(chunk_header, ""))
  }
  # Rewrite lines
  y <- x
  regex_inline_grouped <- "`r[ ]?#?(([^`]+)\\s*)`"
  i.empty       <- which(x_type == 'empty')
  i.text        <- which(x_type == 'text')
  y[i.chunk.begin] <- make_chunk_header(x[i.chunk.begin], chunk_header)
  y[i.inline]      <- comment(stringr::str_replace_all(x[i.inline], regex_inline_grouped, "{{\\1}}"))
  y[i.text]        <- comment(x[i.text])
  if (length(yaml_markers) > 0) y[i.yaml] <- comment(x[i.yaml])
  y[i.empty]       <- ""
  y[i.chunk.end]   <- ""
  
  y_code <- y[which(stringr::str_detect(x_type, 'chunk'))]
  
  if (!is.null(outfile)){ 
    outfile_name <- stringr::str_replace(outfile, "(.+)\\.R$", "\\1")
    if (output_type == "script") {
      cat(c(y, ""), file = paste0(outfile_name, ".R"), sep = '\n')
    } else if (output_type == "code") {
      cat(c(y_code, ""), file = paste0(outfile_name, ".R"), sep = '\n')
    } else {
      cat(c(y, ""), file = paste0(outfile_name, ".R"), sep = '\n')
      cat(c(y_code, ""), file = paste0(outfile_name, "_code.R"), sep = '\n')
    }
  } else {
    switch(
      output_type,
      'script' = unname(y),
      'code' = unname(y_code)
    )
  }
}

@isteves - thanks! I apparently have a habit of unwittingly re-writing parts of knitr, and this was one of those times.