Defer in R Programming Language

In R, we don’t have a direct equivalent of the defer keyword. However, we can achieve similar functionality using the on.exit() function, which allows us to specify code that will be executed when the current function exits, either normally or as the result of an error.

Let’s see how we can implement a similar example in R:

library(base)

# Suppose we wanted to create a file, write to it,
# and then close when we're done. Here's how we could
# do that with on.exit().

main <- function() {
  # Immediately after getting a file connection with
  # createFile, we set up the closing of that file
  # with on.exit(). This will be executed at the end
  # of the enclosing function (main), after
  # writeFile has finished.
  f <- createFile("/tmp/defer.txt")
  on.exit(closeFile(f))
  writeFile(f)
}

createFile <- function(p) {
  cat("creating\n")
  f <- file(p, "w")
  if (!isOpen(f)) {
    stop("Failed to create file")
  }
  return(f)
}

writeFile <- function(f) {
  cat("writing\n")
  writeLines("data", f)
}

# It's important to check for errors when closing a
# file, even in an on.exit() function.
closeFile <- function(f) {
  cat("closing\n")
  tryCatch({
    close(f)
  }, error = function(e) {
    cat(sprintf("error: %s\n", e$message), file = stderr())
    quit(status = 1)
  })
}

# Run the main function
main()

Running the program confirms that the file is closed after being written.

$ Rscript defer.R
creating
writing
closing

In this R version:

1. We use on.exit() to ensure that closeFile() is called when main() exits.
2. Instead of os.Create(), we use R’s file() function to create a file connection.
3. Error handling is done using tryCatch() in the closeFile() function.
4. We use cat() for printing messages and writeLines() for writing to the file.
5. The stop() function is used for error handling in createFile().

This R code demonstrates how to use on.exit() to ensure cleanup operations are performed, similar to the defer keyword in other languages. It’s particularly useful for closing file connections, releasing resources, or performing any necessary cleanup when a function exits.