Make execution stop on error in RStudio / Interactive R session

Question

When executing a block of code in RStudio, execution doesn't actually stop when an error occurs. For example, if I have the following code in the open editor:

x <- 'test'
stopifnot(is.numeric(x))
print('hello world')

And run it (either with command-return or by clicking the "Run" button), it prints the error but then marches on and executes the print statement.

Is there a way to configure RStudio to not proceed past the error? i.e. make it stop at line 2 above and not proceed to the print statement?

EDIT: Just realized this also happens if I'm sending blocks of code in the standard R GUI using command-R.

Answer 1

One way to solve this is to configure the R 'error' option to run a custom function when an error is encountered.

An example error-handling function can pause execution of R using Sys.sleep and await user input (for instance, breaking-out of the running function by pressing ctrl-C, or Escape in R-Studio on my Mac).

Normally, R will then move-on and execute any subsequent commands. However, we can prevent this by defining a function that will 'eat' all of these so that they don't get run. To make this slicker-still, we can output a 'cursor up' character sequence after each 'eaten' line, which erases each of them as the next one is output, so that the not-run lines don't clutter up the terminal/console.

Example code:

# Define our error-handling pause function:

pause=function(){
    on.exit(eat_input())
    cat("Paused: press ctrl-C (or Escape key in Rstudio) to continue\n")
    cat("(any subsequent code will be ignored)\n")
    Sys.sleep(Inf)
}

# Define our 'eat_input' function to prevent output of subsequent, not-run input:

eat_input=function(){
    cat("\033[1A")
    while((x=readline())!='')cat("\033[1A")
}

# Set the 'error' option to execute our pause function:

options(error=pause)

Try it out:

print("Before the error")

# an error: we want to stop after this
xxx

# some more input: we don't want this to be run or to appear in the console
print("After the error")

To switch-off this behaviour, just re-set the error option to NULL:

options(error=NULL)

Answer 2

I don't think that there is a way to prevent RStudio from running all the lines, when you select a section and press Ctrl+Enter. Rstudio is just running one line after the other. Even if stopifnot() is called inside of a function, all the lines after that function call will still be evaluated.

If your goal is simply to be informed when something goes wrong, before a lot of code is run in vain, maybe you could define a function similar to stopifnot() that will just go into an endless loop, if there is an error. You could then press Esc or the Stop-Button in RStudio to interrupt the program. Something like this:

waitifnot <- function(cond) {
  if (!cond) {
    message(deparse(substitute(cond)), " is not TRUE")  
    while (TRUE) {}
  }
}

Now, you can run your example code with this function:

x <- 'test'
waitifnot(is.numeric(x))
print('hello world')

As expected, hello world is never printed. You will get an error message, telling you that something went wrong, and then the program will wait until you abort it manually.

This won't work well in any situation other than interactive mode. To avoid unpleasant situations, you could also let the function react differently, if it is not used in interactive mode, for instance like this:

waitifnot <- function(cond) {
  if (!cond) {
    msg <- paste(deparse(substitute(cond)), "is not TRUE")
    if (interactive()) {
      message(msg)
      while (TRUE) {}
    } else {
      stop(msg)
    }
  }
}

This function will go into an endless loop only if run in interactive mode. Otherwise, it will simply abort execution by calling stop(). I have checked that this works as expected with Ctrl+Enter or the Source button in RStudio (endless loop) and with Rscript on the Bash command line (abort of the program).