When editing an .R-file I see that ess-r-mode is on ("ESS[R]" in the status line). If an R session is opened in another buffer, I see that inferior-ess-r-mode is on there ("iESS [R:1]").

In the R-session buffer I see that there's TAB completion: it completes a string with possible function/variable names, and within a function's parentheses it gives list of arguments. That's very convenient. In the .R-file buffer, on the other hand, this doesn't happen.

Is it possible to have this kind of completion in the buffer editing the .R-file, that is, in ess-r-mode?

I see that ess-use-company and ess-use-eldoc are both t, at their standard values.

2 Answers 2


As you've discovered, ess-r-mode doesn't provide completion until it's associated with an R process. This is because "In R versions newer than 2.14.1, ESS uses R's built-in completion engine" ESS Manual. Which means there's nothing to complete without an R process connected.

I don't think there's a way to automatically attach a script buffer to an R process - ESS would have to know if you wanted multiple scripts to share the same process, or to each get their own process, or some combination.

My work-around: in practice, I usually want to run code from my script as soon as I open it (such as loading packages or data), which means completion is available as soon as I do. If I want completion before I've written any code at all, I will usually just send a ls() to get R started, and then work from there.


Another work around is to automatically call ess-switch-process (i.e., C-c C-s) when you open a file in r-mode, the following code in your init should do it:

(add-hook 'ess-r-mode-hook 'my-start-r-process)

(defun my-start-r-process ()

or even just the single line:

(add-hook 'ess-r-mode-hook 'ess-switch-process)
  • Thank you for the explanation and work-around! I think I can just write a macro to call C-c C-s directly after the R session is started; or something to that effect.
    – pglpm
    Nov 6, 2023 at 19:08
  • 1
    @pglpm I misunderstood what you meant. You can use a hook to call C-c C-s, I'll update the answer
    – Tyler
    Nov 6, 2023 at 19:37
  • Fantastic, thank you!
    – pglpm
    Nov 6, 2023 at 20:15

I just tried, ess-r-mode do have completion. Maybe check the value of company-backends. I have
(company-R-library company-R-args company-R-objects :separate) in it, did not need to set it myself though.

  • Thank you Sean. I discovered that completion gets activated if I "attach" the file-buffer with an R session, with C-c C-s. I'm trying to understand whether the attachment can be done automatically. But maybe with company I don't need to do this.
    – pglpm
    Nov 4, 2023 at 21:38

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