CERN-ROOT-mode

CERN-ROOT-mode is an Emacs major-mode for interacting with ROOT (https://root.cern/). Specifically, this package provides the facilities to run the ROOT command line interface/REPL.

Installation & Configuration

You can install the package from melpa, or use straight.el and download the package directly from this github repository. Here is one example of installing/configuring the package with straight.el:

(use-package cern-root-mode
  :bind (:map c++-mode-map
	     (("C-c C-c" . cern-root-eval-defun)
	      ("C-c C-b" . cern-root-eval-buffer)
	      ("C-c C-l" . cern-root-eval-file)
	      ("C-c C-r" . cern-root-eval-region)))
  :straight (cern-root-mode :type git :host github :repo "jaypmorgan/cern-root-mode")
  :config
  (setq cern-root-filepath "/path/to/root"))

There are only a few variables provided by the package:

• cern-root-filepath -- (string, default "root") this is the absolute/relative path to the root executable. This could be left as the default "root" value if ROOT is available on your $PATH.
• cern-root-command-option -- (string, default "") these are the command line options used when starting ROOT. By default there are no options.
• cern-root-prompt-regex -- (string, default "^\\[[0-9;^k]+m?") this is the regular expression used to find the input prompt of the ROOT REPL. If there is a customised prompt, this will need to be updated to suit the customisation.
• cern-root-buffer-name -- (string, default "*ROOT*") what to call the ROOT repl buffer when it starts. This could be left as the default *ROOT*, but its there for you to modify as you please.
• cern-root-terminal-backend -- (symbol, default 'terminal) the terminal emulator to run the ROOT instance in. There are only two current defined [vterm, inferior]. By default, the terminal backend is set to inferior. Though, there is still some issues in getting the auto-complete to work correctly. If you really need auto-complete, I would recommend switching the backend to 'vterm (requires you've already installed vterm).

Using the package

Below is listed the various commands provided by the package.

Command	Description
cern-root-run	Start the ROOT REPL in the same window.
cern-root-run-other-window	Start the ROOT REPL in a different window.
cern-root-switch-to-repl	Move the cursor to the ROOT REPL.
cern-root-eval-region	Evaluate a marked region in ROOT.
cern-root-eval-line	Evaluate this line in the REPL.
cern-root-eval-string	(non-interactive function) Evaluate a string in the REPL.
cern-root-eval-defun	Evaluate the current function in ROOT (current as defined by cursor position).
cern-root-eval-defun-maybe	If the cursor is within a function declaration, then evaluate this function, else we'll evaluate this single line.
cern-root-eval-buffer	Evaluate the current buffer in ROOT.
cern-root-eval-file	Evaluate a file (using the '.L' syntax), prompt for a file.
cern-root-change-working-directory	Change the working directory of the root session, prompt for a directory.
cern-root-list-input-history	List the previously input statements and for selection.

Org-mode

CERN-ROOT-mode provides the functionality to execute C++ source code blocks using the ROOT REPL instead of the default C++ executable. This can make the process of writing C++ in org-mode more fluid.

To write C++ code blocks that then are executed using ROOT, specify cern-root as the language in the begin_src header. For example:

#+begin_src cern-root
// write your code here.
#+end_src

Upon executing this code block (such as with C-c C-c), this code will be executed in the ROOT REPL instead.

Code can be written and re-used over multiple code blocks with the use of the :session argument (with an optional name). By providing this argument, a ROOT REPL instance will be created that can be used within the context of many source blocks. For example, let's create another code block with this session argument:

#+begin_src cern-root :session *my-root-session*
void test() {
	// print something
	printf("This is something");
}
#+end_src

When we first run this code block, CERN-ROOT-mode will create a new instance of the ROOT REPL in the buffer named 'my-root-session' (you can switch to this buffer if you want to interact directly outside of the code blocks!). This means that, by specifying the same argument to a different code block, we can call this test function.

#+begin_src cern-root :session *my-root-session*
test();  // call the test function
#+end_src

This gets us a long way to achieving the same functionality as root --notebook, but in Emacs.

Running tests

To test the functionality of the package, some unit tests have been stored in the tests/ directory. To run these unit tests, you can either evaluate the specific buffer in Emacs and run ert (see https://www.gnu.org/software/emacs/manual/html_node/ert/Running-Tests-Interactively.html for more information), or you can them all from the command line using the Makefile:

make test

Comparison with other tools

• inferior-cling: https://github.com/brianqq/inferior-cling. This package hasn't been updated in quite a while, and is light on functionality. CERN-ROOT-mode attempts to go beyond this to make the process of interacting with the REPL more seamless. There are also other ongoing issues such as duplicated input (https://root-forum.cern.ch/t/interactive-input-in-emacs-is-echoed-progressively/24113) in dumb terminals (root-project/cling#99) which have yet to be merged in cling. CERN-ROOT-mode solves these problems while we're waiting for the pull-request to be accepted. CERN-ROOT-mode also aims to support org-babel to allow one to execute C++ source code blocks from within an org-mode document.