Indentation in square brackets

Question

Yet another question about indentation in AUCTeX...

I would like to be able to indent content inside square brackets [...]. It would be useful for tikz option lists, for example, or trees drawn using forest or qtree: both these packages use square brackets for their syntax, which makes it easy to scan in source code in an editor supporting paren-indentation.

Unfortunately, AUCTeX only indents the content of environments and content between braces {...}.

The forest example is also a situation where cheating by adding braces around the content we want indented won't work, because braces will escape all content from being evaluated as a tree.

Example:

\documentclass{article}
\usepackage{forest}

\begin{document}

\begin{forest}
  [A
  [B]
  [C
  [D]
  [E]
  ]
  ]
\end{forest}

\end{document}

Which I would like indented as:

\documentclass{article}
\usepackage{forest}

\begin{document}

\begin{forest}
  [A
    [B]
    [C
      [D]
      [E]
    ]
  ]
\end{forest}

\end{document}

Is there a way?

Answer 1

Setting LaTeX-begin-regexp and LaTeX-end-regexp does not help here because inside LaTeX-indent-level-count it searches a backslash before checking the regexp.

I got it working by modifying TeX-brace-count-line. This solution counts [] as brace {}. Compare it will original function if you want to see the difference.

(defun TeX-brace-count-line ()
  "Count number of open/closed braces."
  (save-excursion
    (let ((count 0) (limit (line-end-position)) char)
      (while (progn
               (skip-chars-forward "^{}[]\\\\" limit)
               (when (and (< (point) limit) (not (TeX-in-comment)))
                 (setq char (char-after))
                 (forward-char)
                 (cond ((eq char ?\{)
                        (setq count (+ count TeX-brace-indent-level)))
                       ((eq char ?\})
                        (setq count (- count TeX-brace-indent-level)))
                       ((eq char ?\[)
                        (setq count (+ count TeX-brace-indent-level)))
                       ((eq char ?\])
                        (setq count (- count TeX-brace-indent-level)))
                       ((eq char ?\\)
                        (when (< (point) limit)
                          (forward-char)
                          t))))))
      count)))