How to customize the delimiter in the minor mode Hideshow for use with JSON data?

Question

The documentation of hs-minor-mode explains:

In C mode and related modes, blocks are delimited by braces, while in Lisp mode they are delimited by parentheses.

I use the mode to collapse objects while reading JSON files. This works fine, because JSON objects use curly braces. But it does not work for JSON arrays, because arrays are delimited by box brackets ([]).

Is it possible to customize the delimiter in order to make curly braces and box brackets valid delimiters to collapse blocks?

Example:

{
  "list": [
    {
      "id": 1
    },
    {
      "id": 2
    },
    {
      "id": 3
    },
    {
      "id": 4
    },
    {
      "id": 5
    }
  ]
}

When I call hs-hide-block from within the array, the object collapses to:

{...}

But I only want to collapse the array:

{
  "list": [...]
}

And I still want to be able to collapse objects.

I am using "GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo version 1.16.0) of 2021-03-28, modified by Debian". JSON files are opened in "JavaScript mode defined in ‘js.el’". I think this is js-mode.

Answer 1

M-x finder-commentary RET hideshow RET says:

Normally, hideshow tries to determine appropriate values for block
and comment definitions by examining the buffer's major mode.  If
there are problems, hideshow will not activate and in that case you
may wish to override hideshow's heuristics by adding an entry to
variable `hs-special-modes-alist'.  Packages that use hideshow should
do something like:

  (add-to-list 'hs-special-modes-alist '(my-mode "{{" "}}" ...))

If you have an entry that works particularly well, consider
submitting it for inclusion in hideshow.el.  See docstring for
`hs-special-modes-alist' for more info on the entry format.

See also variable `hs-set-up-overlay' for per-block customization of
appearance or other effects associated with overlays.  For example:

(setq hs-set-up-overlay
      (defun my-display-code-line-counts (ov)
        (when (eq 'code (overlay-get ov 'hs))
          (overlay-put ov 'display
                       (propertize
                        (format " ... <%d>"
                                (count-lines (overlay-start ov)
                                             (overlay-end ov)))
                        'face 'font-lock-type-face)))))

The default value of hs-special-modes-alist is:

  '((c-mode "{" "}" "/[*/]" nil nil)
    (c++-mode "{" "}" "/[*/]" nil nil)
    (bibtex-mode ("@\\S(*\\(\\s(\\)" 1))
    (java-mode "{" "}" "/[*/]" nil nil)
    (js-mode "{" "}" "/[*/]" nil))

It's not clear to me whether you're using the standard js-mode for JSON or if you're using the json-mode package from GNU ELPA. The latter derives from the former, however that's only relevant here if you're using Emacs 28+ (refer to bug #39354), so you'll have to decide which of these you need:

(push '(js-mode "[[{]" "[]}]" "/[*/]") hs-special-modes-alist)
(push '(json-mode "[[{]" "[]}]") hs-special-modes-alist)

Be sure to revert the JSON buffer for testing.

Answer 2

I may be misunderstanding what you're requesting. I tried using hide/show with some JSON code in js-mode as a quick experiment just now, and hs-minor-mode seemed to work OK.

On the other hand, JSON does not (in general) have any notion of a comment, so you might want to use an impossible-to-match regexp for the COMMENT-START part of the entry for the mode in variable hs-special-modes-alist. You can use the regexp "^\b$" for that - it can't be matched.

(setq hs-special-modes-alist '((c-mode "{" "}" "/[*/]" nil nil)
                               (c++-mode "{" "}" "/[*/]" nil nil)
                               (bibtex-mode ("@\\S(*\\(\\s(\\)" 1))
                               (java-mode "{" "}" "/[*/]" nil nil)
                               ;; (js-mode "{" "}" "/[*/]" nil) ; <=== Remove
                               (js-mode "{" "}" "^\b$" nil) ; <=== Add
                               ))

But in that case, you'd probably really want to use some mode that's only for JSON, and not also for JavaScript (which has comments).

If you need to use some such other mode, you can redefine hs-special-modes-alist to add an entry for the major mode you use for JSON data. For example, I tried with a json-mode from library json-mode.el by Josh Johnston, and that too seemed to work OK (just tested quickly, and again, I'm not sure what you're asking).

(setq hs-special-modes-alist '((c-mode "{" "}" "/[*/]" nil nil)
                               (c++-mode "{" "}" "/[*/]" nil nil)
                               (bibtex-mode ("@\\S(*\\(\\s(\\)" 1))
                               (java-mode "{" "}" "/[*/]" nil nil)
                               (js-mode "{" "}" "/[*/]" nil)
                               (json-mode "{" "}" "^\b$" nil)))

That leaves js-mode as it was, with comment behavior defined.