Understanding Major and Minor Modes

Oct 13, 2025 by wbrum.org 149 views

Introduction

In Emacs, minor-modes are designed to provide additional functionality to the user, often in conjunction with a specific major-mode. However, there are situations where the output of a minor-mode may need to be customized based on the major-mode in use. This article will explore how to achieve this customization, allowing the user to define different default formats for the text inserted by a minor-mode, depending on the major-mode, and also provide an option for the user to override these defaults.

Understanding Major and Minor Modes

Before diving into the customization process, it's essential to understand the difference between major and minor modes in Emacs.

Major Mode: A major mode is a specific editing mode that is activated in a buffer. Examples of major modes include emacs-lisp-mode, python-mode, and latex-mode. Each major mode has its own set of key bindings, syntax highlighting, and other features.
Minor Mode: A minor mode is a secondary mode that can be activated in addition to a major mode. Minor modes are designed to provide additional functionality or customize the behavior of a major mode. Examples of minor modes include auto-fill-mode, flyspell-mode, and linum-mode.

Customizing Minor-Mode Outputs

To customize the output of a minor-mode based on the major-mode, we need to use conditionals in our Emacs Lisp code. Conditionals allow us to execute different code blocks based on specific conditions, such as the major-mode in use.

Here's an example of how to define a minor-mode that inserts text in the current buffer, with different default formats depending on the major-mode:

(defun my-insert-text-minor-mode ()
  "Minor mode to insert text in the current buffer."
  (interactive)
  (setq-local my-insert-text-format (cond
                                        ((eq major-mode 'emacs-lisp-mode) "ELISP")
                                        ((eq major-mode 'python-mode) "PYTHON")
                                        (t "DEFAULT"))))

(defun my-insert-text ()
  "Insert text in the current buffer based on the major-mode."
  (interactive)
  (insert (format "%s" my-insert-text-format)))

(add-hook 'text-mode-hook 'my-insert-text-minor-mode)

In this example, the my-insert-text-minor-mode function is activated when the user enters a text-mode buffer. The my-insert-text-format variable is then set based on the major-mode in use, using a conditional statement. The my-insert-text function inserts the text in the current buffer, using the format defined by my-insert-text-format.

Allowing User Override

While the default format for the text inserted by the minor-mode is determined by the major-mode, it's also essential to provide an option for the user to override these defaults. This can be achieved by adding a user option to the minor-mode.

Here's an updated version of the code that includes a user option to override the default format:

(defcustom my-insert-text-format-alist
  '(("emacs-lisp-mode" . "ELISP")
    ("python-mode" . "PYTHON")
    ("default" . "DEFAULT"))
  "Association list of major-modes and their corresponding formats."
  :type 'alist
  :group 'my-insert-text)

(defun my-insert-text-minor-mode ()
  "Minor mode to insert text in the current buffer."
  (interactive)
  (setq-local my-insert-text-format (or (cdr (assoc major-mode my-insert-text-format-alist))
                                             (cdr (assoc 'default my-insert-text-format-alist)))))

(defun my-insert-text ()
  "Insert text in the current buffer based on the major-mode."
  (interactive)
  (insert (format "%s" (or my-insert-text-format (user-error "No format defined for major-mode %s" major-mode)))))

(add-hook 'text-mode-hook 'my-insert-text-minor-mode)

In this updated version, the my-insert-text-format-alist variable is an association list that maps major-modes to their corresponding formats. The my-insert-text-minor-mode function uses this association list to determine the format for the text inserted by the minor-mode. The user can override these defaults by customizing the my-insert-text-format-alist variable.

Conclusion

Q: What is the difference between a major mode and a minor mode in Emacs?

A: A major mode is a specific editing mode that is activated in a buffer, such as emacs-lisp-mode or python-mode. A minor mode is a secondary mode that can be activated in addition to a major mode, such as auto-fill-mode or flyspell-mode.

Q: How do I define a minor-mode that inserts text in the current buffer, with different default formats depending on the major-mode?

A: You can use the following code as a starting point:

(defun my-insert-text-minor-mode ()
  "Minor mode to insert text in the current buffer."
  (interactive)
  (setq-local my-insert-text-format (cond
                                        ((eq major-mode 'emacs-lisp-mode) "ELISP")
                                        ((eq major-mode 'python-mode) "PYTHON")
                                        (t "DEFAULT"))))

(defun my-insert-text ()
  "Insert text in the current buffer based on the major-mode."
  (interactive)
  (insert (format "%s" my-insert-text-format)))

(add-hook 'text-mode-hook 'my-insert-text-minor-mode)

Q: How do I allow the user to override the default format for the text inserted by the minor-mode?

A: You can add a user option to the minor-mode using the defcustom function. Here's an updated version of the code:

(defcustom my-insert-text-format-alist
  '(("emacs-lisp-mode" . "ELISP")
    ("python-mode" . "PYTHON")
    ("default" . "DEFAULT"))
  "Association list of major-modes and their corresponding formats."
  :type 'alist
  :group 'my-insert-text)

(defun my-insert-text-minor-mode ()
  "Minor mode to insert text in the current buffer."
  (interactive)
  (setq-local my-insert-text-format (or (cdr (assoc major-mode my-insert-text-format-alist))
                                             (cdr (assoc 'default my-insert-text-format-alist)))))

(defun my-insert-text ()
  "Insert text in the current buffer based on the major-mode."
  (interactive)
  (insert (format "%s" (or my-insert-text-format (user-error "No format defined for major-mode %s" major-mode)))))

(add-hook 'text-mode-hook 'my-insert-text-minor-mode)

Q: How do I customize the `my-insert-text-format-alist` variable to override the default format for a specific major-mode?

A: You can customize the my-insert-text-format-alist variable by adding a new association to the list. For example, to override the default format for emacs-lisp-mode, you can add the following code to your Emacs configuration file:

(custom-set-variables
 '(my-insert-text-format-alist
   (append
    '(("emacs-lisp-mode" . "CUSTOM-ELISP"))
    my-insert-text-format-alist)))

Q: How do I remove the `my-insert-text-minor-mode` from a specific major-mode?

A: You can remove the my-insert-text-minor-mode from a specific major-mode by adding the following code to your Emacs configuration file:

(remove-hook 'text-mode-hook 'my-insert-text-minor-mode)

Q: How do I disable the `my-insert-text-minor-mode` globally?

A: You can disable the my-insert-text-minor-mode globally by adding the following code to your Emacs configuration file:

(disable-major-mode 'my-insert-text-minor-mode)

Conclusion

Customizing the output of a minor-mode based on the major-mode in use can be achieved using conditionals in Emacs Lisp code. By defining different default formats for the text inserted by a minor-mode, depending on the major-mode, and providing an option for the user to override these defaults, we can create a more flexible and user-friendly minor-mode. This article has provided a step-by-step guide on how to achieve this customization, using examples and code snippets to illustrate the process.