Before using this major mode, you need to install Lean 4.

To use lean4-mode in Emacs, add the following to your init.el:

;; You need to modify the following line
(setq load-path (cons "/path/to/lean4-mode" load-path))

(setq lean4-mode-required-packages '(dash flycheck lsp-mode magit-section))

(require 'package)
(add-to-list 'package-archives '("melpa" . "https://melpa.org/packages/"))
(package-initialize)
(let ((need-to-refresh t))
  (dolist (p lean4-mode-required-packages)
    (when (not (package-installed-p p))
      (when need-to-refresh
        (package-refresh-contents)
        (setq need-to-refresh nil))
      (package-install p))))

(require 'lean4-mode)

Alternatively if you are a fan of use-package and straight.el you can use:

(use-package lean4-mode
  :straight (lean4-mode
	     :type git
	     :host github
	     :repo "leanprover/lean4-mode"
	     :files ("*.el" "data"))
  ;; to defer loading the package until required
  :commands (lean4-mode))

If you are a doom-emacs user, adding the following to packages.el should work:

(package! lean4-mode :recipe
  (:host github
   :repo "leanprover/lean4-mode"
   :files ("*.el" "data")))

If things are working correctly, you should see the word Lean 4 in the Emacs mode line when you open a file with extension .lean. Emacs will ask you to identify the "project" this file belongs to. If you then type

#check id

the word #check will be underlined, and hovering over it will show you the type of id. The mode line will show FlyC:0/1, indicating that there are no errors and one piece of information displayed.

To view the proof state, run lean4-toggle-info (C-c C-i). This will show the *Lean Goals* buffer (like the Lean infoview pane in VSCode) in a separate window.

Set these with e.g. M-x customize-variable.

• lsp-headerline-breadcrumb-enable: show a "breadcrumb bar" of namespaces and sections surrounding the current location (default: off)

For lsp-mode bindings, see https://emacs-lsp.github.io/lsp-mode/page/keybindings/ (not all capabilities are supported currently).

In the default configuration, the Flycheck annotation FlyC:n/n indicates the number of errors / responses from Lean; clicking on FlyC opens the Flycheck menu.