mojo-mode

Emacs major mode for the Mojo programming language, aiming for feature parity with Modular's VS Code extension.

Derives from python-mode, so indentation, movement, and everything else Python-shaped keeps working, with Mojo's own keywords, types, and tooling on top.

Features vs. the VS Code extension

vscode-mojo	mojo-mode
Syntax highlighting (.mojo, .🔥)	✅ mojo-mode, both extensions
SDK detection (mojo.sdk.path, workspace env, PATH)	✅ mojo-sdk-path, pixi/uv/magic envs, PATH, ~/.modular
Mojo: Refresh SDK Detection	✅ M-x mojo-refresh-sdk
LSP client for mojo-lsp-server	✅ via eglot, auto-started
mojo.lsp.includeDirs	✅ mojo-lsp-include-dirs
mojo.lsp.suppress.diagnostics.in.docstring	✅ mojo-lsp-suppress-diagnostics-in-docstring
Restart / Stop the Mojo LSP Server	✅ mojo-lsp-restart, mojo-lsp-stop
Run Mojo File	✅ mojo-run-file (C-c C-c), errors clickable in compilation-mode
Debug Mojo File in Terminal	✅ mojo-debug-file (C-c C-d), LLDB in a comint buffer
Attach to Process	✅ mojo-debug-attach
Configure Build and Run Args	✅ mojo-set-args (C-c C-a), mojo-build-args
Formatting, mojo.formatting.args	✅ mojo-format-buffer (C-c C-f), mojo-format-on-save
Markdown code-block injection	✅ Markdown fenced blocks and Org src blocks
Command palette entries	✅ Mojo menu + keymap
Mojo file icon	✅ 🔥 registered with nerd-icons (mojo-icon)
DAP-based debugging UI	➖ comint LLDB instead (see below)
Test explorer, MLIR dump, LSP traffic recording	❌ not implemented
—	➕ mojo-repl (C-c C-z), no VS Code equivalent
—	➕ imenu index for struct / trait / fn / comptime

Install

Emacs 29.1+ (needs eglot, defvar-keymap, replace-buffer-contents).

;; Emacs 29+ built-in
(package-vc-install '(mojo-mode :url "https://github.com/caius72/mojo-mode"))

;; straight.el
(straight-use-package '(mojo-mode :host github :repo "caius72/mojo-mode"))

;; elpaca
(elpaca (mojo-mode :host github :repo "caius72/mojo-mode"))

Doom Emacs — packages.el:

(package! mojo-mode :recipe (:host github :repo "caius72/mojo-mode"))

then doom sync. Nothing else is required: .mojo files pick up the mode, and eglot starts if a mojo-lsp-server is found.

Configuration

(use-package mojo-mode
  :mode ("\\.mojo\\'" "\\.🔥\\'")
  :custom
  ;; Pin an SDK instead of auto-detecting it (like mojo.sdk.path)
  (mojo-sdk-path "~/.pixi/envs/mojo")
  (mojo-format-on-save t)
  (mojo-lsp-include-dirs '("~/src/mypkgs")))

Without mojo-icon, nerd-icons would show the Python icon, since mojo-mode derives from python-mode. The default is the 🔥 emoji; a Nerd Font glyph avoids emoji-font sizing quirks in the modeline:

(setq mojo-icon (nerd-icons-faicon "nf-fa-fire"))   ; or nil to keep Python's

Everything is auto-detected by default. The SDK is looked up in this order:

1. mojo-sdk-path
2. a workspace environment found by walking up from the current file — .pixi/envs/default, .magic/envs/default, .venv, venv, env (disable with mojo-prefer-workspace-env)
3. mojo on PATH
4. ~/.modular

So a per-project uv or pixi install is used automatically, without setting anything.

Keys

Key	Command
C-c C-c	mojo-run-file
C-c C-b	mojo-build-file
C-c C-d	mojo-debug-file
C-c C-f	mojo-format-buffer
C-c C-z	mojo-repl
C-c C-a	mojo-set-args
C-c C-r	mojo-lsp-restart

In Doom Emacs and Spacemacs the same commands are registered under the major-mode leader, SPC m (M-SPC m outside normal state), with which-key labels — no configuration needed:

Key	Command
SPC m r	run file
SPC m b	build file
SPC m d / SPC m D	debug file / attach to process
SPC m a	configure run args
SPC m f	format buffer
SPC m z	REPL
SPC m l r / l s / l e	restart server / stop server / refresh SDK

Set mojo-setup-leader-keys to nil to opt out, or edit mojo-leader-bindings and call M-x mojo-setup-leader-keys to change them.

Debugging

mojo-debug-file runs mojo debug <file>, i.e. the LLDB command line, in a comint buffer. A dape (DAP) integration would give breakpoints in the fringe and an inspector; the SDK ships lldb-dap, so it is a small addition — it is left out until someone actually wants it.

Tests

emacs -Q -batch -L . -l test/mojo-mode-tests.el -f ert-run-tests-batch-and-exit

The formatting test is skipped unless a real Mojo SDK is on PATH.

License

MIT.