Sema for Emacs

Sema major mode for Emacs — a Lisp with first-class LLM primitives.

Syntax highlighting, Lisp-aware indentation, and an interactive REPL for editing .sema files, with optional LSP support.

Install

MELPA

The MELPA recipe (melpa-recipe) is pending acceptance. Until it lands, use the from-source install below.

Once available on MELPA:

;; M-x package-install RET sema-mode
(package-install 'sema-mode)

With use-package:

(use-package sema-mode
  :ensure t
  :mode "\\.sema\\'")

From source

Clone the repo and add it to your load-path:

(add-to-list 'load-path "/path/to/emacs-sema")
(require 'sema-mode)

Or, on Emacs 29+, install straight from GitHub with use-package :vc:

(use-package sema-mode
  :vc (:url "https://github.com/sema-lisp/emacs-sema" :rev :newest)
  :mode "\\.sema\\'")

Doom Emacs — in packages.el:

(package! sema-mode
  :recipe (:host github :repo "sema-lisp/emacs-sema"))

Features

• Syntax highlighting — special forms, standard-library builtins (including the llm/*, agent/*, conversation/*, and tool/* primitives), keyword literals (:foo), booleans (#t/#f/true/false), nil, character literals, numbers, strings, and ; / #| ... |# comments.
• Lisp-aware indentation — Sema-specific indent rules layered over lisp-mode indentation.
• REPL integration — start an inferior sema REPL and send the region, last sexp, or whole buffer to it.
• imenu — navigate functions, variables, macros, agents, tools, and record types.
• Electric pairs — auto-close (), [], {}, and "".
• LSP — sema-mode registers sema lsp with eglot automatically (M-x eglot); an lsp-mode recipe is below.

Requirements

• Emacs 25.1 or newer.

• The sema binary on your PATH for the REPL, sema-run-file, and LSP. Install it from the Sema project (cargo install sema). Point sema-program at a non-default location if needed:

  (setq sema-program "/path/to/sema")

Key Bindings

Key	Command	Description
C-c C-z	sema-repl	Start or switch to the Sema REPL
C-c C-e	sema-send-last-sexp	Send sexp before point to REPL
C-c C-r	sema-send-region	Send selected region to REPL
C-c C-b	sema-send-buffer	Send entire buffer to REPL
C-c C-l	sema-run-file	Run current file with sema

LSP Support

Sema ships with a built-in language server (sema lsp) providing completions, hover docs, go-to-definition, find references, rename, signature help, diagnostics, document symbols, and code lens (run expressions).

eglot (built-in since Emacs 29)

sema-mode registers the server with eglot automatically — just run M-x eglot in a .sema buffer. For automatic startup, add:

(add-hook 'sema-mode-hook #'eglot-ensure)

lsp-mode

(with-eval-after-load 'lsp-mode
  (add-to-list 'lsp-language-id-configuration '(sema-mode . "sema"))
  (lsp-register-client
   (make-lsp-client
    :new-connection (lsp-stdio-connection '("sema" "lsp"))
    :activation-fn (lsp-activate-on "sema")
    :server-id 'sema-lsp)))

The Sema LSP server also emits a custom sema/evalResult notification for inline evaluation results, which requires a custom client handler — see the Sema documentation for details.

Submitting to MELPA (maintainer note)

The ready-to-submit recipe lives in melpa-recipe. To publish, open a PR against melpa/melpa adding it as recipes/sema-mode. Lint locally first:

emacs -Q --batch -f batch-byte-compile sema-mode.el
emacs -Q --batch -l package-lint -f package-lint-batch-and-exit sema-mode.el
emacs -Q --batch --eval '(checkdoc-file "sema-mode.el")'

Links

• Website — sema-lang.com
• Playground — sema.run
• Documentation — sema-lang.com/docs
• Grammar — tree-sitter-sema
• Repository — sema-lisp/emacs-sema

License

MIT © Helge Sverre