Emit an emacs major mode

With coloring! Next up: formatting but that might be hard.

grammar.py

@@ -439,4 +439,4 @@ if __name__ == "__main__":
     ts_path = Path(__file__).parent / "tree-sitter-fine"
     emit_tree_sitter_grammar(grammar, ts_path)
     emit_tree_sitter_queries(grammar, ts_path)
-    emit_emacs_major_mode(grammar, ts_path)
+    emit_emacs_major_mode(grammar, ts_path / "fine.el")

parser/emacs.py

@@ -5,6 +5,13 @@ import pathlib
 import textwrap
 
 from parser.tree_sitter import terminal_name
+from parser.generated_source import (
+    begin_manual_section,
+    end_manual_section,
+    merge_existing,
+    sign_generated_source,
+    signature_token,
+)
 
 from . import parser
 
@@ -116,5 +123,116 @@ def gather_faces(grammar: parser.Grammar):
     return feature_string
 
 
-def emit_emacs_major_mode(grammar: parser.Grammar, path: pathlib.Path | str):
+def emit_emacs_major_mode(grammar: parser.Grammar, file_path: pathlib.Path | str):
+    if isinstance(file_path, str):
+        file_path = pathlib.Path(file_path)
+
     face_var = gather_faces(grammar)
+
+    contents = f"""
+;;; {file_path.name} --- Major mode for editing {grammar.name} --- -*- lexical-binding: t -*-
+
+;; NOTE: This file is partially generated.
+;;       Only modify marked sections, or your modifications will be lost!
+;;       {signature_token()}
+
+;; {begin_manual_section('commentary')}
+
+;; This is free and unencumbered software released into the public domain.
+;; Anyone is free to copy, modify, publish, use, compile, sell, or distribute this
+;; software, either in source code form or as a compiled binary, for any purpose,
+;; commercial or non-commercial, and by any means.
+;;
+;; In jurisdictions that recognize copyright laws, the author or authors of this
+;; software dedicate any and all copyright interest in the software to the public
+;; domain. We make this dedication for the benefit of the public at large and to
+;; the detriment of our heirs and successors. We intend this dedication to be an
+;; overt act of relinquishment in perpetuity of all present and future rights to
+;; this software under copyright law.
+;;
+;; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+;; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+;; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+;; AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+;; ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+;; WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+;;; Commentary:
+;; (Nobody has written anything about the major mode yet.)
+
+;; {end_manual_section()}
+
+;;; Code:
+(require 'treesit)
+
+;; {begin_manual_section('prologue')}
+
+;; {end_manual_section()}
+
+{face_var}
+
+(defun {grammar.name}-ts-setup ()
+  "Setup for {grammar.name}-mode."
+
+  ;; {begin_manual_section('setup_prologue')}
+  ;; {end_manual_section()}
+
+  ;; Set up the font-lock rules.
+  (setq-local treesit-font-lock-settings
+              (apply #'treesit-font-lock-rules
+                     {grammar.name}-font-lock-rules))
+
+  ;; {begin_manual_section('feature_list')}
+  ;; NOTE: This list is just to get you started; these are some of the standard
+  ;;       features and somewhat standard positions in the feature list. You can
+  ;;       edit this to more closely match your grammar's output. (The info page
+  ;;       for treesit-font-lock-feature-list describes what it does nicely.)
+  (setq-local treesit-font-lock-feature-list
+              '((comment definition)
+                (keyword string)
+                (assignment attribute builtin constant escape-sequence number type)
+                (bracket delimiter error function operator property variable)))
+  ;; {end_manual_section()}
+
+  ;; {begin_manual_section('setup_epilogue')}
+  ;; If you want to set up more do it here.
+  ;; {end_manual_section()}
+
+  (treesit-major-mode-setup))
+
+;;;###autoload
+(define-derived-mode {grammar.name}-mode prog-mode "{grammar.name}"
+  "Major mode for editing {grammar.name} files."
+
+  (setq-local font-lock-defaults nil)
+  (when (treesit-ready-p '{grammar.name})
+    (treesit-parser-create '{grammar.name})
+    ({grammar.name}-ts-setup)))
+
+
+;; {begin_manual_section('eplogue')}
+
+;; {end_manual_section()}
+;;; {file_path.name} ends here
+""".lstrip()
+
+    # Sign the contents to give folks a way to check that they haven't been
+    # messed with.
+    contents = sign_generated_source(contents)
+
+    # Try to pull existing file contents out and merge them with the
+    # generated code. This preserves hand-editing in approved areas.
+    try:
+        with open(file_path, "r", encoding="utf-8") as file:
+            existing_contents = file.read()
+            contents = merge_existing(existing_contents, contents)
+    except Exception:
+        pass
+
+    # Ensure that parent directories are created as necessary for the output.
+    if not file_path.parent.exists():
+        file_path.parent.mkdir(parents=True, exist_ok=True)
+
+    # And write the file!
+    with open(file_path, "w", encoding="utf-8") as file:
+        file.write(contents)