build: adopt Arity and enforce R formatting in CI

.github/workflows/main.yml

@@ -19,6 +19,7 @@ on:
       - '!.oxfmtrc.jsonc'
       - '!.oxlintrc.jsonc'
       - '!**/.lintr'
+      - '!arity.toml'
       - '!.vscode/**'
       - '!.agents/**'
       - '!.claude/**'

.github/workflows/r.yml

@@ -5,8 +5,15 @@ on:
     branches:
       - main
     paths: &r-paths
+      - '**/*.R'
       - 'R/**'
       - 'sess/**'
+      - 'arity.toml'
+      - 'package.json'
+      - 'pnpm-lock.yaml'
+      - 'pnpm-workspace.yaml'
+      # Arity file discovery honours .gitignore.
+      - '.gitignore'
       - '.github/workflows/r.yml'
   pull_request:
     branches:
@@ -22,7 +29,33 @@ permissions:
   contents: read
 
 jobs:
-  lint:
+  format:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - uses: pnpm/action-setup@73e46992e05b19abd31f2f3c20e0e7370d762115 # v6.1.1
+      - uses: actions/setup-node@949feb2413d6458794dcd2491c4babbbce0c15c1 # v7.1.0
+        with:
+          node-version: 24
+          cache: 'pnpm'
+      - run: pnpm install --frozen-lockfile
+      - name: Check R formatting
+        run: pnpm run format:r:check
+
+  lint-arity:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+      - uses: pnpm/action-setup@73e46992e05b19abd31f2f3c20e0e7370d762115 # v6.1.1
+      - uses: actions/setup-node@949feb2413d6458794dcd2491c4babbbce0c15c1 # v7.1.0
+        with:
+          node-version: 24
+          cache: 'pnpm'
+      - run: pnpm install --frozen-lockfile
+      - name: Lint R code with Arity
+        run: pnpm run lint:r
+
+  lint-lintr:
     runs-on: ubuntu-latest
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}

CONTRIBUTING.md

@@ -31,7 +31,8 @@ Run `pnpm run format` to format TypeScript files with Oxfmt, and
 `pnpm run format:check` to check formatting without changing files. CI runs the
 same check for pull requests and pushes to `main`. Formatting currently applies
 to TypeScript files only (`.ts`, `.tsx`, `.mts`, and `.cts`); other file types,
-including JavaScript, JSON, Markdown, YAML, R, C, and EJS files, are not formatted.
+including JavaScript, JSON, Markdown, YAML, C, and EJS files, are not formatted.
+R files are formatted separately, as described below.
 
 The checked-in `.oxfmtrc.jsonc` is shared by the CLI and the recommended Oxc
 VS Code extension, and its `ignorePatterns` restrict both to TypeScript files.
@@ -46,6 +47,14 @@ Workspace settings select Oxc as the formatter and enable format on save for
 TypeScript. Run `pnpm run lint` and `pnpm run typecheck` separately to check
 TypeScript code.
 
+R files are formatted and linted with [Arity](https://arity.cc), which
+`pnpm install` provides, so no R installation is needed. Run `pnpm run format:r`
+to format them, `pnpm run format:r:check` to check formatting without changing
+files, and `pnpm run lint:r` to lint them. `pnpm exec arity lint --fix .` applies
+the safe fixes. The configuration is in `arity.toml`. CI runs both checks, and
+also runs lintr with the `.lintr` files in `R/` and `sess/`; layout is left to
+the formatter, so lintr does not check line length or indentation.
+
 ## Building bundled sess
 
 Builds require Git and a checkout with HEAD. `scripts/prepare-sess.js` fingerprints

R/.lintr

@@ -1,6 +1,6 @@
 linters: linters_with_defaults(
-  line_length_linter(120),
-  indentation_linter(4),
+  line_length_linter = NULL,
+  indentation_linter = NULL,
   return_linter = NULL,
   object_name_linter = NULL,
   object_usage_linter = NULL)