adding-grammar

Adding a Grammar to KotlinTextMate

Overview

Adding a new grammar requires: grammar file, corpus files, golden snapshot generation, and test registration. Missing any step causes silent gaps in conformance coverage.

Placeholders used below: <lang> = corpus directory name (always lowercase, e.g., javascript), <Grammar> = grammar filename without .tmLanguage.json (preserve original casing, e.g., JavaScript), <Label> = human-readable name for test display (e.g., JavaScript).

Prerequisites

• Node.js >= 21.2 (for the snapshot generator)

Steps

0. Obtain the grammar file

Source the .tmLanguage.json from VS Code's built-in extensions: microsoft/vscode repo under extensions/<lang>-basics/syntaxes/. Copy to shared-assets/grammars/. Use the exact original filename — casing varies per grammar (e.g., JavaScript.tmLanguage.json uppercase, kotlin.tmLanguage.json lowercase). If the file is JSONC (has trailing commas or comments), strip them to make valid JSON.

Also check if vscode-textmate/test-cases/themes/syntaxes/ or vscode-textmate/benchmark/ already has the grammar.

1. Create corpus files

Create tools/generate-golden/corpus/<lang>/ with hand-crafted source files exercising the language's syntax.

Check vscode-textmate/test-cases/themes/tests/ first — it has small, focused test files (e.g., test.js, test.jsx) that make excellent corpus files. Also check vscode-textmate/benchmark/ for larger corpus files.

Prefer 1-3 small focused files over one large dump. Existing corpora use 1-2 files each (JSON: 1 file, Kotlin: 2, Markdown: 2, JavaScript: 1).

2. Register in generator

Add entry to GRAMMARS in tools/generate-golden/generate.mjs:

<lang>: { file: "<Grammar>.tmLanguage.json", scope: "<scope.name>" },

Find the scope name in the grammar file's top-level scopeName field.

3. Run generator

cd tools/generate-golden && npm install && npm run generate

Produces core/src/test/resources/conformance/golden/<lang>.snapshot.json.

4. Add to GoldenSnapshotTest

Add parameter row in core/src/test/kotlin/dev/textmate/conformance/GoldenSnapshotTest.kt:

arrayOf("<Label>", "grammars/<Grammar>.tmLanguage.json", "conformance/golden/<lang>.snapshot.json"),

5. Add sentinel test

Add test in core/src/test/kotlin/dev/textmate/conformance/SentinelPatternTest.kt:

@Test
fun `<Label> grammar has 0 sentinel patterns`() {
    val onigLib = JoniOnigLib()
    loadAndCompileGrammar("grammars/<Grammar>.tmLanguage.json", onigLib)
    assertEquals("<Label> should have no sentinel patterns", 0, onigLib.sentinelPatternCount)
}

If the grammar uses backreferences inside lookbehinds (like Markdown's strikethrough), expect >0 sentinels. Run the test first with 0 — if it fails, the actual count tells you the correct value.

Important: loadAndCompileGrammar() tokenizes fixed representative lines. Lazy compilation means only exercised rules get compiled. You MUST add at least one representative line for the new language to the lines list inside loadAndCompileGrammar() in SentinelPatternTest.kt, otherwise 0 sentinels passes vacuously.

6. Run tests

./gradlew :core:test --tests "dev.textmate.conformance.*"

7. Commit

Commit all new/changed files: grammar file, corpus, snapshot, generator config, test code.

Key File Locations

Common Mistakes

• Forgetting sentinel test — sentinels silently degrade tokenization; always verify count
• Not checking vscode-textmate clone — reusable corpus files often exist in test-cases/themes/tests/
• Wrong grammar file casing — use exact filename (e.g., JavaScript.tmLanguage.json not javascript.tmLanguage.json)
• Generator without npm install — node_modules not committed; always install first
• Cross-grammar includes won't resolve — generator uses loadGrammar: () => null; sub-grammars (like source.js.regexp) won't load, and that's expected — the snapshot captures this behavior