4D v20 Development Skill

This skill provides expert knowledge for developing in 4D v20. It includes embedded official documentation and best practice guides tailored for version 20.

📚 Documentation Router

Topic	Reference File	Description
Language Syntax	language-syntax.md	var, operators, control flow, v20 specifics.
Data Types	data-types.md	Collections, Objects, Variants, typing rules.
ORDA	orda-modern.md	Entity Selections, DataClass, optimization.
Queries	query-patterns.md	Placeholders, formula queries, optimization.
Error Handling	error-handling.md	ON ERR CALL, Try/Catch (check availability).
Web & REST	web-and-rest.md	Web Server, REST API, HTTPRequest.
Forms & UI	forms-and-ui.md	Form objects, events, list boxes.

⚠️ Critical v20 Rules

1. Variable Declaration: ALWAYS use var $name : Type. Avoid C_TEXT unless modifying legacy code.
2. Assignment: Use := for assignment found in legacy/standard mode. v20 allows = in some contexts but := is safer for compatibility.
   • Note: If "Use standard assignment =" option is on, = is fine. Default to := to be safe.
3. Collections vs Arrays: Prefer Collections ([]) and Objects ({}) over Arrays (ARRAY TEXT).
4. Query Literals: Use placeholders (:1, :2) or : $var. NEVER concat strings into queries (SQL injection checks).
   • Safe: ds.Users.query("name = :1", $name)
5. Null: Use Null keyword. Check with Unknown value or Assert helper if needed.
6. Linked Collections: When using [table], remember it returns a selection. To get a collection, use ds.Table.all().toCollection().

🚫 v21+ Features to AVOID (v21-only)

Do NOT use the following features unless you are certain the project is v20 R7+ or v21:

• HTTP Classes: 4D.IncomingMessage, 4D.OutgoingMessage (v20 R6+). Use On Web Connection / WEB GET BODY for standard v20 servers.
• File Handles: 4D.File.Handle (v21). Use File and Document to text / Text to document or File.getContent().
• Singleton Classes: shared singleton Class. Use Storage for singletons in v20.
• Try/Catch: Available in v20 R4+. If base v20, use ON ERR CALL. Verify exact v20 R-release.

🔍 Searching Documentation

The official 4D v20 documentation is embedded in the docs/ directory.
To find information about a command or class:

1. Search References: Check references/ first for high-level patterns.
2. Grep Docs: Use grep_search or find_by_name in docs/ folder.
   • Example: grep -r "HTTP Request" docs/
3. API Index: See api-index.md for a map of standard commands.

REST Contract

When building REST APIs in v20:

• Success: { "ok": true, "data": ... }
• Error: { "ok": false, "error": { "code": 123, "message": "..." } }
• Use Web Server class or On Web Connection database method.