# 4d V21

> >

- **Type:** Skill
- **Install:** `agentstack add skill-ganbin-4d-development-skill-4d-development-skill`
- **Verified:** Yes — security-reviewed for prompt injection and unsafe behavior
- **Seller:** [Ganbin](https://agentstack.voostack.com/s/ganbin)
- **Installs:** 0
- **Category:** [Agent Skills](https://agentstack.voostack.com/c/agent-skills)
- **Latest version:** 0.1.0
- **License:** MIT
- **Upstream author:** [Ganbin](https://github.com/Ganbin)
- **Source:** https://github.com/Ganbin/4d-development-skill

## Install

```sh
agentstack add skill-ganbin-4d-development-skill-4d-development-skill
```

Requires the [AgentStack CLI](https://agentstack.voostack.com/docs/cli). Works with Claude Code, Cursor, and any MCP-compatible agent.

## About

# 4D Development Expert (v21)

## Skill Version

- **4D Version**: v21
- **Skill Version**: 1.0
- **Docs**: Full official 4D v21 documentation embedded in `docs/` (3,387 files)

This skill targets **4D v21** specifically. Do not assume features from other versions exist unless verified in the embedded documentation.

---

## How to Use This Skill

**CRITICAL**: For any 4D task, ALWAYS prefer reading the embedded documentation files (`docs/`) over relying on training data. The `docs/` folder contains the authoritative 4D v21 reference.

**Workflow:**
1. Check this file for critical rules and routing
2. Read the relevant `references/` file for curated knowledge and patterns
3. If more detail is needed, read the specific `docs/` file pointed to by the reference
4. For edge cases, use grep to search across `docs/`
5. Only fall back to training data if `docs/` doesn't cover the topic

**Priority reading**: Always check `references/manual-insights.md` — it contains real-world corrections from code reviews that override documentation.

---

## Local Conventions

Before providing 4D guidance, check if a `local/` directory exists in this skill folder with project-specific conventions:

```bash
ls local/*.md 2>/dev/null
```

If files exist, read them first. The `local/` directory (gitignored) can contain:
- Company-specific naming conventions and documentation standards
- Project-specific database schemas
- Version overrides (e.g., if project uses 4D v19.2 instead of v21)

---

## Critical Syntax Rules

These are the most common sources of bugs in 4D code. Know them by heart.

### 1. Assignment vs Comparison

```4d
// WRONG: = is comparison, returns True/False
$name = Request("Enter name")        // Compares, doesn't assign!
If ($input = Request("Name"))        // WRONG: compares, doesn't assign

// CORRECT: := is assignment
$name:=Request("Enter name")        // Assigns
$input:=Request("Name")
If ($input # "")                     // Then compare separately
```

**Rule**: `:=` assigns. `=` compares. Never mix them.

### 2. Object Properties Are Case-Sensitive

```4d
// Variables: case-INSENSITIVE
$MyVar:="test"
$myvar:="changed"            // Same variable!

// Object properties: case-SENSITIVE
$obj.Name:="John"
$obj.name:="Jane"            // Different properties!
```

### 3. Collection vs Array Indexing

```4d
// Collections: 0-based
$col:=New collection("A"; "B"; "C")
$first:=$col[0]              // "A"

// Arrays: 1-based with special element zero
ARRAY TEXT($arr; 3)
$arr{1}:="A"                 // First element
$arr{0}:="default"           // Special element zero
```

### 4. Null Queries Require Literal Syntax

```4d
// WRONG: Null as placeholder doesn't work
$result:=ds.Users.query("email = :1"; Null)

// CORRECT: literal null in query string
$result:=ds.Users.query("email = null")
$active:=ds.Users.query("email != null")
```

### 5. Linked Collection Queries

```4d
// WRONG: conditions can match DIFFERENT collection elements
ds.Users.query("projects[].status = 'active' AND projects[].budget > 1000")

// CORRECT: [a] links conditions to the SAME element
ds.Users.query("projects[a].status = 'active' AND projects[a].budget > 1000")
```

### 6. Numeric Object Properties Are Always Real

```4d
$obj:=New object("count"; 5)
Value type($obj.count)  // Returns Is real, NEVER Is longint
```

### 7. Decimal Separator Is Always Period

```4d
$price:=19.99    // CORRECT — always period
$price:=19,99    // WRONG — two separate numbers!
```

### 8. 4D has a strict left-to-right precedence

```4d
if ($length > 1+$i) // Runtime error: $length>1 -> true -> true+$i -> error
if ($length > (1+$i)) // No error

$result:=3+4*5 // 35
$result:=3+(4*5) // 23
```

---

## Quick Decision Router

### By Task

| Task | Read First | Then If Needed |
|------|-----------|----------------|
| Syntax errors, operators | [language-syntax.md](references/language-syntax.md) | `docs/Concepts/operators.md` |
| Data types, conversions | [data-types.md](references/data-types.md) | `docs/Concepts/data-types.md` |
| ORDA, entity classes | [orda-modern.md](references/orda-modern.md) | `docs/ORDA/ordaClasses.md` |
| Database queries | [query-patterns.md](references/query-patterns.md) | `docs/API/DataClassClass.md` |
| Error handling | [error-handling.md](references/error-handling.md) | `docs/Concepts/error-handling.md` |
| Legacy/classic code | [classic-patterns.md](references/classic-patterns.md) | `docs/Concepts/arrays.md` |
| Forms, events, UI | [forms-and-ui.md](references/forms-and-ui.md) | [events-index.md](references/events-index.md) |
| Web server, REST API | [web-and-rest.md](references/web-and-rest.md) | [rest-index.md](references/rest-index.md) |
| Specific class API | [api-index.md](references/api-index.md) | `docs/API/{ClassName}Class.md` |
| Specific command | [commands-index.md](references/commands-index.md) | `docs/commands/{name}.md` |
| Legacy command | [legacy-commands-index.md](references/legacy-commands-index.md) | `docs/commands-legacy/{name}.md` |

### By Error / Symptom

| Error or Symptom | Read |
|-----------------|------|
| "Type mismatch" or wrong type | [data-types.md](references/data-types.md) |
| "Cannot use = to assign" / silent assignment bug | [language-syntax.md](references/language-syntax.md) |
| Query returns wrong results | [query-patterns.md](references/query-patterns.md) |
| "Null query" not working | [manual-insights.md](references/manual-insights.md) |
| Entity/EntitySelection methods | [orda-modern.md](references/orda-modern.md) then [api-index.md](references/api-index.md) |
| Form events not firing | [forms-and-ui.md](references/forms-and-ui.md) then [events-index.md](references/events-index.md) |
| Process variables not working | [classic-patterns.md](references/classic-patterns.md) |
| REST endpoint 404 or auth error | [web-and-rest.md](references/web-and-rest.md) |
| Transaction or save error | [error-handling.md](references/error-handling.md) |
| `local` keyword confusion | [manual-insights.md](references/manual-insights.md) |
| Need to find a specific 4D command | [commands-index.md](references/commands-index.md) |
| Need to find a legacy command | [legacy-commands-index.md](references/legacy-commands-index.md) |

---

## Docs Navigator

The `docs/` folder contains the **full official 4D v21 documentation** (3,387 files, 48MB). Use this section to find the right file.

### Category Quick Lookup

| Need | Directory | File Pattern | Example Path |
|------|-----------|-------------|--------------|
| Class API reference | `docs/API/` | `{Class}Class.md` | `docs/API/CollectionClass.md` |
| ORDA concepts | `docs/ORDA/` | `{topic}.md` | `docs/ORDA/entities.md` |
| Modern commands | `docs/commands/` | `{command-name}.md` | `docs/commands/dialog.md` |
| Commands by theme | `docs/commands/theme/` | `{Theme}.md` | `docs/commands/theme/JSON.md` |
| Language concepts | `docs/Concepts/` | `{topic}.md` | `docs/Concepts/classes.md` |
| REST endpoints | `docs/REST/` | `${endpoint}.md` | `docs/REST/$filter.md` |
| Form events | `docs/Events/` | `on{Event}.md` | `docs/Events/onClicked.md` |
| Form objects | `docs/FormObjects/` | `{type}_overview.md` | `docs/FormObjects/listbox_overview.md` |
| Web server | `docs/WebServer/` | `{topic}.md` | `docs/WebServer/sessions.md` |
| Legacy commands | `docs/commands-legacy/` | `{command-name}.md` | `docs/commands-legacy/alert.md` |
| Settings | `docs/settings/` | `{topic}.md` | `docs/settings/web.md` |
| Project structure | `docs/Project/` | `{topic}.md` | `docs/Project/architecture.md` |
| AI Kit | `docs/aikit/` | various | `docs/aikit/` |
| ViewPro | `docs/ViewPro/` | various | `docs/ViewPro/` |
| WritePro | `docs/WritePro/` | various | `docs/WritePro/` |

### Search Patterns

When you need to find something specific across `docs/`:

```bash
# Find a command by name
grep -rl "title: CommandName" docs/commands/ docs/commands-legacy/

# Find a class method
grep -rl "\.methodName" docs/API/

# Find usage of a specific function
grep -rl "functionName" docs/ --include="*.md"

# Find a form event
ls docs/Events/on*.md

# Find a form object type
ls docs/FormObjects/*_overview.md

# Find a REST endpoint
ls docs/REST/\$*.md

# Find settings for a topic
grep -rl "keyword" docs/settings/

# Find a legacy command by theme
grep -rl "CommandName" docs/commands/theme/
```

### Index Files

For structured navigation of large categories, read these generated indexes:

| Index | Content | Files Indexed |
|-------|---------|---------------|
| [api-index.md](references/api-index.md) | All API classes with key methods | 42 |
| [commands-index.md](references/commands-index.md) | Modern commands + theme categories | 65 + 76 |
| [concepts-index.md](references/concepts-index.md) | Language concept files | 29 |
| [orda-index.md](references/orda-index.md) | ORDA documentation structure | 10 |
| [rest-index.md](references/rest-index.md) | REST API endpoints | 36 |
| [events-index.md](references/events-index.md) | Form and system events | 61 |
| [form-objects-index.md](references/form-objects-index.md) | Form object types | 54 |
| [webserver-index.md](references/webserver-index.md) | Web server documentation | 14 |
| [legacy-commands-index.md](references/legacy-commands-index.md) | Legacy commands by theme | 1,211 |
| [all-categories-index.md](references/all-categories-index.md) | Master navigation | 32 categories |

---

## Reference Files Guide

### Curated Knowledge (hand-written, with code examples)

| File | Covers | Priority |
|------|--------|----------|
| [language-syntax.md](references/language-syntax.md) | Assignment, operators, control flow, methods, classes, strings, formulas | High — read for any syntax question |
| [data-types.md](references/data-types.md) | All types, conversions, null/undefined, collections, objects, pointers | High — read for type errors |
| [orda-modern.md](references/orda-modern.md) | ORDA architecture, entity classes, computed attributes, shared objects, signals | High — read for new feature development |
| [query-patterns.md](references/query-patterns.md) | Query syntax, placeholders, null queries, linked collections, formulas, performance | High — read for any query work |
| [error-handling.md](references/error-handling.md) | Try/Catch, ON ERR CALL, transactions, Throw, logging | Medium — read when handling errors |
| [classic-patterns.md](references/classic-patterns.md) | Arrays, process variables, pointers, sets, migration strategies | Medium — read for legacy code |
| [forms-and-ui.md](references/forms-and-ui.md) | Forms, events, form objects, list boxes, subforms, form classes | Medium — read for UI work |
| [web-and-rest.md](references/web-and-rest.md) | Web server, REST API, HTTP handlers, sessions, authentication | Medium — read for web/REST work |
| [manual-insights.md](references/manual-insights.md) | Real-world corrections from code reviews — overrides other sources | **Always check** for edge cases |

### Common Workflows

**New Feature Development:**
1. [orda-modern.md](references/orda-modern.md) for architecture patterns
2. [query-patterns.md](references/query-patterns.md) for database operations
3. [error-handling.md](references/error-handling.md) for robust error management
4. Specific `docs/API/` files for class method details

**Legacy Code Maintenance:**
1. [classic-patterns.md](references/classic-patterns.md) for understanding legacy code
2. [orda-modern.md](references/orda-modern.md) for modernization options
3. [language-syntax.md](references/language-syntax.md) for syntax questions

**Debugging:**
1. Check [Critical Syntax Rules](#critical-syntax-rules) above first
2. [language-syntax.md](references/language-syntax.md) for syntax errors
3. [data-types.md](references/data-types.md) for type errors
4. [query-patterns.md](references/query-patterns.md) for query issues
5. [manual-insights.md](references/manual-insights.md) for known gotchas

**Web/REST Development:**
1. [web-and-rest.md](references/web-and-rest.md) for server and REST patterns
2. [orda-modern.md](references/orda-modern.md) for exposed functions
3. [rest-index.md](references/rest-index.md) for specific endpoint docs

---

## External Resources

When the embedded docs don't cover your case:

- **Official docs**: https://developer.4d.com/docs/
- **Community forum**: https://discuss.4d.com/
- **Blog** (feature deep-dives): https://blog.4d.com/
- **GitHub depot** (code examples): https://github.com/4d-depot

## Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

- **Author:** [Ganbin](https://github.com/Ganbin)
- **Source:** [Ganbin/4d-development-skill](https://github.com/Ganbin/4d-development-skill)
- **License:** MIT

Install and usage instructions live in the source repository linked above.

## Pricing

- **Free** — Free

## Security capabilities

Automated source analysis of v0.1.0 — what this tool can access:

- **Network access:** no
- **Filesystem access:** no
- **Shell / process execution:** no
- **Environment & secrets:** no
- **Dynamic code execution:** no

*"Yes" means the capability is present in the source — more access means more to trust, not that it is unsafe.*


## Versions

- **0.1.0** — security scan: passed — Imported from the upstream source.

## Links

- Listing page: https://agentstack.voostack.com/l/skill-ganbin-4d-development-skill-4d-development-skill
- Seller: https://agentstack.voostack.com/s/ganbin
- Browse the marketplace: https://agentstack.voostack.com/browse

---
Listed on AgentStack — the marketplace for AI agent skills and MCP servers. Every listing is security-reviewed. Creators keep 70%.