# Template Field Types

## Overview

Templates support explicit type declarations for frontmatter fields. This ensures correct formatting and enables proper LLM schema generation.

## Syntax

```json
{
  "fieldname": "strategy|strategy:type"
}
```

The type is specified after a colon `:` at the end of the strategy chain.

## Supported Types

| Type | Description | YAML Output | Example |
|------|-------------|-------------|---------|
| `text` | String value (default) | `field: value` | Title, description |
| `number` | Integer or float | `field: 42` | Count, score |
| `checkbox` | Boolean value | `field: true` | Published, active |
| `date` | Date only (YAML ISO date) | `field: '2025-10-23'` | Birth date, deadline |
| `datetime` | Date and time (YAML ISO 8601) | `field: '2025-10-23T19:54:00+02:00'` | Created, modified |
| `list` | Array of strings | `field: [item1, item2]` | Tags, keywords, authors |

If no type is specified, `text` is assumed.

## Examples

### Basic Template with Types

```json
{
  "title": "heur|orig:text",
  "created": "heur|orig:datetime",
  "changed": "heur|orig:datetime",
  "authors": "heur|orig:list",
  "version": "heur|orig:text",
  "tags": "heur|orig|ai default:list",
  "published": "orig|lit:false:checkbox",
  "word_count": "heur:number"
}
```

### Text Fields

**Template:**
```json
{
  "title": "heur|orig:text",
  "summary": "orig|ai default:text",
  "author_name": "heur|lit:Anonymous:text"
}
```

**Output:**
```yaml
---
title: My Document
summary: This is a summary
author_name: Anonymous
---
```

### Number Fields

**Template:**
```json
{
  "priority": "orig|lit:5:number",
  "version_number": "heur:number",
  "word_count": "heur:number"
}
```

**Output:**
```yaml
---
priority: 5
version_number: 2
word_count: 1532
---
```

**Literal conversion:**
- `lit:42:number` → `42` (integer)
- `lit:3.14:number` → `3.14` (float)
- Invalid numbers default to string

### Checkbox Fields

**Template:**
```json
{
  "published": "orig|lit:false:checkbox",
  "draft": "lit:true:checkbox",
  "featured": "orig|lit:false:checkbox"
}
```

**Output:**
```yaml
---
published: false
draft: true
featured: false
---
```

**Literal conversion:**
- `true`, `1`, `yes`, `on` → `true`
- Anything else → `false`

### Date Fields

**Template:**
```json
{
  "birth_date": "orig:date",
  "deadline": "heur:date"
}
```

**Output:**
```yaml
---
birth_date: '2025-10-23'
deadline: '2025-12-31'
---
```

**Format:** ISO 8601 date (`YYYY-MM-DD`)

### Datetime Fields

**Template:**
```json
{
  "created": "heur:datetime",
  "modified": "heur:datetime",
  "published_at": "orig:datetime"
}
```

**Output:**
```yaml
---
created: '2025-10-23T19:54:00+02:00'
modified: '2025-10-23T20:15:30+02:00'
published_at: '2025-10-23T18:00:00+00:00'
---
```

**Format:** ISO 8601 with timezone

### List Fields

**Template:**
```json
{
  "tags": "heur|orig|ai default:list",
  "authors": "heur|orig:list",
  "keywords": "orig|lit:[]:list",
  "categories": "ai categorize:list"
}
```

**Output:**
```yaml
---
tags:
- machine_learning
- python
- tutorial
authors:
- John Doe
- Jane Smith
keywords: []
categories:
- technology
- programming
---
```

**Literal list:**
```json
{
  "reviewers": "lit:[]:list"
}
```

Creates empty list: `reviewers: []`

## Type with Strategies

### Literal Values

```json
{
  "status": "lit:draft:text",
  "priority": "lit:3:number",
  "urgent": "lit:false:checkbox",
  "default_tags": "lit:[]:list"
}
```

### Heuristic Values

Heuristics automatically infer appropriate types:

```json
{
  "created": "heur:datetime",        // From git (ISO 8601)
  "changed": "heur:datetime",        // From git (ISO 8601)
  "authors": "heur:list",            // From git (list of names)
  "version": "heur:text",            // From git (tag or hash)
  "title": "heur:text",              // From # heading
  "tags": "heur:list"                // From normalized tags
}
```

### Original Values

Preserves the original type if it matches:

```json
{
  "tags": "orig:list",               // Keep original tags list
  "published": "orig:checkbox",      // Keep original boolean
  "score": "orig:number"             // Keep original number
}
```

### AI Values

LLM generates values conforming to the specified type:

```json
{
  "summary": "ai summarize:text",
  "tags": "ai default:list",
  "relevance_score": "ai evaluate:number",
  "needs_review": "ai check:checkbox"
}
```

The type determines the JSON schema sent to the LLM.

## Complex Templates

### Blog Post

```json
{
  "title": "heur|orig:text",
  "date": "heur:datetime",
  "author": "heur|lit:Blog Team:text",
  "category": "orig|ai categorize:text",
  "tags": "orig|ai default:list",
  "published": "orig|lit:false:checkbox",
  "featured": "orig|lit:false:checkbox",
  "word_count": "heur:number",
  "reading_time": "heur:number",
  "excerpt": "orig|ai excerpt:text"
}
```

### Documentation

```json
{
  "title": "heur|orig:text",
  "version": "heur:text",
  "created": "heur:datetime",
  "updated": "heur:datetime",
  "authors": "heur:list",
  "reviewers": "orig|lit:[]:list",
  "status": "orig|lit:draft:text",
  "tags": "heur|orig|ai default:list",
  "api_version": "orig:text",
  "deprecated": "orig|lit:false:checkbox"
}
```

### Task/Todo

```json
{
  "title": "heur|orig:text",
  "created": "heur:datetime",
  "due_date": "orig:date",
  "priority": "orig|lit:3:number",
  "completed": "orig|lit:false:checkbox",
  "assigned_to": "orig|lit:[]:list",
  "tags": "orig:list",
  "estimated_hours": "orig:number"
}
```

## Type Validation

### Automatic Conversion

When using literal values, types are automatically converted:

```json
{
  "published": "lit:true:checkbox"      // String "true" → boolean true
}
```

### Invalid Values

If conversion fails, value is kept as string:

```json
{
  "count": "lit:invalid:number"         // "invalid" → "invalid" (string)
}
```

## LLM Schema Generation

Types control the JSON schema sent to LLMs:

**Template:**
```json
{
  "title": "ai generate:text",
  "tags": "ai generate:list",
  "relevance": "ai score:number",
  "approved": "ai check:checkbox"
}
```

**Generated Schema:**
```json
{
  "type": "object",
  "properties": {
    "title": {"type": "string"},
    "tags": {"type": "array", "items": {"type": "string"}},
    "relevance": {"type": "number"},
    "approved": {"type": "boolean"}
  }
}
```

This ensures LLM returns correctly typed values.

## Best Practices

### 1. Always Specify Types for Lists

```json
{
  "tags": "heur|orig:list",              // ✅ Explicit
  "tags": "heur|orig"                    // ❌ Ambiguous
}
```

### 2. Use Checkbox for Booleans

```json
{
  "published": "orig|lit:false:checkbox", // ✅ Boolean
  "published": "orig|lit:no:text"         // ❌ String
}
```

### 3. Datetime for Timestamps

```json
{
  "created": "heur:datetime",             // ✅ Full timestamp
  "created": "heur:text"                  // ❌ Generic
}
```

### 4. Number for Numeric Data

```json
{
  "priority": "orig|lit:5:number",        // ✅ Number
  "priority": "orig|lit:5:text"           // ❌ String "5"
}
```

### 5. List Default Values

```json
{
  "keywords": "orig|lit:[]:list",         // ✅ Empty list
  "keywords": "orig:list"                 // ⚠️ May be null
}
```

## Migration from Untyped Templates

### Before (Untyped)

```json
{
  "title": "heur|orig",
  "tags": "heur|orig|ai default",
  "published": "lit:false"
}
```

### After (Typed)

```json
{
  "title": "heur|orig:text",
  "tags": "heur|orig|ai default:list",
  "published": "lit:false:checkbox"
}
```

**Note:** Untyped templates still work (default to `text`), but typed templates are recommended for clarity and correctness.

## Troubleshooting

### List Shows as String

**Problem:** `tags: "tag1, tag2"` instead of `tags: [tag1, tag2]`

**Solution:** Specify `:list` type and ensure rules split comma-separated values:
```json
{
  "tags": "heur|orig:list"
}
```

### Boolean Shows as String

**Problem:** `published: "true"` instead of `published: true`

**Solution:** Use `:checkbox` type:
```json
{
  "published": "orig|lit:false:checkbox"
}
```

### Number Shows as String

**Problem:** `priority: "5"` instead of `priority: 5`

**Solution:** Use `:number` type:
```json
{
  "priority": "orig|lit:5:number"
}
```

### Date Format Wrong

**Problem:** `date: "10/23/2025"` instead of `date: '2025-10-23'`

**Solution:** Use `:date` or `:datetime` type and ensure source provides ISO 8601 format.

## See Also

- [LITERAL_VALUES.md](LITERAL_VALUES.md) - Literal value syntax
- [CONFIGURATION.md](CONFIGURATION.md) - Template configuration
- [USAGE.md](USAGE.md) - Template usage examples
- [RULES_GUIDE.md](RULES_GUIDE.md) - Rule writing guide