# LinkML YAML Best Practices Rule

## Rule: Follow LinkML Conventions for Valid, Interoperable Schema Files

### 1. equals_expression Anti-Pattern

`equals_expression` is for dynamic formula evaluation (e.g., `"{age_in_years} * 12"`). Never use it for static value constraints.

**WRONG:**
```yaml
slot_usage:
  has_type:
    equals_expression: '["hc:ArchiveOrganizationType"]'
  hold_record_set:
    equals_expression: '["hc:Fonds", "hc:Series"]'
```

**CORRECT** (single value):
```yaml
slot_usage:
  has_type:
    equals_string: "hc:ArchiveOrganizationType"
```

**CORRECT** (multiple allowed values - if classes):
```yaml
slot_usage:
  hold_record_set:
    any_of:
      - range: UniversityAdministrativeFonds
      - range: StudentRecordSeries
      - range: FacultyPaperCollection
```

**CORRECT** (multiple allowed values - if literals):
```yaml
slot_usage:
  status:
    equals_string_in:
      - "active"
      - "inactive"
      - "pending"
```

### 2. Declare All Used Prefixes

Every CURIE prefix used in the file must be declared in the `prefixes:` block.

**WRONG:**
```yaml
prefixes:
  linkml: https://w3id.org/linkml/
  skos: http://www.w3.org/2004/02/skos/core#
slot_usage:
  has_type:
    equals_string: "hc:ArchiveOrganizationType"  # hc: not declared!
```

**CORRECT:**
```yaml
prefixes:
  linkml: https://w3id.org/linkml/
  hc: https://nde.nl/ontology/hc/
  skos: http://www.w3.org/2004/02/skos/core#
default_prefix: hc
slot_usage:
  has_type:
    equals_string: "hc:ArchiveOrganizationType"
```

### 3. Import Referenced Classes

When using external classes in `is_a`, `range`, or other references, import them.

**WRONG:**
```yaml
imports:
  - linkml:types
classes:
  AcademicArchive:
    is_a: ArchiveOrganizationType      # Not imported!
    slot_usage:
      related_to:
        range: WikidataAlignment       # Not imported!
```

**CORRECT:**
```yaml
imports:
  - linkml:types
  - ../classes/ArchiveOrganizationType
  - ../classes/WikidataAlignment
classes:
  AcademicArchive:
    is_a: ArchiveOrganizationType
    slot_usage:
      related_to:
        range: WikidataAlignment
```

### 4. Quote Regex Patterns and Annotation Values

**Regex patterns:**
```yaml
# WRONG
pattern: ^Q[0-9]+$

# CORRECT
pattern: "^Q[0-9]+$"
```

**Annotation values (must be strings):**
```yaml
# WRONG
annotations:
  specificity_score: 0.1

# CORRECT
annotations:
  specificity_score: "0.1"
```

### 5. Remove Unused Imports

Only import slots and classes that are actually used in the file.

**WRONG:**
```yaml
imports:
  - ../slots/has_scope    # Never used in slots: or slot_usage:
  - ../slots/has_score
  - ../slots/has_type
```

**CORRECT:**
```yaml
imports:
  - ../slots/has_score
  - ../slots/has_type
```

### 6. Slot Usage Requires Slot Presence

A slot referenced in `slot_usage:` must either be:
- Listed in the `slots:` array, OR
- Inherited from a parent class via `is_a`

**WRONG:**
```yaml
classes:
  MyClass:
    slots:
      - has_type
    slot_usage:
      has_type: {...}
      identified_by: {...}    # Not in slots: and not inherited!
```

**CORRECT:**
```yaml
classes:
  MyClass:
    slots:
      - has_type
      - identified_by
    slot_usage:
      has_type: {...}
      identified_by: {...}
```

## Checklist for Class Files

- [ ] All prefixes used in CURIEs are declared
- [ ] `default_prefix` set if module belongs to that namespace
- [ ] All referenced classes are imported
- [ ] All used slots are imported
- [ ] No `equals_expression` with static JSON arrays
- [ ] Regex patterns are quoted
- [ ] Annotation values are quoted strings
- [ ] No unused imports
- [ ] `slot_usage` only references slots that exist (via slots: or inheritance)