The BAML Type System

BAML's type system is the foundation for reliable AI applications. It ensures LLM outputs are always valid and usable.

Core Types

Primitives

Type	Description	Example
string	Text	"hello"
int	Integer	42
float	Decimal	3.14
bool	Boolean	true

Complex Types

// Enums - constrained choices
enum Status {
  pending
  approved
  rejected
}

// Classes - structured objects
class User {
  name string
  email string
  age int?  // Optional field
}

// Arrays
class Response {
  items string[]
  users User[]
}

Type Safety at Runtime

Unlike traditional type systems that only work at compile time, BAML validates types at runtime when the LLM responds:

LLM Output → BAML Parser → Validated Type → Your Code

This is called Schema-Aligned Parsing.

Why This Matters

1. No more JSON parsing errors - BAML handles malformed outputs
2. Guaranteed structure - Your code always receives the expected type
3. Provider independence - Same types work across OpenAI, Anthropic, etc.

Learn More

• Schema-Aligned Parsing - How BAML parses and validates outputs
• Classification Example - See types in action
• BAML Syntax Reference - Complete type syntax