Classes and Enums

Classes and enums are user-defined composite types for structured data.

Classes

Classes define complex data structures with named properties.

Syntax

class ClassName {
    property1 Type1
    property2 Type2
    // ...
}

warning

Properties have no : between name and type.

Example

BAML

class Foo {
    property1 string
    property2 int?
    property3 Bar[]
    property4 MyEnum
}

Python Equivalent

from pydantic import BaseModel

class Foo(BaseModel):
    property1: str
    property2: Optional[int] = None
    property3: List[Bar]
    property4: MyEnum

TypeScript Equivalent

import z from "zod";

const FooZod = z.object({
    property1: z.string(),
    property2: z.number().int().nullable().optional(),
    property3: z.array(BarZod),
    property4: MyEnumZod,
});

type Foo = z.infer<typeof FooZod>;

Property Rules

• Must start with a letter
• Contain only letters, numbers, and underscores
• Must be unique within the class
• Classes can be recursively defined

Field Attributes

Attribute	Type	Description
@alias	string	Renames field for LLM (keeps original name in code)
@description	string	Adds context to the field in the prompt

class MyClass {
    property1 string @alias("name") @description("The name of the object")
    age int? @description("The age of the object")
}

Class Attributes

Attribute	Description
@@dynamic	Allows adding fields at runtime

class MyClass {
    property1 string
    property2 int?

    @@dynamic  // allows adding property3 float[] at runtime
}

Inheritance

BAML does not support inheritance. Use composition instead:

class Address {
    street string
    city string
}

class Person {
    name string
    address Address  // composition
}

Enums

Enums define a set of named constants, useful for classification tasks.

Syntax

enum EnumName {
    Value1
    Value2
    Value3
}

Example

BAML

enum MyEnum {
    Value1
    Value2
    Value3
}

Python Equivalent

from enum import StrEnum

class MyEnum(StrEnum):
    Value1 = "Value1"
    Value2 = "Value2"
    Value3 = "Value3"

TypeScript Equivalent

enum MyEnum {
    Value1 = "Value1",
    Value2 = "Value2",
    Value3 = "Value3",
}

Value Rules

• May have any number of values
• Values may not be duplicated or empty
• Values may not contain spaces or special characters
• Values must not start with a number

Enum Attributes

Attribute	Type	Description
@@alias	string	Name rendered in prompt
@@dynamic	-	Allows modifying values at runtime

enum MyEnum {
    Value1
    Value2
    Value3

    @@alias("My Custom Enum")
    @@dynamic  // allows skipping Value2 at runtime
}

Value Attributes

Attribute	Type	Description
@alias	string	Renames value for LLM
@description	string	Adds context to the value
@skip	-	Excludes value from prompt and parsing

enum MyEnum {
    Value1 @alias("complete_summary") @description("Answer in 2 sentences")
    Value2
    Value3 @skip
    Value4 @description(#"
        This is a long description that spans multiple lines.
        It can be useful for providing more context to the value.
    "#)
}

Dynamic Types

For adding fields/values at runtime, see Dynamic Types.

See Also

• Types Overview
• @alias Attribute
• @description Attribute
• @skip Attribute
• Dynamic Types