Functions

Functions are the main entry points into a BAML program. Every function declares a name, a parameter list, a return type, and a body. The body can compute a value directly or use a client and prompt to ask a model for the declared result.

Anatomy of a function

The smallest useful function returns a typed value directly.

function ReturnNumber(value: int) -> int {
  value
}

function ReturnNumber(value: int) -> int {
  value
}

ReturnNumber is the function name, value is a named parameter, and the arrow introduces the return type. The final expression in the body becomes the result.

The typed boundary

Inputs and outputs can use primitives, optional values, lists, enums, classes, and other named types. Prefer a named class when downstream code cares about more than one field.

class ReceiptItem {
  name string
  quantity int
  price float
}

class Receipt {
  items ReceiptItem[]
  total float?
}

class ReceiptItem {
  name string
  quantity int
  price float
}

class Receipt {
  items ReceiptItem[]
  total float?
}

Optionality is part of the contract. Use float? only when the application can genuinely handle a missing value.

The compiler rejects a body whose value does not match the declared return type. This deliberately invalid example is checked for the expected E0001 diagnostic during documentation validation.

function ReturnNumber(value: int) -> string {
  value
}

function ReturnNumber(value: int) -> string {
  value
}

Model-backed functions

A model-backed function selects a configured client and supplies a prompt. Including ${ctx.output_format()} gives the model the output instructions derived from the return type.

function ExtractReceipt(text: string) -> Receipt {
  client: "openai/gpt-4o-mini"
  prompt: `
    Extract the receipt from the following text.

    ${text}

    ${ctx.output_format()}
  `
}

function ExtractReceipt(text: string) -> Receipt {
  client: "openai/gpt-4o-mini"
  prompt: `
    Extract the receipt from the following text.

    ${text}

    ${ctx.output_format()}
  `
}

Call from an application

After generation, the TypeScript client exposes the function under the same name. Its promise resolves to the generated Receipt type.

import { b } from "./baml_client"

const receipt = await b.ExtractReceipt(input)
console.log(receipt.items)

Where to go next

Use the function reference for a condensed syntax view, or build a complete flow in the structured extraction tutorial.