Skip to main content

Optional Types

Optional types represent values that might or might not be present.

Syntax​

Type?

Examples​

Basic Optional​

int?           // optional integer
string?        // optional string
MyClass?       // optional class instance

Complex Optionals​

(MyClass | int)?     // optional union
(int | string)?      // optional union of primitives

Usage​

In Class Properties​

class Person {
    name string
    age int?              // age is optional
    email string?         // email is optional
}

In Function Parameters​

function DescribeCircle(radius: int, pi: float?) -> string {
    client "openai/gpt-4o-mini"
    prompt #"
        Describe a circle with radius {{ radius }}.
        Use pi as {{ pi or 3.14159 }}.
    "#
}

In Return Types​

function FindUser(id: string) -> User? {
    client "openai/gpt-4o-mini"
    prompt #"
        Find user with ID {{ id }}.
        Return null if not found.
        {{ ctx.output_format }}
    "#
}

Language Equivalents​

BAMLPythonTypeScriptGo
int?Optional[int]number | null*int
string?Optional[str]string | null*string
MyClass?Optional[MyClass]MyClass | null*MyClass

Key Points​

  • Optional types default to None/null when not provided
  • Use ? suffix to make any type optional
  • Arrays cannot be optional (Type[]? is invalid)
  • For optional arrays, use an empty array [] as the default

See Also​