Union Types

Union types let a value be one of several types. They are useful when you want flexible inputs or results without defining a full enum.

Defining a Union

type Result union { i32, str };

You can also use an anonymous union type directly in a variable declaration or function signature:

let value: union { i32, str } = 42;

Assigning Values

A value can be implicitly assigned to a union if it matches one of the variants:

type Result union { i32, str };

let ok: i32 = 42;
let a: Result = ok;  // ✅ i32 is a variant

let err: str = "failed";
let b: Result = err; // ✅ str is a variant

Using Union Values

To use a union value as a specific type, narrow it with the is operator:

type Result union { i32, str };

fn print_result(r: Result) {
    if r is i32 {
        let n: i32 = r;  // r narrowed to i32
        io::Println(n);
    } else {
        let s: str = r;  // r narrowed to str
        io::Println(s);
    }
}

Outside of a narrowed branch, the compiler treats the value as the full union type.

When to Use Unions

• Accept multiple input types for a function
• Return either a value or an error message
• Model data that can legitimately be different shapes

If you need tagged variants with distinct names, prefer enums.

Related

• Type Compatibility and Casting
• Enums