Type Aliases

Named aliases for builtins and object-shaped hash types.

Syntax

type Age = int;
type User = { id: int, name: str, active: bool };
type ExactUser = exact { id: int, name: str, active: bool };

Example

type User = { id: int, name: str, active: bool };

user: User = {
    id: 1,
    name: "Echo",
    active: true
};

say(user);

{"id": 1, "name": "Echo", "active": true}

Notes

• Use aliases in declarations and function parameters.
• Open object aliases check required fields and types; extra fields are allowed.
• exact { ... } requires exactly the listed fields and types. Exactness nests. It does not freeze the hash.
• Exact → compatible open is assignable. Open → exact is not.
• Builtin type names and alias names cannot be redefined.
• Aliases are not new runtime types.

Common Mistakes

• Expecting open object aliases to reject extra fields
• Treating exact as immutability (const freezes; exact does not)
• Treating aliases as nominal runtime types

Exact object example

type User = exact { id: int, name: str };
user: User = { id: 1, name: "Echo" };

Extra field → E3208. Missing field → E3209.

See Also

• Variables and Types
• Functions
• Hashes