Wrap domain values in their own types

How to do it

Validate the value once, in the type's constructor or factory, and trust it everywhere after. Two IDs then cannot be swapped by mistake, because their types differ. Use branded types in TypeScript, NewType or a frozen dataclass in Python, and a readonly class in PHP. Store money as the money rule says, never as a float.

Example

type OrderId = string & { readonly brand: 'OrderId' };
type CustomerId = string & { readonly brand: 'CustomerId' };

function refund(order: OrderId, customer: CustomerId) { /* ... */ }
refund(customerId, orderId); // Fails to compile, because the arguments are swapped

OrderId = NewType("OrderId", str)

@dataclass(frozen=True)
class EmailAddress:
    value: str

    def __post_init__(self):
        if "@" not in self.value:
            raise InvalidEmail(self.value)

How to check

A type test passes a swapped ID to a typed function and marks the line as an expected error. Examples are // @ts-expect-error in TypeScript and # type: ignore[arg-type] with mypy --warn-unused-ignores. The checker fails if the error ever disappears. A unit test gives each factory an invalid value, such as "not-an-email", and expects an error.