loom. Senior judgement, compiled.

class CreateUserUseCase(UseCase[User, User]):
    rules = [
        Rule.check(F(CreateUser).email, via=_email_must_be_valid),
        Rule.forbid(lambda _, __, exists: exists,
                    message="email already exists")
            .from_params("email_exists"),
    ]

    async def execute(
        self,
        cmd: CreateUser = Input(),
        email_exists: bool = Exists(User, from_command="email",
                                    against="email"),
    ) -> User:
        return await self.main_repo.create(cmd)

• Exists checks a database condition before execute runs.
• Rule.check and Rule.forbid declare invariants, not if blocks.
• A wrong plan fails at start-up with a structured error, not in production.