One operation, one usecase

Start with a namespace directory matching namespace tasks; below src/main/vernac. This self-contained example requires Spring context and transaction support in the Java application, including a configured transaction manager (as provided by the persistence demo). Generation itself can be inspected without starting Spring. Save it as src/main/vernac/tasks/describe.vernac:

namespace tasks;
id TaskId;
value Title(String);

usecase DescribeTask(TaskId, Title? title)
    returns (TaskId, Title? title)
validates {
    require(self.title().isEmpty() || !self.title().orElseThrow().string().isBlank(),
            "A supplied title must not be blank");
}
behavior {
    execute {
        return describe(taskId, title);
    }
    private Result describe(TaskId id, Title? title) {
        return Result.of(id, title);
    }
}

Compile using your normal Maven build. Inject tasks.usecase.DescribeTask into the application’s caller. Invoke execute(id, null) for no title. The result’s title() returns Optional.empty(); toString describes both components. The public execute argument is nullable, and title inside behavior/private helpers is nullable too. A blank supplied title fails before execute’s implementation body runs.

Try replacing the two result fields with returns Title?, changing the body to return Optional.ofNullable(title); and removing the private helper. The public return type becomes Optional

To access persistence add uses TaskRepository tasks and invoke self.tasks().byId(…) and self.tasks().save(…) in execute. The repository and aggregate must already be declared. Dependencies arrive through constructor injection. A usecase called through an injected Spring bean establishes/joins a REQUIRED transaction.

Restore the original tuple result and helper before the next step. To move Java out of the model, replace execute’s block with:

execute implemented by custom.DescribeTaskImplementation;

For the original result contract that class supplies:

public static DescribeTask.Result execute(tasks.usecase.access.DescribeTaskAccess self, TaskId taskId, @Nullable Title title) {
    return DescribeTask.Result.of(taskId, title);
}

Place the class in package custom and import tasks.domain.TaskId, tasks.domain.Title, tasks.usecase.DescribeTask and org.jspecify.annotations.Nullable. It does not need @Component. The first parameter is the generated Access self; declared uses dependencies are available through its getters. Use your actual namespace for DescribeTaskAccess. The generated validation and transaction wrapper remain in force.

Read the complete usecase contract for optional results, private helpers, naming, imports and transaction limitations.