Domain Layer

The domain layer defines the rules of the task system. It does not know about HTTP or SQL. It only knows business concepts.

Key files

• src/domain/task.rs - Task, TaskStatus, transition rules.
• src/domain/types.rs - validation for title, description, priority.
• src/domain/policy.rs - authorization hooks.

Task status and transitions

TaskStatus is an enum with three values:

• PLANNED
• IN_PROGRESS
• COMPLETED

Transitions are controlled by a pure function:

PLANNED -> IN_PROGRESS (allowed)
IN_PROGRESS -> COMPLETED (allowed)
Other transitions (not allowed)

This logic is implemented in src/domain/task.rs and used by commands.

Code example: status enum and transitions

From src/domain/task.rs:

#![allow(unused)]
fn main() {
pub enum TaskStatus {
    Planned,
    InProgress,
    Completed,
}

pub fn can_transition(from: TaskStatus, to: TaskStatus) -> bool {
    matches!(
        (from, to),
        (TaskStatus::Planned, TaskStatus::InProgress)
            | (TaskStatus::InProgress, TaskStatus::Completed)
    )
}
}

Value types and validation

src/domain/types.rs defines types like TaskTitle and TaskPriority. Each type has a parse function that checks rules and returns an error if the input is invalid.

Why this matters:

• Validation is consistent everywhere.
• Rules are easy to test without a database.

Code example: title validation

From src/domain/types.rs:

#![allow(unused)]
fn main() {
pub fn parse(raw: &str) -> Result<Self, TaskTitleError> {
    let s = raw.trim();
    if s.is_empty() {
        return Err(TaskTitleError::Empty);
    }
    if s.chars().count() > 200 {
        return Err(TaskTitleError::TooLong);
    }
    Ok(Self(s.to_string()))
}
}

Policy hooks

src/domain/policy.rs defines functions like can_create_task. In the reference implementation they are permissive, but they provide an explicit seam for real authorization.

Code example: policy seam

From src/domain/policy.rs:

#![allow(unused)]
fn main() {
pub fn can_delete_task(_principal: &Principal) -> bool {
    true
}

pub fn can_view_tasks(_principal: &Principal) -> bool {
    true
}
}

Exercise

• Add a new validation rule for title length and update the error message.
• Add a new policy rule that only allows deletes for admins (fake a role check for now).

Next: Data Layer and SQL.