Todo CLI in Rust 2. Immutable domain and typed errors by layer

Welcome back.

Read this chapter in Spanish: ES

With boundaries in place, we tackled the core question: what makes a task valid, and how do we enforce that every single time.

Code references:

• src/tasks/domain/task.rs
• src/tasks/domain/errors.rs

Modeling Task around invariants

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Task holds:

• id
• title
• status
• created_at
• modified_at

The real value is not the fields. It is the transition model.

Immutable transitions (self -> Result<Self>)

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We use:

pub fn mark_done(self) -> DomainResult<Self>
pub fn mark_todo(self) -> DomainResult<Self>
pub fn edit_title(self, title: String) -> DomainResult<Self>

Why this instead of &mut self

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With &mut self, business rules often get scattered:

• one flow updates status,
• another flow updates timestamps,
• another forgets validation.

With immutable transitions, every state change goes through one validated gate.

Example of a composed rule

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In mark_as, one transition enforces all of this:

• rejects same-state transitions,
• keeps created_at stable,
• updates modified_at only when transition is valid.

This prevents subtle audit bugs where timestamps change even when state does not.

Layered error taxonomy

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By responsibility:

• Domain: DomainError
• Repository: RepoError
• Application: ApplicationError
• CLI: CliError

Why not one global error type

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If everything ends up as “something failed”, you lose both technical and business context.

Real case:

• done <id> with a missing task is not I/O failure.
• It is a business error (TaskNotFound).

Typing that explicitly improves debugging and test precision.

Use cases as orchestrators, not rule owners

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Use cases follow a stable flow: find -> transition -> save.

Examples:

• mark_task_done.rs
• mark_task_todo.rs

Use cases coordinate. Domain decides validity.

Closing

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This is the line between “code that runs” and “code that guarantees behavior”.

Next chapter: CLI design with clap, strict input contracts, and output for both humans and automation.