Domain Modeling

Value: Communication -- domain types make code speak the language of the business. They turn implicit knowledge into explicit, compiler-verified contracts that humans and AI can reason about.

Purpose

Teaches how to build rich domain models that prevent bugs at compile time rather than catching them at runtime. Covers primitive obsession detection, parse-don't-validate, making invalid states unrepresentable, and semantic type design. Independently useful for any code review or design task, and provides the principles that domain review checks for in the TDD cycle.

Practices

Avoid Primitive Obsession

Do not use raw primitives (String, int, number) for domain concepts. Create types that express business meaning.

Do:

fn transfer(from: AccountId, to: AccountId, amount: Money) -> Result<Receipt, TransferError>

Do not:

fn transfer(from: String, to: String, amount: i64) -> Result<(), String>

When reviewing code, flag every parameter, field, or return type where a primitive represents a domain concept. The fix is a newtype or value object that validates on construction.

Parse, Don't Validate

Validate at the boundary. Use strong types internally. Never re-validate data that a type already guarantees.

1. •Accept raw input at system boundaries (user input, API responses).
2. •Parse it into a domain type that enforces validity at construction.
3. •Pass the domain type through the system. No further validation needed.

# Boundary: parse raw input into domain type
email = Email(raw_input)  # raises if invalid

# Interior: trust the type
def send_welcome(email: Email) -> None:
    # No need to validate -- Email guarantees validity

If you find validation logic deep inside business logic, it belongs at the construction boundary instead.

Make Invalid States Unrepresentable

Use the type system to make illegal combinations impossible to construct.

Problem -- boolean flags create invalid combinations:

struct User { email: Option<String>, email_verified: bool }
# Can have email_verified=true with email=None

Solution -- encode state in the type:

enum User {
    Unverified { email: Email },
    Verified { email: Email, verified_at: Timestamp },
}

When reviewing code, ask: "Can this type represent a state that is meaningless in the domain?" If yes, redesign it.

Semantic Types Over Structural Types

Name types for what they ARE in the domain, not what they are made of.

The test: if two fields have the same structural type, the compiler cannot catch you swapping them. Semantic types prevent this.

// BAD: title and name are both NonEmptyString -- swappable
{ title: NonEmptyString, name: NonEmptyString }

// GOOD: distinct types catch mix-ups at compile time
{ title: UserTitle, name: UserName }

Structural types are useful as building blocks that semantic types wrap. The semantic type adds domain identity; the structural type provides reusable validation.

Newtypes for Identifiers

Every identifier gets its own type. Never use raw String or int for IDs.

struct AccountId(Uuid);
struct UserId(Uuid);

// Compiler catches: transfer(user_id, account_id) won't compile
fn transfer(from: AccountId, to: AccountId, user: UserId) -> Result<(), Error>

Ergonomic Conversions

Make construction validated and extraction easy.

• •Construction (IN): Always through a validating constructor. No automatic conversion from primitives.
• •Extraction (OUT): Provide Display, AsRef, Into or equivalent so the type is convenient to use. Getting the inner value out should be trivial.

Never provide an automatic conversion FROM a primitive -- that bypasses validation and undermines parse-don't-validate.

Exhaustive Matching

Use enums with exhaustive match/switch to ensure all cases are handled. Never use a catch-all default for domain states -- it silently swallows new variants.

Veto Authority

When reviewing code (whether in a TDD cycle or a standalone review), you have authority to reject designs that violate these principles. When exercising this authority:

1. •State the specific violation (e.g., "primitive obsession: email is String, should be Email type").
2. •Propose the alternative with a concrete type definition.
3. •Explain the impact in one sentence.
4. •If the other party disagrees, engage substantively for up to two rounds. Then escalate to the human.

Do not back down from valid domain concerns to avoid conflict. Do not silently accept designs that violate these principles.

Enforcement Note

This skill provides advisory guidance on domain modeling quality. It cannot mechanically prevent an agent from using primitives or creating invalid state representations. On harnesses with plugin support, enforcement plugins can add file-type restrictions (domain agent edits only type definitions) and mandatory review gates. On other harnesses, these principles are followed by convention and verified through code review. For available enforcement plugins, see the Harness Plugin Availability table.

Verification

After applying domain modeling principles, verify:

• • No primitive types (String, int, number) used for domain concepts
• • All identifiers use newtype wrappers, not raw primitives
• • Invalid states are unrepresentable (no contradictory field combinations)
• • Validation occurs at construction boundaries, not deep in business logic
• • Types are named for domain meaning (semantic), not structure
• • Two fields with the same underlying type cannot be accidentally swapped
• • Enum matching is exhaustive with no catch-all defaults for domain states

If any criterion is not met, create or refine the domain type before proceeding.

Dependencies

This skill works standalone. For enhanced workflows, it integrates with:

• •tdd-cycle: Domain review is a mandatory checkpoint in the TDD cycle. This skill provides the principles that review checks for.
• •code-review: Domain integrity is stage 3 of the three-stage review. This skill defines what to look for.
• •architecture-decisions: Architectural patterns (event sourcing, CQRS, hexagonal) affect where domain boundaries fall.

Missing a dependency? Install with:

npx skills add jwilger/agent-skills --skill tdd-cycle