Functional TypeScript Code Review

Review server-side TypeScript code against the functional domain modeling principles defined in the functional-ts skill. This review uses the same knowledge base as functional-ts — every checklist item below corresponds to a section of that skill, and links to the authoritative description there.

Review Procedure

1. Load the principle knowledge first. Before reading any code under review, read the following so that findings cite the canonical principle:
   • ../functional-ts/SKILL.md — the principle index
   • ../functional-ts/error-handling.md
   • ../functional-ts/boundary-defense.md
   • ../functional-ts/state-modeling.md
   • The validation library guide matching the project's package.json under ../functional-ts/validation-libraries/ (zod.md / valibot.md / arktype.md)
   • The Result library guide matching the project's package.json under ../functional-ts/result-libraries/ (neverthrow.md / byethrow.md / fp-ts.md / option-t.md)
2. Read the files under review.
3. Walk through the checklist below in the order of the principles. The numbering mirrors functional-ts/SKILL.md.
4. When a violation is found, report it with the relevant principle, the reason it matters, and a fix.
5. When something is not a violation but has room for improvement, communicate it as a suggestion.

Checklist

The checklist mirrors the structure of ../functional-ts/SKILL.md. Each item links back to the authoritative description.

1. Type-Driven Domain Modeling

1.1 Are domain states modeled as Discriminated Unions?

Reference: ../functional-ts/SKILL.md §1 "Represent State with Discriminated Unions"

Flag: a single type with many optional properties and a string state field (e.g. { state: string; driverId?: string; startTime?: Date }). Suggest splitting into per-state types unioned together so state-specific properties become required.

1.2 Is kind used as the unified discriminant?

Reference: ../functional-ts/SKILL.md §1 "Use kind as the unified discriminant"

Flag: discriminant property names other than kind (type, status, state, _tag, …). Suggest renaming to kind for codebase consistency.

1.3 Are classes used for domain models?

Reference: ../functional-ts/SKILL.md §1 "Represent State with Discriminated Unions", and the Companion Object pattern.

If class defines domain entities or value objects, suggest migrating to Discriminated Union + Companion Object. Class inheritance required by an external library is a legitimate exception.

1.4 Is the Companion Object pattern followed?

Reference: ../functional-ts/SKILL.md §1 "Companion Object Pattern"

Check that:

• A type's related operations live on a const of the same name as the type.
• Branded Type validation schemas are exposed as .schema on the companion object, not as standalone XxxSchema exports.
• Domain logic is not scattered as free-standing xxxAssignDriver helpers when a companion object would naturally own them.

1.5 Is interface used for domain types?

Reference: ../functional-ts/SKILL.md §1 "Use type (not interface)"

Declaration merging silently changes a type's shape. Domain types must be type. interface is acceptable only for library type augmentation.

1.6 Is method notation used inside type definitions?

Reference: ../functional-ts/SKILL.md §1 "Use function property notation (not method notation)"

Method notation (save(task: Task): Promise<void>) makes parameters bivariant, allowing a narrower implementation (save(task: DoingTask): …) to type-check at injection sites. Suggest function property notation (save: (task: Task) => Promise<void>).

1.7 Are Branded Types applied to semantically distinct primitives?

Reference: ../functional-ts/SKILL.md §1 "Distinguish meaning with Branded Types", plus the project's validation library guide under ../functional-ts/validation-libraries/.

Flag: string / number used directly for IDs and semantically distinct values (UserId, OrderId, Email, money amounts, …). Verify that brands use the validation library's brand feature when one is present (so as casts are unnecessary), or the unique symbol pattern when no library is used.

1.8 Are domain objects Readonly<>?

Reference: ../functional-ts/SKILL.md §1 "Ensure immutability with Readonly<>"

Flag: domain object types defined without Readonly<…> (or readonly per-property). State changes should produce new objects, not mutate properties.

1.9 Is the "one concept per file" rule followed?

Reference: ../functional-ts/SKILL.md §1 "File structure: one concept per file"

Flag: catch-all files (types.ts, models.ts, domain.ts) aggregating many domain types, especially when companion objects live elsewhere. Barrel files (index.ts) must only re-export.

2. State Transitions via Functions

Reference: ../functional-ts/SKILL.md §2 and ../functional-ts/state-modeling.md

2.1 Do state transitions constrain source states by argument type?

Flag: a transition function whose argument type is the union (TaxiRequest) instead of the specific source state (Waiting). The wider type allows callers to apply the transition to invalid source states.

2.2 Do switch statements over Discriminated Unions have assertNever?

Reference: ../functional-ts/SKILL.md §2 "Exhaustiveness Checking"

Flag: switch on kind without default: return assertNever(x). Without it, adding a new variant will not produce a compile error.

3. Error Handling — Railway Oriented Programming

Reference: ../functional-ts/SKILL.md §3, ../functional-ts/error-handling.md, and the project's Result library guide under ../functional-ts/result-libraries/.

3.1 Are exceptions thrown in the domain layer?

Flag: throw in entities, value objects, or use cases. Suggest migrating to Result. Acceptable: throw inside assertNever (unreachable) and unexpected failures in the infrastructure layer.

3.2 Are error types Discriminated Unions?

Flag: Error subclasses, free-form string error codes, or Result<T, string>. Suggest a Discriminated Union ({ kind: "DriverNotAvailable"; driverId } | { kind: "RequestAlreadyAssigned" }) so callers can branch exhaustively.

3.3 Are Result chains used instead of nested if/else?

Verify that the project uses the matching Result library API (.map, .andThen, Result.do, …) rather than unwrapping immediately into branching code. Cite the matching guide under ../functional-ts/result-libraries/ for the correct combinator.

4. Boundary Defense

Reference: ../functional-ts/SKILL.md §4, ../functional-ts/boundary-defense.md, and the project's validation library guide under ../functional-ts/validation-libraries/.

4.1 Is schema validation present at every external boundary?

Flag: API handlers, DB-result mappers, queue/message handlers, file/config loaders, or env-var readers that treat raw data as domain types without parsing it through a validation library schema (Zod / Valibot / ArkType).

4.2 Are as type assertions used?

Reference: ../functional-ts/SKILL.md §4 "Do not use type assertions (as)"

Flag every as and verify it falls into one of these acceptable cases:

• External data: must be replaced by a validation schema parse.
• as inside a Branded Type factory: acceptable when no validation library is used (unique symbol pattern).
• Internal data: type inference should resolve it; if not, the type design is likely wrong.

4.3 Do PII fields use Sensitive<T>?

Reference: ../functional-ts/SKILL.md §4 "PII Protection", ../functional-ts/boundary-defense.md

Flag: fields plausibly carrying personal information (name, email, phone, address, government IDs, payment details, health/diagnostic information, IP addresses) that are bare string/number rather than Sensitive<T>. Pay special attention to objects that may appear in logs or error messages. Verify that the validation schema auto-wraps such fields with Sensitive.of.

5. Declarative Style

Reference: ../functional-ts/SKILL.md §5, ../functional-ts/state-modeling.md

5.1 Are array operations declarative?

Flag: for / for…of loops that build up arrays imperatively when filter / map / reduce would express the intent directly. Suggest defining predicates on the companion object (e.g., tasks.filter(Task.isActive)).

5.2 Are domain events emitted as immutable records?

Flag: state-change code that mutates a shared event log, or that omits domain events entirely when the state-modeling guidance calls for them. Events should be Readonly<{ eventId; eventAt; eventName; payload; aggregateId }> and recorded separately from the repository.

6. Test Data

Reference: ../functional-ts/SKILL.md §6

6.1 Is as const satisfies Type used for fixtures?

Flag: test fixtures typed with : Type = or with as Type, which widen discriminant literals to string. Suggest as const satisfies Type so kind keeps its literal type.

How to Write Findings

Each finding should include:

1. What the problem is: the specific location in the code (path:line).
2. Why it is a problem: the principle (with a link back to ../functional-ts/...) and the risk of violating it.
3. How to fix it: a code example showing the corrected version.

### Use of method notation

`src/repository/task-repository.ts:15`

`save(task: Task): Promise<void>` uses method notation. Per
[`../functional-ts/SKILL.md` §1 "Use function property notation"](../functional-ts/SKILL.md),
parameters become bivariant under method notation, so a narrower implementation such as
`save(task: DoingTask): Promise<void>` will pass type checking at the injection site.

Suggested fix:
\`\`\`typescript
type TaskRepository = {
  save: (task: Task) => Promise<void>;
};
\`\`\`

Severity

Severity	Item	Reason
High	as type assertions (4.2)	Direct cause of runtime errors
High	Unprotected PII (4.3)	Risk of compliance violations
High	Missing schema validation at external boundaries (4.1)	Direct cause of runtime errors
High	Missing Branded Types on semantically distinct primitives (1.7)	Cross-domain ID confusion at runtime
Medium	Class usage (1.3)	Reduced type safety when extended
Medium	Optional-property state modeling instead of Discriminated Union (1.1)	Invalid states become representable
Medium	Use of throw in domain layer (3.1)	Inconsistent error handling
Medium	Non-Discriminated-Union error types (3.2)	Callers cannot branch exhaustively
Medium	Missing assertNever (2.2)	New variants slip through unhandled
Medium	State transitions accepting the union type (2.1)	Invalid transitions compile
Medium	Catch-all type files (1.9)	Circular dependencies, separation of types from behavior
Medium	Companion Object violations / standalone schema export (1.4)	Implementation detail leakage
Low	Method notation (1.6)	Issue only manifests under specific conditions
Low	interface usage for domain types (1.5)	Declaration merging accidents are rare
Low	Non-Readonly<> domain types (1.8)	Mutation is usually caught in review even without the type
Low	Discriminant other than kind (1.2)	Stylistic inconsistency rather than a defect
Low	Imperative array loops (5.1)	Readability rather than correctness
Low	Missing domain events (5.2)	Depends on whether event sourcing is in scope
Low	Fixtures without as const satisfies (6.1)	Caught by tests in practice