Kamae — Functional Domain Modeling in TypeScript

Six topic files cover the principles. Read only the file(s) relevant to the current task. The library guides under result-libraries/ and validation-libraries/ are read on demand based on the project's package.json.

Step 0: Load applicable rules

Before any other step, glob and Read rules in priority order:

1. .claude/rules/*.md (project-level overrides at the working-tree root)
2. ~/.claude/rules/*.md (user-global preferences)
3. ../../rules/defaults/*.md relative to this SKILL.md (plugin defaults)

For each file:

• Read the YAML frontmatter. Skip the rule unless applies-to is kamae or *.
• Group by name. For each name, keep only the highest-tier instance (1 > 2 > 3); within a tier the lexicographically last filename wins.
• Apply the body of each surviving rule throughout the remaining steps. A library-preference rule overrides Step 1 detection; a convention rule shapes generated code; an override rule replaces guidance from a specific topic file.

If no rules are found, proceed with the plugin defaults already documented in ../../rules/defaults/.

See ../../rules/README.md for the rule format.

Before applying the topic and library guides, inspect nearby files in the same package for an established, consistent coding and import style. Follow that local convention when it exists; use Kamae's guide conventions only when the repository does not establish one.

Step 1: Detect project libraries

Read package.json once. Note which Result library and validation library are present:

• Result libraries — match the first present in priority neverthrow > byethrow > fp-ts > option-t. Load the matching guide under result-libraries/ when error-handling is in scope.
• Validation libraries — match the first present in priority zod > valibot > arktype. Load the matching guide under validation-libraries/ when boundary or branded-type work is in scope.

If none are present, ask the user before proceeding.

Step 2: Apply the topic relevant to the task

Each topic below is one file. Read it lazily — only the file(s) you need for the current task.

Type-Driven Domain Modeling — domain-modeling.md

Represent states with discriminated unions using kind as the unified discriminant. Use type (not interface), Companion Object pattern, branded types via the project's validation library, Readonly<>, function property notation, and one-concept-per-file structure.

Separate read contracts (resolvers) from write contracts (stores), and prefer one method per contract. Give each consumer only the operations it needs; an event store does not require a resolver or a CRUD repository. Keep I/O at workflow edges and pass values into pure domain decisions.

Place these domain-facing ports beside the concepts they serve in the domain layer. Do not introduce a dedicated port/ or ports/ directory, including inside domain/. Keep concrete I/O adapters outside the domain; use cases and adapters import the domain-owned contracts. Read the domain-modeling guide when defining dependency contracts or choosing their file locations.

State Transitions — state-modeling.md

Express transitions with pure functions. Argument types constrain valid source states; return types make targets explicit. Invalid transitions become compile errors. Use assertNever for exhaustiveness.

Error Handling — error-handling.md

• Model expected business failures as use-case-specific Result error unions.
• Include an external failure in the domain Result error union only when the workflow has a documented recovery decision.
• Let unexpected infrastructure failures and contract/invariant violations reach the application error boundary. Preserve library contracts: fp-ts uses a separate execution failure channel inside TaskEither and rethrows unexpected faults at a native Promise boundary; see the fp-ts guide.
• A private control-flow sentinel is allowed when its associated boundary catches only that sentinel and rethrows all other errors.

Boundary Defense — boundary-defense.md

Validate every external input (API requests, DB results, file/queue/env) with a schema at runtime. Trust types inside the domain. Do not use type assertions — as const and as const satisfies Type are the only allowed forms; when the type is unknown, parse through a validation-library schema instead. Apply Sensitive<T> to PII fields; the validation schema auto-wraps them.

When a runtime schema already defines a boundary representation, derive its TypeScript type from the schema instead of restating the same shape.

Declarative Style — declarative-style.md

Use filter / map / reduce with companion-object predicates instead of imperative loops. Model domain events as immutable records.

Test Data — test-data.md

Define fixtures with as const satisfies Type to preserve discriminant literal types and prevent widening.

Examples

Worked end-to-end examples are in examples/. Read them only when the topic guide cites a specific example.

Applying These Principles

These are recommendations, not strict rules. Use judgment based on context. If you deviate from a principle, state the reason in a comment. Justifiable reasons include: external library requires class inheritance, immutable object creation cost is a measured performance concern, or a different pattern has been adopted by team agreement.