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.

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.

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

Treat errors as values via Result. Define error types as discriminated unions so callers branch exhaustively. Do not throw exceptions in domain code.

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.

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.