# Capability Spec: i18n-formatter-list (i18n-formatter-list) **Package**: `@suaveplan/i18n-formatter-list` **Status**: ACTIVE > Every requirement below carries a `**Verified by:**` line at authoring time. (See `openspec/AGENTS.md` Section 13.) ## Requirements ### Requirement: ListFormatter class wrapping `Intl.ListFormat` The package SHALL export a `ListFormatter` class that wraps `Intl.ListFormat` with construction-time Zod validation of options and a `format(values: readonly string[], locale: string): string` method. Construction with malformed options SHALL throw `I18nError(I18N.INVALID_FORMATTER_OPTIONS)`. #### Scenario: format with default options (long conjunction) - **GIVEN** `const f = new ListFormatter()` - **WHEN** `f.format(["a", "b", "c"], "en-GB")` is called - **THEN** the result equals `"a, b, and c"` #### Scenario: invalid options throw INVALID_FORMATTER_OPTIONS - **WHEN** `new ListFormatter({ type: "invalid" as never })` is called - **THEN** the call throws `I18nError` with code `"I18N.INVALID_FORMATTER_OPTIONS"` **Verified by:** `packages/i18n/i18n-formatter-list/src/formatter.test.ts::"ListFormatter formats with default options"` AND `packages/i18n/i18n-formatter-list/src/formatter.test.ts::"ListFormatter rejects invalid options with INVALID_FORMATTER_OPTIONS"` ### Requirement: `formatList` standalone helper The package SHALL export a `formatList(values, locale, options?)` standalone helper that delivers the same behaviour as `ListFormatter.format` without requiring class construction. Defaults: `style: "long"`, `type: "conjunction"`. #### Scenario: standalone helper formats with disjunction type - **WHEN** `formatList(["x", "y"], "en-GB", { type: "disjunction" })` is called - **THEN** the result equals `"x or y"` #### Scenario: standalone helper twins class output - **WHEN** both `formatList(v, l, opts)` and `new ListFormatter(opts).format(v, l)` are called for the same `(v, l, opts)` - **THEN** the results are identical **Verified by:** `packages/i18n/i18n-formatter-list/src/formatter.test.ts::"formatList formats disjunction"` AND `packages/i18n/i18n-formatter-list/src/formatter.test.ts::"formatList output twins ListFormatter.format output"` ### Requirement: `Intl.ListFormat` `RangeError` maps to `UNSUPPORTED_LOCALE` The package SHALL catch every `RangeError` thrown by the `Intl.ListFormat` constructor and re-throw it as `I18nError(I18N.UNSUPPORTED_LOCALE)` with the original error attached as `cause` and the locale string in `context`. #### Scenario: bogus locale throws UNSUPPORTED_LOCALE - **WHEN** `formatList(["a"], "xx-bogus-tag-!!!")` is called - **THEN** the call throws `I18nError` with code `"I18N.UNSUPPORTED_LOCALE"` AND the error's `cause` is the original `RangeError` **Verified by:** `packages/i18n/i18n-formatter-list/src/formatter.test.ts::"unsupported locale throws UNSUPPORTED_LOCALE with cause"` ### Requirement: Per-(locale, style, type) cache The package SHALL maintain an internal `Map` keyed by `${locale}|${style}|${type}`. Repeated calls with identical `(locale, style, type)` SHALL reuse the same `Intl.ListFormat` instance. The package SHALL export a `clearListFormatterCache()` test-only helper. #### Scenario: identical key reuses the same Intl.ListFormat - **GIVEN** the cache is empty - **WHEN** `formatList(["a"], "en-GB")` is called twice in sequence - **THEN** only one `Intl.ListFormat` constructor invocation occurs #### Scenario: distinct type creates distinct cache entries - **WHEN** `formatList(["a"], "en-GB", { type: "conjunction" })` and `formatList(["a"], "en-GB", { type: "disjunction" })` are called - **THEN** two distinct `Intl.ListFormat` instances are created and cached **Verified by:** `packages/i18n/i18n-formatter-list/src/formatter.test.ts::"identical key reuses cached Intl.ListFormat"` AND `packages/i18n/i18n-formatter-list/src/formatter.test.ts::"distinct type creates distinct cache entries"` ### Requirement: OTel histogram `i18n.format.list.duration_ms` The package SHALL emit an OTel histogram named exactly `i18n.format.list.duration_ms` (unit `ms`) on every successful `formatList` call, tagged with `locale`, `style`, and `type`. #### Scenario: histogram emitted on every call - **GIVEN** an OTel test exporter capturing histogram observations - **WHEN** `formatList(["a", "b"], "en-GB", { style: "long", type: "conjunction" })` is called - **THEN** exactly one observation appears under `i18n.format.list.duration_ms` with `locale`, `style`, and `type` attributes **Verified by:** `packages/i18n/i18n-formatter-list/src/observability.test.ts::"emits i18n.format.list.duration_ms histogram on every call"` ### Requirement: OTel cache-hit counter `i18n.format.cache_hit` The package SHALL emit an OTel counter `i18n.format.cache_hit` tagged with `formatter: "list"` on every cache hit. The counter SHALL NOT increment on the first (cache-miss) call. #### Scenario: cache-hit counter increments only on second call - **GIVEN** an OTel test exporter capturing counter observations and an empty cache - **WHEN** `formatList(["a"], "en-GB")` is called twice in sequence - **THEN** the counter `i18n.format.cache_hit{formatter="list"}` shows exactly one increment **Verified by:** `packages/i18n/i18n-formatter-list/src/observability.test.ts::"cache-hit counter increments only on second call with same options"` ### Requirement: Ergonomic helpers — `formatConjunction`, `formatDisjunction` The package SHALL export `formatConjunction(values, locale, style?)` and `formatDisjunction(values, locale, style?)` ergonomic shortcuts pre-binding `formatList` with the corresponding `type`. #### Scenario: formatConjunction renders comma + and - **WHEN** `formatConjunction(["a", "b", "c"], "en-GB")` is called - **THEN** the result equals `"a, b, and c"` #### Scenario: formatDisjunction renders or - **WHEN** `formatDisjunction(["x", "y"], "en-GB")` is called - **THEN** the result equals `"x or y"` **Verified by:** `packages/i18n/i18n-formatter-list/src/helpers.test.ts::"formatConjunction renders a, b, and c"` AND `packages/i18n/i18n-formatter-list/src/helpers.test.ts::"formatDisjunction renders x or y"` ### Requirement: Subpath exports The package SHALL declare subpath exports in `package.json` so consumers can import only the surfaces they need: - `@suaveplan/i18n-formatter-list` — root barrel - `@suaveplan/i18n-formatter-list/formatter` — `ListFormatter` class + `formatList` + types + schema only - `@suaveplan/i18n-formatter-list/helpers` — `formatConjunction`, `formatDisjunction` only #### Scenario: every documented subpath resolves - **WHEN** every documented subpath is imported in a consumer test fixture - **THEN** every import resolves with no `Module not found` errors **Verified by:** `packages/i18n/i18n-formatter-list/src/index.test.ts::"every documented subpath export resolves"` ### Requirement: Side-effect-free package The package SHALL declare `"sideEffects": false` in `package.json`. #### Scenario: package.json declares sideEffects false - **WHEN** `package.json` is parsed - **THEN** the `sideEffects` field equals `false` **Verified by:** `packages/i18n/i18n-formatter-list/src/index.test.ts::"package.json declares sideEffects false"`