🔀 Merge feature/doc-tests into main

♻️ Run doc-test generation as an explicit CI step
Drop the `pretest:ci` lifecycle hook and call `create:doc-tests` from the `build` job before `test:ci`. The hook hid the step from the job log and made `test:ci` behave differently under npm than when run directly.
2026-09-24 13:51:41 +00:00 · 2026-09-24 13:08:45 +00:00 · 2026-09-24 12:52:42 +00:00 · 2026-09-24 12:17:24 +00:00 · 2026-09-24 12:13:36 +00:00 · 2026-09-24 11:53:29 +00:00
35 changed files with 5428 additions and 1107 deletions
@@ -77,6 +77,13 @@ jobs:
            - run: npm ci
            - run: npm run build
            - run: npm run check
+            # Compile the prose examples into gitignored tests. Kept as an
+            # explicit step (not a `pretest:ci` hook) so it is visible in the
+            # job log and `test:ci` stays a plain command.
+            - run: npm run create:doc-tests
+            # Fails the build below 100% coverage on `src/` (`c8 --all --100`);
+            # the same run produces the report published below. See
+            # development/ci.md § Coverage threshold.
            - run: npm run test:ci
            # Publish this tag's coverage to the self-hosted pages server,
            # served read-only at
@@ -1,6 +1,10 @@
 node_modules
 dist
 coverage
+
+# Generated doc-tests (scripts/create-doc-tests.ts); the folder is kept via .gitkeep
+src/doc-test/__generated__/*.test.ts
+!src/doc-test/__generated__/.gitkeep
 *.log
 *.tsbuildinfo
 *.local
@@ -34,7 +34,9 @@
                "no-unused-expressions": "off",
                "no-empty-file": "off",
                "import/no-nodejs-modules": "off",
-                "eslint/no-magic-numbers": "off"
+                "eslint/no-magic-numbers": "off",
+                "unicorn/no-null": "off",
+                "typescript/no-floating-promises": "off"
            }
        },
        {
@@ -9,8 +9,9 @@ first-action facts. Do not restate evolving prose here — it will drift.

 - Project: F#-style pattern matching for TypeScript/ESM. Node `>=26` (pinned via `.node-version`), ESM-only (no CommonJS shim).
 - **While iterating:** `npm run test` (`check:tsc` + the unit suite) for fast feedback on the files you changed.
- **Optional code intelligence:** this repo installs `@spences10/pi-lsp` (pinned in `.pi/settings.json`) as a project-local pi extension. It talks to the repo's own TypeScript 7 via `tsc --lsp --stdio` and exposes **read-only** tools — `lsp_hover`, `lsp_definition`, `lsp_references`, `lsp_find_symbol`, `lsp_document_symbols`, `lsp_diagnostics(_many)`. Prefer `lsp_references` over `grep -w` for widely-colliding identifiers (`matches`, `type`, …); use `lsp_hover` to read inferred types on generic-heavy code. It has no rename / code-action / apply-edit surface — the write side is pi's `edit` tool + `check:tsc`. Treat empty LSP output as _inconclusive_, not success: **`npm run test` / `npm run verify` remain the sole authoritative gate** (see the next bullet). The server keeps running across that gate with a ~5 min idle timeout and registers no file watchers, so if you change `tsconfig.json` / `package.json` mid-session its diagnostics can be stale — when LSP output disagrees with `check:tsc`, trust `check:tsc` and restart pi (or wait out the idle timeout) before concluding the LSP is wrong.
+- **Optional code intelligence:** this repo installs `@spences10/pi-lsp` (pinned in `.pi/settings.json`) as a project-local pi extension. It talks to the repo's own TypeScript 7 via `tsc --lsp --stdio` and exposes **read-only** tools — `lsp_hover`, `lsp_definition`, `lsp_references`, `lsp_find_symbol`, `lsp_document_symbols`, `lsp_diagnostics(_many)`. **When you are looking for a symbol, reach for the LSP before `rg`/`grep`** — `lsp_references` / `lsp_find_symbol` / `lsp_definition` / `lsp_document_symbols` are semantic and cross-file, so they see shadowing, imports and overloads that a text search cannot; use `lsp_hover` to read inferred types on generic-heavy code. Use `rg` for what the LSP cannot see — doc prose, string literals, config, task lists, file discovery — and reconcile the two sets before editing (symbols from the LSP, strings and prose from `rg`). It has no rename / code-action / apply-edit surface — the write side is pi's `edit` tool + `check:tsc`. Treat empty LSP output as _inconclusive_, not success: **`npm run test` / `npm run verify` remain the sole authoritative gate** (see the next bullet). The server keeps running across that gate with a ~5 min idle timeout and registers no file watchers, so if you change `tsconfig.json` / `package.json` mid-session its diagnostics can be stale — when LSP output disagrees with `check:tsc`, trust `check:tsc` and restart pi (or wait out the idle timeout) before concluding the LSP is wrong.
 - **Definition of done — run this before you call the work finished:** `npm run verify`. If all green, commit. If red, look at the output, fix the root cause, and re-run.
+- **Touched a `ts`-tagged fence in `README.md` / `CONTRIBUTING.md`?** Run `npm run create:doc-tests` first: it regenerates the gitignored tests under `src/doc-test/__generated__/`, formats and type-checks them, so `verify` executes the documented example. CI runs it in an explicit step before `test:ci`; the fast local tiers do not. See [development/docs.md](./development/docs.md).
 - **On commit:** write a good message (see [CONTRIBUTING.md § Commit messages](./CONTRIBUTING.md#commit-messages)). Lefthook's pre-commit hook already runs the fast, offline, staged-file checks — don't run them by hand. If the hook fails on style, `npm run fix`, restage, recommit.
 - **Document decisions where the next maintainer will look:** rationale, rejected alternatives and known issues go in `development/<category>.md` (see [development/README.md](./development/README.md)); the actionable rule stays in [CONTRIBUTING.md](./CONTRIBUTING.md) and links to it. Write each fact once — never copy the rule into `development/` or the reason into `CONTRIBUTING.md` — and change both in the same commit when a rule changes.
 - **`npm run maintain` is NOT part of the feature loop.** Its scans are advisory, never a gate; run them only on an explicit maintenance / update-deps branch.
@@ -25,10 +26,11 @@ first-action facts. Do not restate evolving prose here — it will drift.
 Don't silence the type system to force a green run. As an agent these are forbidden:

 - `// @ts-nocheck`, `// @ts-ignore`, `// @ts-expect-error`
- `// oxlint-disable` / `// oxlint-disable-next-line` — the sole exception is the fixed file-level `typescript/no-floating-promises` header at the top of a `*.test.ts` file, spelled exactly as [CONTRIBUTING.md § Rules the tools don't enforce](./CONTRIBUTING.md#rules-the-tools-dont-enforce) prescribes
+- `// oxlint-disable` / `// oxlint-disable-next-line`
+- editing `.oxlintrc.json` to silence a finding (e.g. turning `typescript/no-floating-promises` off)
 - `as` casts used to push an expression through (type-aware oxlint already flags unsafe assertions)

-Fix the root cause with the type system instead — narrowing, generics, `satisfies`, conditional / mapped types, utility types (`NonNullable`, `Exclude`, …). TypeScript can express it; that's the intended tool. The `oxlint-disable`-location rule in [CONTRIBUTING.md § Rules the tools don't enforce](./CONTRIBUTING.md#rules-the-tools-dont-enforce) is a **human** last-resort convention (so a reviewer can spot a deliberate suppression) — it is not permission for you to add one, except the one fixed `*.test.ts` file-level header named there. If the types genuinely cannot express something, stop and surface the conflict (commit message / handover) rather than suppress it.
+Fix the root cause with the type system instead — narrowing, generics, `satisfies`, conditional / mapped types, utility types (`NonNullable`, `Exclude`, …). TypeScript can express it; that's the intended tool. Suppressions — a source `oxlint-disable` **or** a `.oxlintrc.json` entry — are a **human** last resort, not a tool for you. If the types genuinely cannot express something, stop and surface the conflict (commit message / handover) rather than suppress it.

 The same applies to the checks themselves: **never `git commit --no-verify`** (or otherwise skip a pre-commit / pre-push hook). The checks are fast and offline, so a redundant run is fine — bypassing a hook to get green is the identical anti-pattern. If a commit already skipped a hook, redo it through one: `git reset --soft HEAD~1 && git commit -C <skipped-sha>`.

@@ -7,6 +7,49 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0

 ## [Unreleased]

+- document the public API in the README: the four factories, when to use each,
+  what the `W` (widening) suffix means, and examples that bind the handler-map
+  function once
+- add TSDoc to the four public matcher factories
+
+## [0.8.1] - 2026-09-23
+
+- gate CI at 100% coverage: `test:ci` runs c8 with `--all --100` over `src/`,
+  and a test loads the `index.ts` barrel so it is measured
+- pin the duplicate-tag union collapse (members sharing a tag dispatch through
+  one handler) with tests
+
+## [0.8.0] - 2026-09-23
+
+- reject a universe that mixes a literal with a broad type (e.g.
+  `"a" | \`x-${number}\``), which the finite-literal gate let through
+- narrow the tagged-union matcher's handler parameters for a union-valued or
+  optional discriminant, and its fallback to the unhandled tags, instead of
+  passing `never`
+
+## [0.7.1] - 2026-09-23
+
+- upgrade dependencies
+
+## [0.7.0] - 2026-09-23
+
+- reject broad universes (`string`, `number`, template literals) and
+  value/stringification collisions (`true | "true"`, `1 | "1"`) at the factory
+- type handler parameters by the matched member, so a
+  standalone `"true"` universe is typed `"true"` rather than `true`
+
+## [0.6.0] - 2026-09-22
+
+- add `getTaggedUnionMatcher` / `getTaggedUnionMatcherW` for discriminated unions
+- rename the primitive matcher to primitive-union: `getMatcher` /
+  `getMatcherW` → `getPrimitiveUnionMatcher` / `getPrimitiveUnionMatcherW`, and
+  `src/primitive.ts` / `src/primitive.test.ts` → `src/primitive-union.*`
+
+## [0.5.0] - 2026-09-21
+
+- reject a fallback when the handler map already covers the universe
+- allow boolean, null and undefined in the matcher universe
+
 ## [0.4.0] - 2026-09-20

 - move the `_` fallback out of handlers
@@ -63,7 +106,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0

 - basic setup

-[Unreleased]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.4.0...main
+[Unreleased]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.8.1...main
+[0.8.1]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.8.0...0.8.1
+[0.8.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.7.1...0.8.0
+[0.7.1]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.7.0...0.7.1
+[0.7.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.6.0...0.7.0
+[0.6.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.5.0...0.6.0
+[0.5.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.4.0...0.5.0
 [0.4.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.3.0...0.4.0
 [0.3.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.2.0...0.3.0
 [0.2.0]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.1.8...0.2.0
@@ -22,6 +22,8 @@ the rules so agents and humans don't diverge.
 - **Test:** `npm run test`, `npm run test:ci`
 - **Watch:** `npm run watch` - re-runs tests on file save, humans only
 - **Checks:** `npm run check`, `npm run fix`
+- **Doc tests:** `npm run create:doc-tests` — compile the `ts`-tagged fences
+  in the prose docs into executed, gitignored tests
 - **Verify:** `npm run verify` — the definition of done
 - **Maintenance:** `npm run maintain` — advisory only
 - **Individual fixes:** `npm run fix:oxfmt`, `npm run fix:oxlint`
@@ -41,7 +43,7 @@ faster tiers catch less, slower tiers are more thorough":
 | `npm run verify`                 | manual                  | Definition of done: `npm run check` + unit tests, one shot                                                                                              | ~6s   |
 | `npm run fix`                    | manual                  | Auto-resolve fixable issues (lint, format)                                                                                                              | ~3s   |
 | `npm run maintain`               | manual / CI (advisory)  | `maintain:knip` + `maintain:outdated` (whole-project + network scans)                                                                                   | ~10s  |
-| CI build (auto)                  | on push to `main` / tag | `build` job (build + correctness + packaging) — see [.gitea/workflows/ci.yml](./.gitea/workflows/ci.yml)                                                | ~30s+ |
+| CI build (auto)                  | on push to `main` / tag | `build` job (build + correctness + coverage + packaging) — see [.gitea/workflows/ci.yml](./.gitea/workflows/ci.yml)                                     | ~30s+ |
 | CI maintain (auto, non-blocking) | on push to `main`       | `npm run maintain` — reports, never fails the build                                                                                                     | ~10s  |
 | CI publish (auto)                | on tag                  | packaging checks + `publish:publint` / `publish:attw`, then the Gitea release page and `npm publish` (skipped, and the job failed, without `NPM_TOKEN`) | ~15s  |

@@ -66,9 +68,12 @@ before the implementation. The loop is **type → red → green → refactor**:
 4. **Refactor** — with the type system and the tests as the safety net, then
   `npm run verify` as the definition-of-done gate.

-Every test pairs an `expectTypeOf(...)` with an `assert.*`; keep them together.
+Every test pairs an `expectTypeOf(...)` with an `assert.*`; keep them together
+— including the expectations inside handler bodies, which pair with an
+assertion on the value dispatch passed, not only the type it inferred (see
+[development/testing.md § Handler arguments](./development/testing.md#handler-arguments)).
 The autocomplete tests (`src/util/__tests__/lsp-completion.test.ts` for the
-helper, `src/primitive.test.ts` for the matcher's popup) are the
+helper, `src/primitive-union.test.ts` for the matcher's popup) are the
 exception —
 the language server, not the type system, is the oracle (see
 [development/testing.md § Autocomplete](./development/testing.md#autocomplete)).
@@ -84,6 +89,17 @@ Per [AGENTS.md § Never do](./AGENTS.md#never-do), reach green honestly — fix
 types, never suppress the checks you can't make pass. Full rationale:
 [development/testing.md](./development/testing.md).

+## Documentation examples
+
+Every `ts`-tagged fence in `README.md` / `CONTRIBUTING.md` is compiled into an
+executed test, so a documented example cannot drift from the API. Describe each
+fence with the paragraph directly above it (that text becomes the test title),
+and keep its library import self-contained;
+`npm run create:doc-tests` regenerates, formats and type-checks the tests under
+`src/doc-test/__generated__/`. CI runs it in an explicit step before `test:ci`,
+so run it yourself before `npm run verify` when you touched a fence. Why:
+[development/docs.md](./development/docs.md).
+
 ## Code style and formatting

 `oxfmt` is the formatter and `oxlint` is the linter (with type-aware rules).
@@ -112,17 +128,21 @@ intended to run. A `<prefix>:<name>` script is implicitly aggregated by a
 `<prefix>` script (if one exists) and run by the corresponding lefthook hook or
 CI step. Pick the prefix that matches the script's lifecycle:

- `create:*` — front doors of the repo's own workflow; these mutate git state
-  rather than the source. `create:branch` opens a unit of work, `create:finish`
+- `create:*` — front doors of the repo's own workflow; these produce or mutate
+  workflow artifacts (git state, generated doc-tests) rather than the
+  hand-written source. `create:branch` opens a unit of work, `create:finish`
  closes the branch half, `create:release` closes the release half
-  (maintainer-only). No bare `create` aggregator on purpose.
+  (maintainer-only), and `create:doc-tests` regenerates the compiled prose
+  examples. No bare `create` aggregator on purpose.
 - `check:*` — read-only verification; never modifies files. Aggregated by
  `npm run check`.
 - `fix:*` — mutating counterpart of a `check:*` script. Aggregated by
  `npm run fix`; the diff is the review surface.
 - `test:*` — test scripts. `test` is the canonical entry point (`check:tsc` +
  unit tests); `test:unit` skips the typecheck for fast local iteration;
-  `test:ci` adds c8 coverage.
+  `test:coverage` runs c8 over the hand-written tests only; `test:doc` runs the
+  generated doc examples without coverage; `test:ci` chains the two and fails
+  below 100% coverage on `src/` (CI-only; `verify` stays coverage-free).
 - `watch:*` — long-running watchers for the manual inner dev loop. Aggregated by
  `watch`.
 - `maintain:*` — advisory repo-maintenance scans: read-only, but whole-project
@@ -155,12 +175,9 @@ reaches for by default:
  [Script prefix convention](#script-prefix-convention).
 - **`oxlint-disable` directives live in source, not `.oxlintrc.json`.** The
  trade-off must sit next to the code it silences. This is a _human_ last-resort
-  convention; agents must not add these — see
-  [AGENTS.md § Never do](./AGENTS.md#never-do). The sole agent exception is the
-  fixed file-level header at the top of a `*.test.ts` file, spelled exactly:
-  `/* oxlint-disable typescript/no-floating-promises -- expectTypeOf() is a sync
-type-assertion library that the type-aware linter misidentifies as a promise
-*/` — any other suppression stays human-last-resort. (why:
+  convention; agents must not add one, nor edit `.oxlintrc.json` to silence a
+  finding (e.g. `typescript/no-floating-promises`) — see
+  [AGENTS.md § Never do](./AGENTS.md#never-do). (why:
  [development/tooling.md](./development/tooling.md#oxlint-disable-directives-live-next-to-the-code))
 - **Don't put slow / network / whole-project scans in `check` or pre-commit.**
  Advisory scans are not correctness gates; they belong under `maintain:`. (why:
@@ -1,6 +1,6 @@
 MIT License

-Copyright (c) 2025 tmu
+Copyright (c) 2026 tmu

 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal
@@ -9,7 +9,7 @@ ordinary objects whose `matches` method is a TypeScript type guard, so narrowing
 composes the way any other guard does. It is deliberately not a regex engine and
 not a macro: there is no transpiler and no DSL to learn, and the type-level
 contract is the feature — see [development/library.md](./development/library.md)
-for the design decisions and the known limitations.
+for the design decisions and [Caveats](#caveats) for the limits.

 ## Requirements

@@ -21,11 +21,140 @@ for the design decisions and the known limitations.

 ## API

-Yet to be implemented
+The package exports four factories. Two axes pick one:
+
+- **Universe** — a _primitive-union_ matcher matches a value that is itself a
+  finite union (`"yes" | "no"`); a _tagged-union_ matcher matches an object
+  discriminated by a property (`{ kind: … }`).
+- **Return** — the _strict_ variant gives every handler one common return type
+  `R`; the _widening_ variant (`W`) widens the return value to the union of the
+  handler returns.
+
+| Factory                          | Use when                                                                  | Return                |
+| -------------------------------- | ------------------------------------------------------------------------- | --------------------- |
+| `getPrimitiveUnionMatcher<T>()`  | the value is the union and all handlers return the same type              | one common `R`        |
+| `getPrimitiveUnionMatcherW<T>()` | the value is the union and handlers return different types                | union of the handlers |
+| `getTaggedUnionMatcher<T>()`     | the value is a discriminated object and all handlers return the same type | one common `R`        |
+| `getTaggedUnionMatcherW<T>()`    | the value is a discriminated object and handlers return different types   | union of the handlers |
+
+Bind the function that takes the handler map to a `match…` variable once and
+reuse it; the examples below do this, so the builder is allocated once.
+
+### Primitive-union matchers
+
+`getPrimitiveUnionMatcher<T>()` takes the finite universe `T` and returns a
+builder. Calling the builder with a handler map keyed by `T`'s members returns a
+matcher: a function from `T` to the common return type.
+
+```ts
+import { getPrimitiveUnionMatcher } from "tiny-pattern-ts";
+
+const matchAnswer = getPrimitiveUnionMatcher<"yes" | "no">();
+
+const reply = matchAnswer({
+    yes: () => "agreed",
+    no: () => "declined",
+});
+
+const answer = reply("yes");
+assert.equal(answer, "agreed");
+```
+
+Add a fallback as the second argument to leave members unhandled; the fallback
+receives the remainder:
+
+```ts
+import { getPrimitiveUnionMatcher } from "tiny-pattern-ts";
+
+const matchLabel = getPrimitiveUnionMatcher<"yes" | "no" | "maybe">();
+
+const label = matchLabel(
+    { yes: () => "agreed", no: () => "declined" },
+    (other) => `not sure: ${other}`, // other: "maybe"
+);
+
+const answer = label("yes");
+assert.equal(answer, "agreed");
+
+const fallback = label("maybe");
+assert.equal(fallback, "not sure: maybe");
+```
+
+`getPrimitiveUnionMatcherW` is the same builder, but the matcher's return type
+is the union of the handler return types rather than one common `R`.
+
+### Tagged-union matchers
+
+`getTaggedUnionMatcher<T>()` takes a discriminated union `T`. The returned
+function takes the discriminant property's name and returns the handler-map
+builder, keyed by that property's tags.
+
+```ts
+import { getTaggedUnionMatcher } from "tiny-pattern-ts";
+
+type Shape =
+    { kind: "circle"; radius: number } | { kind: "square"; side: number };
+
+const matchShape = getTaggedUnionMatcher<Shape>()("kind");
+
+const area = matchShape({
+    circle: (s) => Math.PI * s.radius ** 2,
+    square: (s) => s.side ** 2,
+});
+
+const circleArea = area({ kind: "circle", radius: 2 });
+assert.equal(circleArea, Math.PI * 4);
+
+const squareArea = area({ kind: "square", side: 3 });
+assert.equal(squareArea, 9);
+```
+
+`getTaggedUnionMatcherW` is the widening counterpart, exactly as in the
+primitive-union pair. The discriminant key is restricted to properties whose
+values are tags; see [Caveats](#caveats) for the supported tags and the
+`boolean` / `null` / `undefined` key projection.
+
+### The universe
+
+The primitive universe `T` must be a finite union of literals with no
+value/stringification collision. A broad member (`string`, `number`, a template
+literal) or a colliding pair (`true | "true"`, `1 | "1"`) is rejected at the
+factory. The reasons and the rejected alternatives are in
+[development/library.md](./development/library.md).
+
+## Caveats
+
+- **Only finite universes are supported.** The factory must be given a finite
+  union of literals; `string`, `number` and template literals are rejected. This
+  is what lets the exhaustive overload be proven, so the runtime `dispatch`
+  throw stays unreachable through the typed API.
+- **A value and its stringification must not both be present.** Object keys
+  stringify, so a universe containing both a member and the string it
+  stringifies to — `1 | "1"`, `true | "true"`, `null | "null"` — is rejected at
+  the factory. Either form alone is fine, and one value's string form may
+  coexist with a _different_ value's bare form (`"true" | false`).
+- **`symbol` and `bigint` are not supported.** A `symbol` brand is a
+  compile-time phantom with nothing to match at runtime, and a `bigint` is not a
+  valid property key; neither satisfies the matcher's universe constraint.
+- **`NaN` and `-0` cannot be matched specifically.** They have no literal type,
+  so both stay part of `number`.
+
+### Why open universes are rejected
+
+An open universe — one carrying a broad member, as in
+`type Units = "s" | "ms" | "min" | (string & {})` — is not a dispatch concern.
+If the values arrive from outside the program, parse them at the boundary down
+to a finite union and match the narrowed result; the openness never reaches the
+matcher. If the domain is genuinely extensible, the right shape is a runtime
+`Map` of handlers, where "no handler" is a lookup, not a pattern. Either way an
+open matcher would abandon the one guarantee this library exists to give —
+provable exhaustiveness — to automate what a `switch` and a default arm already
+cover. The type-level cost of supporting open universes is recorded in
+[development/library.md](./development/library.md#supported-universes).

 ## License

-MIT © 2025 tmu. See [LICENSE](./LICENSE).
+MIT © 2026 tmu. See [LICENSE](./LICENSE).

 ## Contributing

@@ -6,54 +6,21 @@ Backlog and tracking for tiny-pattern-ts. Managed in vscode-todotasks format.

 Setup:
 ☐ Split off template into separate package => pi --session 01a07dde-7050-7054-bb36-1606d7eb2bc3 @low
-✔ straighten oxc rules @done
-    ✔ oxc forbids ternary => remove oxc rule @done
-    ✔ oxc wants comments to start comments with a capital letter => remove oxc rule @done
-    ✔ there already exists a /* */ block comment in primitive.ts, => change to multi-line comment @done
-
-✔ Remove example code files and its documentation and its exports from index.ts @high @done
-    ✔ remove src/match.ts and its documentation @high @done
-    ✔ remove src/pattern.ts and its documentation @high @done
-    ✔ remove src/index.test.ts and its documentation @high @done
-    ✔ exports from `src/index.ts` should only be the public API surface @done
-

 v1.0:
-☐ API surface is stable and fully typed
-    ☐ Finalize public exports in `src/index.ts`
-    ☐ Document all exported types and functions
-    ☐ Add JSDoc for public APIs
-☐ Test coverage meets threshold
-    ☐ Achieve 100% branch coverage on `src/primitive.ts`
-    ☐ Achieve 100% branch coverage on `src/index.ts`
-
-Testing:
-✔ Cover autocomplete with real completion test cases in the suite @medium @done
-    → `src/util/__tests__/lsp-completion.ts` (`#test-utils/…`) is the helper; the suite asserts its labels (see development/testing.md § Autocomplete)
-    → wire it into `node --test` so a test asserts the offered labels
-    → note: `Parameters<typeof factory>[0]` resolves only the *last* overload; use `@ts-expect-error` call sites for factory negatives, not `not.toExtend<Parameters<…>>`
-✔ Drive the LSP completion helper through the LSP protocol library instead of a hand-rolled JSON-RPC client @medium @done
-    ✔ Add `vscode-languageserver-protocol` and record the tooling decision @done
-    ✔ Rewrite `src/util/__tests__/lsp-completion.ts` onto `createMessageConnection` and typed requests @done
-    ✔ Update `development/testing.md § Autocomplete` for the new client @done
-    ✔ Run `npm run verify` and check the task off @done
+✔ API surface is stable and fully typed @done
+    ✔ Finalize public exports in `src/index.ts` @done
+    ✔ Document all exported types and functions @done
+    ✔ Add JSDoc for public APIs @done
+✔ Test coverage meets threshold @done
+    ✔ Achieve 100% branch coverage on `src/primitive-union.ts` @done
+    ✔ Achieve 100% branch coverage on `src/index.ts` @done

 Matcher:
-✔ Clean up: adopt the 3-overload matcher (`src/prototype-ac2.ts`) and delete the prototypes @high @done
-    → `getMatcher` / `getMatcherW`, each with overloads `ExhaustiveLoose` → `Fallback` → `Handlers` (order is load-bearing)
-    → fold into `src/primitive.ts` / the public API; drop `src/prototype*.ts`
-✔ `_` should receive only the unhandled `T` keys, not all of `T` @medium @done
-    → the fallback is now a second argument: `(handlers, (s) => …)`, `s: Exclude<T, keyof handlers>`
-☐ A fallback for an already-exhaustive handler map must be a compile error @medium
-    → `(handlers, fallback)` with `handlers` covering all of `T` is accepted; the redundant fallback should be rejected (currying would allow the guard)
-
-Bugs:
-✔ TS 7 LSP server logs `context canceled` on stderr at shutdown @done
-    → `handleExit` returns `io.EOF`, cancelling the background context while `Session.updateWatches` is still in flight; the bare error is flushed to stderr and the server exits 1
-    → close stdin after `shutdown` instead of sending `exit`; the server exits cleanly (code 0, no output), kill kept as a fallback
-Enhancements:
-☐ Allow boolean literals in primitive union patterns (e.g. `true: () => "yes"`) @medium
-☐ Are there other primitive types that should be supported in union patterns? (e.g. `bigint`, `symbol`) @medium
+✔ when using a union type as a property, the current behavior of tagged union matcher is @done
+    to pass never to handler parameters
+    → new matcher function needed or can be fixed in tagged union matcher
+    ✔ optional discriminant (`{ type?: "x" }`) is the same hole: the boolean/nullish change now admits the `undefined` tag, so the factory accepts the key, but `Extract<T, Record<K, V>>` still passes `never` to both the `x` and `undefined` handlers @done

 Documentation:
 ☐ Bring README.md back to its previous form — synopsis and examples restored, in the correct place
@@ -61,13 +28,11 @@ Documentation:
    → previous Examples order: literal/exhaustive, typeof, structural/discriminated unions, when, any
 ☐ Create `examples/` directory with runnable snippets
 ☐ Add comparison section vs. other TS pattern-matching libs in Readme.md
+    ☐ Why do we do this? => exhaustiveness encoded type safe
+    ☐ Why this form? little syntax, data last, very small, autocomplete, strict typing in the handler; for more features use ts-pattern
 ☐ Write migration guide for users coming from discriminated unions
 ☐ Create backlog tasks for implementation
-✔ Document the matcher design paths in `development/` — union vs overload merge, inferred universe (`NoInfer`), conditional `RequireKeys`, cases-first — and why each was abandoned @done
-☐ Validate code fences in Markdown (start with README.md) — compile the TypeScript examples against `src/` so the docs cannot drift from the API
-
-Workflow:
-✔ Resolve the finish/push tension: `create:finish` leaves `main` ahead of its upstream while `create:branch` refuses until `main` matches upstream — decide whether `finish` should push or `branch` should compare only `BEHIND` (see development/workflow.md) @done
+✔ Validate code fences in Markdown (start with README.md) — compile the TypeScript examples against `src/` so the docs cannot drift from the API @done

 Maintenance:
 ☐ Serve CI coverage over a tiny self-hosted webserver (replace the zip artifact) @low
@@ -81,3 +46,4 @@ Maintenance:
    ☐ Add a minimal dir-listing webserver to the gitea docker setup for serving landing page (reuse existing reverse proxy)
    ☐ CI writes landing page to a shared volume keyed by project + tag (e.g. `/landing/tiny-pattern-ts/<tag>/`)
    ☐ Browse to `…/tiny-pattern-ts/index.html` in the browser
+☐ Add testing with TypeScript 5.0 baseline in CI
@@ -43,7 +43,9 @@
        "silverwind",
        "idris",
        "todotasks",
-        "connor"
+        "connor",
+        "injective",
+        "injectivity"
    ],
    "ignorePaths": ["dist", "node_modules", "coverage", "*.svg", ".gitignore"]
 }
@@ -24,6 +24,7 @@ One file per category:
 | [workflow.md](./workflow.md)     | Branching and merging, script prefixes, feedback tiers, commit messages |
 | [tooling.md](./tooling.md)       | Toolchain choices and configuration, editor setup                       |
 | [testing.md](./testing.md)       | Test strategy and type-driven development                               |
+| [docs.md](./docs.md)             | Validating the Markdown code fences in the prose docs                   |
 | [ci.md](./ci.md)                 | CI pipeline, runner image, coverage serving                             |
 | [publishing.md](./publishing.md) | Release and npm publishing                                              |

@@ -97,6 +97,39 @@ Leave `act_runner`'s `force_pull` disabled.
  (`docker rmi gitea.e1nsnull.de/tmu/act-ci:<version>`); do not reach for
  force-pull.

+## Coverage threshold
+
+#### Decision (2026-09)
+
+`npm run test:ci` fails below 100% statements / branches / functions / lines
+across `src/**/*.ts` (`c8 --all --include "src/**/*.ts" --100`). The gate rides
+the `build` job; `npm run verify` stays coverage-free.
+
+#### Why
+
+- The types are the feature, so an untested branch is a hole in the contract,
+  not a metric to trade off; 100% is the only threshold that means "no hole".
+- `--all` counts a `src/` file no test imports. Without it c8 reports only the
+  files the suite happened to load, so a new untested module is invisible and
+  the threshold passes vacuously.
+- The gate rides `test:ci`, which `build` already runs — no new job or step.
+- `verify` stays fast and local; the slower coverage run is a CI-only tier (see
+  [workflow.md § Feedback tiers](./workflow.md#feedback-tiers)).
+
+#### Rejected
+
+- Per-file thresholds: a global 100% already forces every counted file to 100%.
+- A `check:coverage` script: it would re-run the suite or read c8's temp dir,
+  and no `check:*` script runs tests.
+- `--all` without `--include`: it would also sweep `scripts/`, which is not the
+  shipped surface.
+
+#### Known issue
+
+- `src/matcher-shared.ts` is types only, so its runtime image is empty; c8 still
+  lists it under `--all`. It carries a file-level `/* c8 ignore start */` with
+  the reason. Adding runtime code there means removing that directive.
+
 ## Coverage serving

 #### Decision (2026-09)
@@ -0,0 +1,61 @@
+# Docs
+
+Why the prose documentation is maintained the way it is. The actionable rules
+are in [CONTRIBUTING.md](../CONTRIBUTING.md); this file records the rationale.
+
+## Validating Markdown code fences
+
+#### Decision (2026-11)
+
+Compile every `ts / `typescript fence in the prose docs into a real
+`node:test` case under `src/doc-test/__generated__/`, typechecked by a scoped
+`tsc` project and executed by `node --test`.
+
+#### Why
+
+- A documented example is a promise about the API. Left unchecked it drifts the
+  moment a signature changes, and a reader copies broken code.
+- The repo runs TypeScript 7, the native/Go compiler. It exposes **no legacy JS
+  compiler API** (`ts.createProgram`, `ts.transpileModule`, `ts.createSourceFile`
+  are all `undefined`; `Object.keys(require("typescript"))` is
+  `["version", "versionMajorMinor"]`). So `@typescript/vfs`, the type-aware
+  `eslint-plugin-markdown` and `docs-ts` / `@effect/docgen` cannot run here.
+- `node --check` parses as JS and rejects valid TS type annotations, so it is not
+  a gate. The only faithful validator is the `tsc` **CLI**, which means emitting
+  real `.ts` files and letting the existing `check:tsc` / `node --test` pipeline
+  judge them.
+
+#### Rejected
+
+- **A packaged doc-test tool** (see above) — no usable compiler API on TS 7.
+- **Embedding a typecheck in the generator** — duplicates the gate, and would
+  not exercise the repo's own resolution.
+- **`node --check`** — wrong language level.
+
+#### Known issue
+
+- Scanned sources are hard-coded to `README.md` and `CONTRIBUTING.md`. A
+  `docs/` + `examples/` list is the natural extension; `development/` must never
+  be scanned (its fences are illustrative, not compilable).
+- The generator hoists and merges leading imports, rewrites `tiny-pattern-ts` to
+  the `#test-tiny-pattern-ts` source alias, and rejects an example that imports
+  `node:assert` / `node:test` (the prelude already binds both). Titles are the
+  immediately preceding paragraph; a fence with no such paragraph is a fatal
+  error, which keeps every example described.
+- `oxlint src/doc-test` reports "No files found" because the generated
+  `*.test.ts` are gitignored. That is cosmetic: the files are still typechecked
+  and run.
+- The generated tests are `*.test.ts`, which c8's default excludes already keep
+  out of the `--100` gate. Do not add an `--exclude` for them: passing any
+  `--exclude` replaces the defaults, so every hand-written test file and
+  `__tests__/` helper re-enters coverage and the gate fails.
+- Generated examples must not run under `c8`: an example could cover a line no
+  hand-written test reaches, so the coverage gate would pass on documentation
+  alone. `test:coverage` therefore runs c8 over the tracked tests only
+  (`git ls-files 'src/*.test.ts'` — the generated files are untracked), and
+  `test:doc` runs the examples in a separate process without c8. `test:ci`
+  chains the two, so correctness and coverage stay independent.
+- The CI `build` job runs `npm run create:doc-tests` as an explicit step before
+  `npm run test:ci`, not a `pretest:ci` lifecycle hook: the hook hides the step
+  from the job log and makes `test:ci` behave differently under npm than when
+  run directly.
@@ -3,9 +3,41 @@
 The type-level design of the public API and the limitations it carries. The
 user-facing reference is [README § API](../README.md#api).

-The matcher is implemented in `src/primitive.ts` and re-exported from
-`src/index.ts` as `getMatcher` / `getMatcherW`; the rest of the library is
-placeholder code.
+The matchers are implemented in `src/primitive-union.ts` (`getPrimitiveUnionMatcher` /
+`getPrimitiveUnionMatcherW`) and `src/tagged-union.ts` (`getTaggedUnionMatcher` /
+`getTaggedUnionMatcherW`), re-exported from `src/index.ts`; the rest of `src/`
+is implementation detail.
+
+## Public surface
+
+#### Decision (2026-09)
+
+`src/index.ts` exports the four factories and nothing else. Every exported
+function carries TSDoc; the builder types and the `matcher-shared.ts`
+vocabulary stay internal.
+
+#### Why
+
+- The factories are the whole contract: a consumer calls one and never needs to
+  name the builder type it returns.
+- The builder interfaces are already the inferred return types, so they travel
+  into the emitted `.d.ts` regardless. Exporting them would only make them
+  nameable while freezing the internal `Strict` / `Widening` overload split as
+  API.
+- TSDoc travels into the emitted declarations, so editor hovers and the
+  published package document the API without a hand-written `.d.ts`.
+
+#### Rejected
+
+- **Exporting the builder types** (`PrimitiveUnionMatcher`, …). Nameable, but it
+  grows the surface for no call-site benefit and pins the `Strict` / `Widening`
+  split.
+- **Exporting the `matcher-shared.ts` vocabulary** (`Matchable`, `UnaryFn`,
+  `PatternKey`, `Member`, `PatternReturns`, …). They appear in the public
+  signatures, but a consumer never needs to name them; exporting them would
+  freeze plumbing as API.
+- **A hand-written `.d.ts` or a separate API document.** It would drift from the
+  implementation; TSDoc is generated from the source.

 ## Matcher shape

@@ -15,8 +47,8 @@ A factory takes the universe and returns a builder; the builder takes a handler
 map and an optional fallback:

 ```ts
-const matcher = getMatcher<"a" | "b">()({ a: (s) => …, b: (s) => … });
-const fallback = getMatcher<"a" | "b" | "c">()({ a: (s) => … }, (s) => …);
+const matcher = getPrimitiveUnionMatcher<"a" | "b">()({ a: (s) => …, b: (s) => … });
+const fallback = getPrimitiveUnionMatcher<"a" | "b" | "c">()({ a: (s) => … }, (s) => …);
 ```

 Exhaustive or fallback is decided **at the call site**, by whether the second
@@ -24,15 +56,16 @@ argument is present. The fallback's parameter is the remainder
 `Exclude<T, keyof Handled>`. Only the return-strictness axis remains, so there
 are two factories:

- `getMatcher` — one common `R`; the fallback must fit it;
- `getMatcherW` — the union `PatternReturns<Handled> | R`.
+- `getPrimitiveUnionMatcher` — one common `R`; the fallback must fit it;
+- `getPrimitiveUnionMatcherW` — the union `PatternReturns<Handled> | R`.

 Each factory is two overloads whose order is load-bearing:

 1. `Handlers<T, R>` — the exhaustive form, and the contextual type of the
   handler-map popup;
-2. `Handled extends Exact<Partial<Handlers<T, R>>, Handled>` plus
-   `Fallback<T, Handled, R>` — a partial handler map plus the fallback.
+2. `Handled extends Exact<Partial<Handlers<T, R>>, Handled>` intersected with
+   `MustBePartial<T, Handled>`, plus `Fallback<T, Handled, R>` — a partial
+   handler map plus the fallback, rejected when the map already covers `T`.

 #### Why

@@ -41,6 +74,14 @@ Each factory is two overloads whose order is load-bearing:
  handler map can only see all of `T`, never `Exclude<T, keyof Handled>`. A later
  argument is contextually typed from inference on an earlier one, so the split
  is what makes the remainder expressible.
+- **The redundant-fallback guard is an F-bounded constraint.** A map that
+  already covers `T` plus a fallback is rejected by folding
+  `MustBePartial<T, Handled>` into `Handled`'s own constraint. The guard is
+  checked _after_ `Handled` is inferred, so the contextual pass that types the
+  handler callbacks survives. The obvious conditional
+  `Exclude<T, keyof Handled> extends never ? …` in the fallback's parameter
+  type is evaluated while `Handled` is still its constraint and rejects every
+  partial map whose callbacks are context-sensitive.
 - **Overload order keeps both messages.** #1 supplies the contextual type
  (`a, b, c`); #2 accepts a partial map once a fallback is present, so its popup
  is optional (`a?, b?, c?`). A gap without a fallback is reported against #1.
@@ -57,9 +98,9 @@ Each factory is two overloads whose order is load-bearing:
 - **Single-object `_`** (the former shape). `_` sees only all of `T`; the
  remainder is not expressible there, and an exhaustive map plus `_` was
  accepted.
- **Curried handlers-first** — `(handlers)(fallback)`. `Handled` is fixed before
-  the second call, so a redundant-fallback guard would work. Rejected: two calls
-  for the common case.
+- **Curried handlers-first** — `(handlers)(fallback)`. Rejected: two calls for
+  the common case. It is not needed for the redundant-fallback guard, which the
+  F-bounded constraint already provides (see Why).
 - **`this` / HKT self-reference.** `this` is post-construction (method bodies,
  return positions); a parameter's contextual type is pre-construction.
  `keyof this` in an interface method is the interface, not the literal.
@@ -76,14 +117,195 @@ Each factory is two overloads whose order is load-bearing:

 #### Known issue

- A redundant fallback is accepted: when the handler map already covers `T`, the
-  fallback is still allowed. The guard would be
-  `Exclude<T, keyof Handled> extends never ? never : unknown`, but the
-  conditional is evaluated before `Handled` is inferred; currying is the only
-  encoding that fixes it (see Rejected).
 - `PatternReturns` must be
  `ReturnType<Extract<ValueOf<P>, (...args: never[]) => unknown>>` so it survives
  the closed, partly-optional `P` constraints.
 - `Parameters<typeof factory>[0]` resolves only the **last** overload, so it is
  not a sound "rejected" oracle for a factory. Factory-negative tests use
  `@ts-expect-error` call sites (the test file only — the general ban stands).
+
+## Shared internals
+
+#### Decision (2026-09)
+
+`src/matcher-shared.ts` holds the universe-agnostic pieces both matchers use:
+`UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`, the shared
+`Matchable` universe, the `PatternKey` key projection and its `Member` inverse,
+and the `Stringified` / `Collisions` / `UnsupportedReason` / `UnsupportedUniverse`
+/ `UniverseGate` universe gate.
+
+#### Why
+
+- `RedundantFallback`'s property name is the diagnostic, so one definition
+  keeps the two matchers' message from drifting; the other pieces appear
+  verbatim in both public signatures or are the same projection over each
+  matcher's universe.
+- **`Matchable` is one definition, not two.** The primitive-union matcher's
+  universe and the tagged-union matcher's allowed `Tag` values are the same set,
+  so aliasing them keeps the two matchers from drifting apart on what they
+  accept (`symbol`/`bigint` rejected once).
+
+#### Rejected
+
+- **A generic `Matcher<Universe>` over the interface pair, `Handlers`,
+  `Fallback` and `MustBePartial`.** Each is built from its own universe
+  (`Tags`/`MapTaggedUnion` vs the primitive values); abstracting over the
+  F-bounded `Handled` constraint that makes the remainder work risks the
+  contextual typing it exists to preserve. `Matchable`, the `PatternKey` /
+  `Member` projection and the `UniverseGate` are the pieces both universes
+  genuinely share.
+
+## Supported universes
+
+#### Decision (2026-09)
+
+A universe must be a **finite union of literals** with **no
+value/stringification collision**. Broad types (`string`, `number`, a template
+literal) and `"true" | true` / `1 | "1"` are rejected; the factory intersects
+`UniverseGate<T>` (the `UnsupportedUniverse<Reason>` diagnostic) into the
+handler and fallback parameters. `Member<T, K>` replaces the `PatternParam<K>`
+inversion: the handler parameter is the member(s) of `T` whose `PatternKey` is
+`K`, so a standalone `"true"` is `"true"`, not `true`.
+
+#### Why
+
+- **`PatternKey` is not injective.** `"true"` and `true` (and `1` / `"1"`)
+  share a runtime key, so `PatternParam<K>` cannot recover the member.
+  `Member<T, K>` inverts against `T`, which is exact.
+- **Broad types cannot be proven exhaustive.** An index-like map lets a partial
+  object satisfy the exhaustive overload and reaches the `dispatch` throw.
+  Rejecting at the boundary avoids threading an open/closed branch through
+  `Handlers`, `Fallback` and `MustBePartial`.
+- **The finite-literal predicate is `IsLiteral<PatternKey<T>> extends true`.**
+  `IsLiteral` is `boolean` for a union that mixes a literal with a broad type
+  (`"a" | \`x-${number}\``), so `extends false`would treat the mix as
+supported;`extends true` is the check that rejects it.
+- **Collisions are rejected, not merged.** `Member<T, K>` would be sound (the
+  handler gets the union), but the API is one handler per member; rejecting
+  keeps `Member` a singleton and the remainder exact.
+- **The collision predicate is type-checkable.** `Collisions<T> =
+Extract<T, Stringified<T>>` catches numeric collisions too.
+- **The gate is an intersection, not a branch,** so `R` inference and the popup
+  survive; a conditional parameter type would not.
+
+#### Rejected
+
+- **Open universes with a required fallback** (`fix/open-universe-*`): sound,
+  but left the collision hole and added an `IsLiteral` /
+  `OpenUniverseNeedsFallback` branch through every handler type. Findings, kept
+  so they are not re-run: `{}` satisfies an index signature (and `Exact` misses
+  it); an index signature dominates contextual typing; `R` infers only from a
+  non-self-referential parameter type (`{ [K in keyof H]: … R … }` gives
+  `unknown`); an F-bounded guard referencing `keyof Handled` in `Handled`'s own
+  constraint sees the constraint, not the map; all handlers share one `R` (only
+  the fallback widens it); `IsLiteral` is the finite-literal predicate. The
+  user-facing consequence — open universes are a parsing or registry concern,
+  not a dispatch one — is guidance in README § Why open universes are rejected.
+- **`Member<T, K>` without the gate:** sound, but a colliding handler gets a
+  union and `1 | "1"` stays one runtime key.
+- **A round-trip injectivity gate** (`IsEqual<T, PatternParam<PatternKey<T>>>`):
+  over-rejects standalone `"true"` / `"false"` / `"null"` / `"undefined"`.
+- **A case-list / ts-pattern builder:** removes the collision class but drops
+  the object map (footprint, popup) and reimplements an existing library.
+- **Normalize numeric keys to strings:** makes `PatternKey` injective but
+  changes "numeric keys stay numbers" and defeats the numeric dispatch fast
+  path.
+
+#### Known issue
+
+- A multi-collision universe lists every collision in the diagnostic.
+- The `dispatch` throw is unreachable through the typed API; the throw tests
+  widen the factory to `Function` to reach it.
+
+## Tagged-union matcher
+
+#### Decision (2026-09)
+
+`getTaggedUnionMatcher` / `getTaggedUnionMatcherW` mirror the primitive-union pair
+with one extra curried step for the discriminant key:
+
+```ts
+type Shape =
+    | { kind: "circle"; radius: number }
+    | { kind: "square"; side: number };
+
+const area = getTaggedUnionMatcher<Shape>()("kind")({
+    circle: (s) => Math.PI * s.radius ** 2,
+    square: (s) => s.side ** 2,
+});
+const fallback = getTaggedUnionMatcher<Shape>()("kind")(
+    { circle: (s) => … },
+    (s) => …, // s: { kind: "square"; side: number }
+);
+```
+
+The key is a separate call because `K` is inferred from its literal argument and
+`T` is fixed by the first factory; one call could not infer both.
+`Discriminated<T>` restricts the key to properties whose values are tags.
+
+#### Why
+
+- **Same fallback/remainder machinery as the primitive-union matcher.** `HandledTags`
+  recovers the tag values the map handled (`Member<Tags, keyof Handled>`) and
+  `Narrowed<T, K, Exclude<Tags, …>>` is the fallback's parameter; the
+  redundant-fallback guard is the same F-bounded constraint. Only the "universe"
+  changes — `T`'s members instead of primitive values.
+- **`T extends object`, not `Record<PropertyKey, unknown>`.** An `interface` has
+  no implicit index signature, so the `Record` constraint would reject
+  interface-based unions. The runtime reads the tag off `object` with one
+  assertion, the tagged twin of the primitive-union dispatch's `shape as string | number`.
+- **`Narrowed` distributes over `T`, narrowing `K` to the tag.** A member whose
+  `K` cannot take the tag drops out; a duplicated tag yields a union of members
+  instead of dropping one. `T[K] extends V` returns the exact member (a
+  discriminated union's declared interface) untouched, so the mapped form only
+  handles a property that is itself a union.
+- **A union-valued or optional discriminant is supported.** A single shape whose
+  property is a union (`{ color: "red" | "green" | "blue" }`) is narrowed per
+  handler instead of being passed `never`. Matching a defined tag on an optional
+  property (`{ type?: "x" }`) proves the key is present, so it becomes required
+  (`{ type: "x" }`); the `undefined` tag narrows it to `{ type?: never }` under
+  `exactOptionalPropertyTypes` (absence) or `{ type?: undefined }` when the
+  property explicitly admits `undefined`.
+- **A `boolean` / `null` / `undefined` tag goes through the shared
+  `PatternKey` / `Member` projection.** `Discriminated` admits those tags
+  (they are in `Tag`), but they cannot key a mapped type, so the handler map is
+  keyed by the stringified form (`true` → `"true"`) and `Member` inverts it
+  against the tag set to recover the member. This is the same projection the
+  primitive-union matcher uses over its universe, which is why it lives in
+  `matcher-shared.ts`.
+
+#### Known issue
+
+- A tag need not be unique across the union. Two members sharing one is not a
+  soundness hole: they select a single runtime key, so one handler receiving
+  their union is the only correct behavior — the key is simply not a
+  discriminant. The gate rejects only the distinct-value collision
+  (`true | "true"`), where two values share a key and `Member` can no longer
+  invert it; see § Supported universes. Pinned by the duplicate-tag tests in
+  `src/tagged-union.test.ts`.
+
+## Primitive universe
+
+#### Decision (2026-09)
+
+The universe (`Matchable`) is `string | number | boolean | null | undefined`,
+with `boolean` admitted as `true | false`.
+
+`boolean`/`null`/`undefined` are not property keys, so handler-map keys are a
+projection (`PatternKey`: each member stringified) and `Member` inverts it
+against the universe, so callbacks receive the real member (`true`, not
+`"true"`; the standalone string `"true"` stays `"true"`). The popup offers
+`true`, `false`, `null`, `undefined` by name (verified over LSP). The same
+projection is shared with the tagged-union matcher; see § Tagged-union matcher.
+The supported universes are constrained as described in § Supported universes.
+
+#### Why
+
+- Runtime dispatch indexes with the raw `shape`; `handlers[true]` coerces to
+  `"true"` at runtime exactly as `String` would. The `shape as string | number`
+  assertion only placates `TS2538` and buys the number fast path (an explicit
+  `String()` defeats V8's numeric-key path: measured ~2× on number-keyed
+  dispatch).
+- `symbol`/`bigint`/`NaN`/`-0` are rejected, and a member colliding with its
+  stringification is rejected by the gate: user-facing, stated once in
+  [README § Caveats](../README.md#caveats).
@@ -28,14 +28,49 @@ before the implementation.
 - Runtime-first (classic red/green): it verifies the value, not the contract,
  and the contract is the product.
 - Testing the type only: it would not catch handler dispatch or the `_`
-  fallback (see `src/primitive.test.ts`).
+  fallback (see `src/primitive-union.test.ts`).

 The runner is `node --test --strip-types "src/**/*.test.ts"` and the tiers are
 in
 [CONTRIBUTING.md § Development commands](../CONTRIBUTING.md#development-commands).
 `c8` uses V8 coverage, so the `--strip-types` source is instrumented without a
 build step, and the runner relies on the `.ts` import-extension convention (see
-[tooling.md](./tooling.md#source-imports-use-ts-extensions)).
+[tooling.md](./tooling.md#source-imports-use-ts-extensions)). CI gates that
+coverage at 100% (see [ci.md § Coverage threshold](./ci.md#coverage-threshold)).
+
+## Handler arguments
+
+#### Decision (2026-09)
+
+A handler's `expectTypeOf(shape)` is always paired with an assertion on the
+argument `dispatch` actually passed: `assert.equal` where the handler runs for
+one shape, `assert.ok(s === … || s === …)` over the set a `_` fallback accepts
+(`assert` is imported as `strict`, so each comparison is `Object.is`). Where a
+test should also prove that the _exact_ value reached the handler unchanged,
+the fallback returns the shape verbatim and the call site asserts it.
+
+#### Why
+
+- The parameter's type is what the compiler inferred from the pattern; the
+  argument is what the runtime passed. Only the second can drift, and the keys
+  whose property name differs from their value (`true`, `null`, `1`) are
+  exactly where it can — see [library.md](./library.md).
+- `String(shape)` at the call site keeps every type expectation and every
+  return-value assertion green; the argument assertions fail (10 of the 33
+  tests). Without them the suite never looks at the passed argument.
+- Returning the shape verbatim costs a widening pattern nothing: the remainder
+  type joins the union of handler returns in place of a marker literal, so the
+  test still shows the widening it is named for.
+
+#### Rejected
+
+- A recorded `unknown[]` of every fallback call compared with `deepEqual`:
+  strong, but it couples the assertion to call order, and the sink sits three
+  blocks away from the value it observes.
+- `typeof` checks: they cannot separate `2` from its key text `"2"`, which is
+  the drift a fallback with a numeric remainder can hit.
+- One expected value asserted inline in a fallback: its argument is a _set_ of
+  shapes, so only the disjunction holds on every call.

 ## AAA ordering

@@ -133,7 +168,7 @@ The script prints the labels the server offers at a `/*COMPLETE*/` marker inside
 `<file>` (the marker is stripped before the document is sent). Its `LspSession`
 is imported by `src/util/__tests__/lsp-completion.test.ts` — which tests the
 helper itself against inline documents, never the library's code — and by
-`src/primitive.test.ts`, where the same probe asserts the matcher's popup;
+`src/primitive-union.test.ts`, where the same probe asserts the matcher's popup;
 the CLI is for manual inspection.

 #### Why
@@ -164,11 +199,16 @@ the CLI is for manual inspection.
  delay is needed.
 - The server answers some requests with a string id (`client/registerCapability`);
  the client must tolerate `string | number` ids or the server stalls.
+- `LspSession.close()` sends `shutdown` and then closes stdin instead of
+  sending `exit`. The TS 7 Go server's `handleExit` returns `io.EOF`, cancelling
+  the background context while a watch update is still in flight, and logs a
+  bare `context canceled` before exiting 1; EOF on stdin exits 0 with no output.
+  The kill stays as a fallback for a server that does not exit.

 ## Known issues

- The type-aware linter misidentifies `expectTypeOf()` as a floating promise, so
-  test files that use it (`src/primitive.test.ts`) carry a file-level
-  `oxlint-disable typescript/no-floating-promises` with an explanatory comment.
-  It is a known false positive, not a rule worth disabling project-wide (see
+- The type-aware linter misidentifies `expectTypeOf()` as a floating promise.
+  It is a known false positive, so `typescript/no-floating-promises` is off for
+  `**/*.test.ts` in the `.oxlintrc.json` override rather than repeated as a
+  file-level header (see
  [tooling.md § oxlint-disable directives live next to the code](./tooling.md#oxlint-disable-directives-live-next-to-the-code)).
@@ -106,26 +106,34 @@ Type-aware oxlint is enabled via `options.typeAware: true` in `.oxlintrc.json`

 #### Decision (2026-09)

-A type-aware rule that false-positives is silenced with a source-level
-`oxlint-disable` directive (see `src/primitive.ts`,
-`src/primitive.test.ts`), not by turning the rule off in `.oxlintrc.json`.
+A type-aware rule that false-positives **at one site** is silenced with a
+source-level `oxlint-disable` directive (see `src/primitive-union.ts`). A rule that is
+wrong for a whole **file class** is turned off in a `.oxlintrc.json` `overrides`
+entry instead — e.g. `typescript/no-floating-promises` (synchronous
+`expectTypeOf` reads as an unhandled promise) and `unicorn/no-null` (intentional
+`null` inputs) for `**/*.test.ts`. The same exemption is not repeated as a
+file-level header in every affected file.

 #### Why

- The disable sits next to the code it silences, visible to anyone reading the
-  source.
- The rule stays on everywhere else, so only the mis-firing line is exempted.
+- A one-site disable sits next to the code it silences, visible to anyone
+  reading the source, and the rule stays on everywhere else.
+- A file-class rule is a property of the file class, not of one line; the
+  override states it once, where the rest of the file-class config lives.

 #### Rejected

- A project-wide disable in `.oxlintrc.json` for a false positive: it hides the
-  exemption from the reader of the affected code and switches the rule off
-  repo-wide for a one-site problem.
+- A project-wide disable in `.oxlintrc.json` for a one-site false positive: it
+  hides the exemption from the reader of the affected code and switches the rule
+  off repo-wide for a one-site problem.
+- A repeated file-level `oxlint-disable` header for a file-class false positive:
+  the copies drift and scatter one config decision across the tree.

 #### Known issue

- A source-level disable is a _human_ last resort. AI agents must not add one;
-  they fix the type at its root (see [AGENTS.md § Never do](../AGENTS.md#never-do)).
+- Both placements are _human_ last resorts. AI agents must neither add a source
+  disable nor edit `.oxlintrc.json`; they fix the type at its root (see
+  [AGENTS.md § Never do](../AGENTS.md#never-do)).

 ### Unwanted stylistic rules are turned off in the config

@@ -188,6 +196,28 @@ joining the oxfmt-superseded rules already off.
  are part of the public API.
 - The narrower scope keeps the signal high without config-file boilerplate.

+### `knip` lists `src/index.ts` as an entry
+
+#### Decision (2026-09)
+
+`knip.json` declares `"entry": ["src/index.ts", "scripts/*.ts"]`.
+
+#### Why
+
+- Supplying `entry` **replaces** knip's default entry detection, which otherwise
+  derives the public entry from `package.json` `exports`. Adding `scripts/*.ts`
+  there therefore dropped the library entry, so knip resolved the package through
+  its `dist/index.js` output and reported the unreferenced source entry file
+  `src/index.ts` as an unused file.
+- Naming the source entry restores the link between the public API and the
+  source graph without pointing knip at build output.
+
+#### Rejected
+
+- `paths` mapping `dist/index.*` back to `src/index.ts`: more config to model a
+  relation the explicit entry states directly, and it would break whenever the
+  build layout changes.
+
 ### `maintain:outdated` ignores `@types/node`

 #### Decision (2026-09)
@@ -96,7 +96,8 @@ aggregator.

 #### Why

- Both members create something real: a branch, a release.
+- Every member creates something real: a branch, a release, the compiled
+  doc-tests.
 - It joined both lists in [CONTRIBUTING.md](../CONTRIBUTING.md) alongside its
  first members, so it could not go invisible the way the retired `use:` prefix
  did.
@@ -1,5 +1,5 @@
 {
    "$schema": "./node_modules/knip/schema.json",
-    "entry": ["scripts/*.ts"],
+    "entry": ["src/index.ts", "scripts/*.ts"],
    "ignoreDependencies": ["@runwisp/pubv"]
 }
@@ -1,6 +1,6 @@
 {
    "name": "tiny-pattern-ts",
-    "version": "0.4.0",
+    "version": "0.8.1",
    "description": "Pattern matching for TypeScript/ESM environments (F#-style, not regex)",
    "keywords": [
        "adt",
@@ -28,7 +28,8 @@
    "type": "module",
    "sideEffects": false,
    "imports": {
-        "#test-utils/*": "./src/util/__tests__/*"
+        "#test-utils/*": "./src/util/__tests__/*",
+        "#test-tiny-pattern-ts": "./src/index.ts"
    },
    "exports": {
        ".": {
@@ -47,7 +48,8 @@
        "check:oxfmt": "oxfmt --check ${LEFTHOOK_FILES:-.}",
        "check:oxlint": "oxlint ${LEFTHOOK_FILES:-src scripts}",
        "check:tsc": "tsc",
-        "clean": "rm -rf dist coverage",
+        "clean": "rm -rf dist coverage src/doc-test/__generated__/*.test.ts",
+        "create:doc-tests": "node --strip-types scripts/create-doc-tests.ts && sh -c 'oxfmt src/doc-test/__generated__/*.test.ts' && tsc -p src/doc-test/tsconfig.json",
        "fix": "npm run fix:oxlint && npm run fix:oxfmt",
        "fix:oxfmt": "oxfmt ${LEFTHOOK_FILES:-.}",
        "fix:oxlint": "oxlint --fix src scripts",
@@ -58,7 +60,9 @@
        "maintain:knip": "knip --include dependencies,exports,files",
        "maintain:outdated": "check-outdated --ignore-pre-releases --ignore-packages @types/node",
        "test": "npm run check:tsc && node --test --strip-types \"src/**/*.test.ts\"",
-        "test:ci": "c8 --reporter=text --reporter=lcov --reporter=html node --test --strip-types \"src/**/*.test.ts\"",
+        "test:ci": "npm run test:coverage && npm run test:doc",
+        "test:coverage": "c8 --all --include \"src/**/*.ts\" --reporter=text --reporter=lcov --reporter=html --100 node --test --strip-types $(git ls-files 'src/*.test.ts')",
+        "test:doc": "node --test --strip-types \"src/doc-test/__generated__/*.test.ts\"",
        "test:unit": "node --test --strip-types \"src/**/*.test.ts\"",
        "verify": "npm run check && npm run test:unit",
        "watch": "npm run watch:test",
@@ -83,7 +87,8 @@
        "expect-type": "1.4.0",
        "knip": "^6.34.0",
        "lefthook": "^2.1.12",
-        "oxfmt": "^0.68.0",
+        "mdast-util-from-markdown": "^2.0.3",
+        "oxfmt": "^0.70.0",
        "oxlint": "^1.83.0",
        "oxlint-tsgolint": "^7.0.2001",
        "publint": "^0.3.24",
@@ -0,0 +1,445 @@
+import fs from "node:fs";
+import path from "node:path";
+
+import type { Code, Heading, Paragraph, PhrasingContent, Root } from "mdast";
+import { fromMarkdown } from "mdast-util-from-markdown";
+
+/**
+ * Compile the TypeScript examples embedded in the prose docs into runnable
+ * tests, so a doc fence that drifts from the API fails CI. Every ```ts fence in
+ * the scanned Markdown becomes a `test(...)` in a generated
+ * `src/doc-test/__generated__/<Name>.test.ts`; the file is typechecked by
+ * `tsc` and executed by `node --test` exactly like a hand-written test.
+ *
+ * Usage: node --strip-types scripts/create-doc-tests.ts
+ *
+ * Why regenerate-then-typecheck instead of a lint plugin: development/docs.md §
+ * Validating Markdown code fences. The repo is TypeScript 7 (the native
+ * compiler), which does not expose the legacy JS compiler API, so the only
+ * faithful check is to emit real `.ts` files and let the existing `check:tsc` /
+ * `node --test` pipeline judge them.
+ */
+
+/** The package's public entry, as spelled inside the examples. */
+const LIBRARY_SOURCE = "tiny-pattern-ts";
+
+/** Matches the library specifier (single- or double-quoted) inside an import. */
+const LIBRARY_SPECIFIER = new RegExp(
+    `(?<quote>["'])${LIBRARY_SOURCE}\\k<quote>`,
+    "g",
+);
+
+/**
+ * A source specifier resolved to `src/index.ts` by `package.json#imports`.
+ * Examples import `from "tiny-pattern-ts"`, which neither `node` nor `tsc`
+ * resolves to source before `dist/` exists, so the generator rewrites it.
+ */
+const PRELUDE_IMPORT_SOURCE = "#test-tiny-pattern-ts";
+
+/** Markdown files to scan, relative to the repo root. */
+const SOURCES = ["README.md", "CONTRIBUTING.md"];
+
+/** Directory the generated tests are written to (gitignored contents). */
+const OUTPUT_DIR = "src/doc-test/__generated__";
+
+/** Fences whose info-string matches one of these are compiled; others skipped. */
+const TYPESCRIPT_LANGS: ReadonlySet<string> = new Set(["ts", "typescript"]);
+
+/**
+ * Modules the generated file already imports. An example that imports one of
+ * these would collide with the prelude binding (duplicate `test` / `assert`), so
+ * it is surfaced as a fatal error and the example is rewritten.
+ */
+const PRELUDE_MODULES: ReadonlySet<string> = new Set([
+    "node:assert",
+    "node:test",
+]);
+
+/** One line of the prelude every generated file starts with. */
+const PRELUDE_TEST = 'import { test } from "node:test";';
+const PRELUDE_ASSERT = 'import { strict as assert } from "node:assert";';
+
+/** Indentation applied to every fence body line inside the `test` callback. */
+const INDENT = "    ";
+
+const EMPTY = "";
+const INITIAL_COUNT = 0;
+const WRITE_INCREMENT = 1;
+const FIRST_ITEM = 0;
+const FAILURE_EXIT_CODE = 1;
+
+/** Strip all leading blank lines so the import scan starts at real content. */
+const LEADING_BLANK_LINES = /^(?:[ \t]*\r?\n)+/;
+/** Collapse the plain text of a title paragraph to one line. */
+const WHITESPACE = /\s+/g;
+/** Split an info-string like `ts title="x"` into its bare language. */
+const LANG_SEPARATOR = /\s+/;
+
+/** One leading `import` statement, including a multi-line named import. */
+const IMPORT_STATEMENT =
+    /^import\s+(?:(?:type\s+)?[\w$*{}\s,]+?\s+from\s+)?["'][^"'\n]+["']\s*;?[ \t]*(?:\r?\n|$)/;
+
+/** `import { a, b } from "src";`, with an optional `type` keyword. */
+const NAMED_IMPORT =
+    /^import\s+(?<isType>type\s+)?\{(?<specifiers>[^}]*)\}\s+from\s+["'](?<source>[^"']+)["']\s*;?$/;
+
+/** The module in a `… from "src"` statement. */
+const FROM_SOURCE = /from\s+["'](?<source>[^"']+)["']/;
+
+/** The module in a side-effect `import "src"` statement. */
+const SIDE_EFFECT_SOURCE = /^import\s+["'](?<source>[^"']+)["']/;
+
+type Block = Root["children"][number];
+
+class DocTestError extends Error {
+    public constructor(message: string) {
+        super(message);
+        this.name = "DocTestError";
+    }
+}
+
+/** Merge named specifiers that target the same module into one import. */
+interface NamedImportGroup {
+    readonly source: string;
+    readonly isType: boolean;
+    readonly names: string[];
+}
+
+/** One compiled fence, ready to be wrapped in a `test(...)`. */
+interface DocCase {
+    readonly title: string;
+    readonly body: string;
+}
+
+/** Accumulator threaded through the Markdown walk. */
+interface BuilderState {
+    readonly named: Map<string, NamedImportGroup>;
+    readonly passthrough: Set<string>;
+    readonly cases: DocCase[];
+    title: string | undefined;
+}
+
+/** Read one capture group, tolerating the type-level `groups` optionality. */
+const groupOf = (match: RegExpExecArray, name: string): string | undefined => {
+    const { groups } = match;
+    return groups === undefined ? undefined : groups[name];
+};
+
+/** Recursively concatenate the literal text of a single inline node. */
+const inlineText = (node: PhrasingContent): string => {
+    if ("value" in node) {
+        return node.value;
+    }
+    if ("children" in node) {
+        return node.children.map(inlineText).join(EMPTY);
+    }
+    return EMPTY;
+};
+
+/** Render a paragraph/heading node to its plain text for use as a test title. */
+const textOf = (node: Paragraph | Heading): string =>
+    node.children.map(inlineText).join(EMPTY).replace(WHITESPACE, " ").trim();
+
+/**
+ * Split a fence into its leading `import` statements and the executable body.
+ * Only a contiguous run of imports at the very top is hoisted; anything after
+ * the first non-import line stays in the body verbatim.
+ */
+const splitImports = (code: string): { imports: string[]; body: string } => {
+    const imports: string[] = [];
+    let rest = code.replace(LEADING_BLANK_LINES, EMPTY);
+    let match = IMPORT_STATEMENT.exec(rest);
+    while (rest.startsWith("import") && match !== null) {
+        const statement = match[FIRST_ITEM] ?? EMPTY;
+        imports.push(statement.trim());
+        rest = rest.slice(statement.length).replace(LEADING_BLANK_LINES, EMPTY);
+        match = IMPORT_STATEMENT.exec(rest);
+    }
+    return { imports, body: rest.trim() };
+};
+
+/** The module an import statement points at, or `undefined` if unreadable. */
+const importSource = (statement: string): string | undefined => {
+    const fromMatch = FROM_SOURCE.exec(statement);
+    if (fromMatch !== null) {
+        return groupOf(fromMatch, "source");
+    }
+    const sideEffectMatch = SIDE_EFFECT_SOURCE.exec(statement);
+    return sideEffectMatch === null
+        ? undefined
+        : groupOf(sideEffectMatch, "source");
+};
+
+/**
+ * Reject an example that imports a harness-provided module, then rewrite the
+ * library specifier to the source alias so the emitted file resolves.
+ */
+const normalizeImport = (statement: string): string => {
+    const source = importSource(statement);
+    if (source !== undefined && PRELUDE_MODULES.has(source)) {
+        throw new DocTestError(
+            `example imports "${source}", which the harness injects; remove the line:\n  ${statement.trim()}`,
+        );
+    }
+    return statement.replace(
+        LIBRARY_SPECIFIER,
+        `$<quote>${PRELUDE_IMPORT_SOURCE}$<quote>`,
+    );
+};
+
+/** Split the `{ a, b }` contents of a named import into trimmed specifiers. */
+const specifiersOf = (raw: string): string[] =>
+    raw
+        .split(",")
+        .map((name) => name.trim())
+        .filter((name) => name.length > INITIAL_COUNT);
+
+/** Add named specifiers to the group for `(isType, source)`, merging. */
+const addNamedImport = (
+    named: Map<string, NamedImportGroup>,
+    ref: Pick<NamedImportGroup, "source" | "isType">,
+    names: readonly string[],
+): void => {
+    const key = `${ref.isType ? "type:" : "value:"}${ref.source}`;
+    const existing = named.get(key);
+    if (existing === undefined) {
+        named.set(key, {
+            source: ref.source,
+            isType: ref.isType,
+            names: [...names],
+        });
+        return;
+    }
+    for (const name of names) {
+        if (!existing.names.includes(name)) {
+            existing.names.push(name);
+        }
+    }
+};
+
+/** Sort each hoisted import into a merged named group or a passthrough set. */
+const collectImports = (
+    statements: readonly string[],
+    named: Map<string, NamedImportGroup>,
+    passthrough: Set<string>,
+): void => {
+    for (const raw of statements) {
+        const statement = normalizeImport(raw);
+        const match = NAMED_IMPORT.exec(statement);
+        if (match === null) {
+            passthrough.add(statement);
+        } else {
+            addNamedImport(
+                named,
+                {
+                    source: groupOf(match, "source") ?? EMPTY,
+                    isType: groupOf(match, "isType") !== undefined,
+                },
+                specifiersOf(groupOf(match, "specifiers") ?? EMPTY),
+            );
+        }
+    }
+};
+
+/** Render the hoisted imports: merged named groups first, then the rest. */
+const renderImports = (
+    named: ReadonlyMap<string, NamedImportGroup>,
+    passthrough: ReadonlySet<string>,
+): string[] => {
+    const lines: string[] = [];
+    for (const group of named.values()) {
+        const keyword = group.isType ? "import type" : "import";
+        lines.push(
+            `${keyword} { ${group.names.join(", ")} } from "${group.source}";`,
+        );
+    }
+    for (const statement of passthrough) {
+        lines.push(statement);
+    }
+    return lines;
+};
+
+/** The bare language of a fence's info-string, e.g. `ts` in `ts title="x"`. */
+const typescriptLang = (code: Code): string =>
+    (code.lang ?? EMPTY).trim().split(LANG_SEPARATOR)[FIRST_ITEM] ?? EMPTY;
+
+/** Set the title a following fence will inherit. */
+const handleTitleNode = (
+    state: BuilderState,
+    node: Paragraph | Heading,
+): void => {
+    state.title = textOf(node);
+};
+
+/** Report and reset a non-TS fence that was skipped. */
+const skipFence = (state: BuilderState, name: string, code: Code): void => {
+    process.stderr.write(
+        `${name}: skipped non-TypeScript fence (lang="${code.lang ?? EMPTY}")\n`,
+    );
+    state.title = undefined;
+};
+
+/** Compile a described TS fence into a case, or reject it. */
+const compileFence = (
+    state: BuilderState,
+    name: string,
+    code: Code,
+): DocCase => {
+    const { title } = state;
+    if (title === undefined) {
+        const lang = typescriptLang(code);
+        throw new DocTestError(
+            `${name}: a \`\`\`${lang} fence has no preceding paragraph or ` +
+                `heading to use as its test title — describe the example.`,
+        );
+    }
+    const { imports, body } = splitImports(code.value);
+    collectImports(imports, state.named, state.passthrough);
+    return { title, body };
+};
+
+/** Compile one TS fence into a case, or reject/skip it. */
+const handleCodeNode = (
+    state: BuilderState,
+    name: string,
+    code: Code,
+): void => {
+    const lang = typescriptLang(code);
+    if (!TYPESCRIPT_LANGS.has(lang)) {
+        skipFence(state, name, code);
+        return;
+    }
+    state.cases.push(compileFence(state, name, code));
+};
+
+/** Route one Markdown block, preserving the "described fence" invariant. */
+const handleNode = (state: BuilderState, name: string, node: Block): void => {
+    if (node.type === "paragraph" || node.type === "heading") {
+        handleTitleNode(state, node);
+    } else if (node.type === "code") {
+        handleCodeNode(state, name, node);
+    } else {
+        // Only a paragraph/heading introduces a fence; any other block breaks
+        // the "immediately preceded" chain.
+        state.title = undefined;
+    }
+};
+
+/** Walk one Markdown file and collect its cases and hoisted imports. */
+const parseDoc = (
+    name: string,
+    markdown: string,
+): Omit<BuilderState, "title"> => {
+    const state: BuilderState = {
+        named: new Map(),
+        passthrough: new Set(),
+        cases: [],
+        title: undefined,
+    };
+    for (const node of fromMarkdown(markdown).children) {
+        handleNode(state, name, node);
+    }
+    return {
+        named: state.named,
+        passthrough: state.passthrough,
+        cases: state.cases,
+    };
+};
+
+/** Indent a fence body one level for the body of the `test` callback. */
+const indentBody = (body: string): string =>
+    body
+        .split("\n")
+        .map((line) =>
+            line.length > INITIAL_COUNT ? `${INDENT}${line}` : line,
+        )
+        .join("\n");
+
+/** Wrap one compiled fence in an executed `test(...)`. */
+const renderCase = (docCase: DocCase): string => {
+    const indented = indentBody(docCase.body);
+    return `test(${JSON.stringify(docCase.title)}, () => {\n${indented}\n});`;
+};
+
+/** Build the prelude + hoisted imports that top every generated file. */
+const buildHeader = (name: string, imports: readonly string[]): string[] => {
+    const header = [
+        "// Generated by scripts/create-doc-tests.ts — do not edit by hand.",
+        `// Source: ${name}`,
+        EMPTY,
+        PRELUDE_TEST,
+        PRELUDE_ASSERT,
+    ];
+    if (imports.length > INITIAL_COUNT) {
+        header.push(EMPTY, ...imports);
+    }
+    header.push(EMPTY);
+    return header;
+};
+
+/** Turn one Markdown file into the source of its generated test file. */
+const buildTestFile = (name: string, markdown: string): string => {
+    const parsed = parseDoc(name, markdown);
+    if (parsed.cases.length === INITIAL_COUNT) {
+        return EMPTY;
+    }
+    const imports = renderImports(parsed.named, parsed.passthrough);
+    const header = buildHeader(name, imports);
+    const blocks = parsed.cases.map(renderCase);
+    return `${header.join("\n")}\n${blocks.join("\n\n")}\n`;
+};
+
+/** Map a source Markdown path to its generated test path under OUTPUT_DIR. */
+const outputFor = (source: string): string =>
+    path.join(
+        OUTPUT_DIR,
+        `${path.basename(source, path.extname(source))}.test.ts`,
+    );
+
+/** Write a generated file when there is content, else clear a stale one. */
+const writeIfAny = (dest: string, name: string, out: string): boolean => {
+    if (out === EMPTY) {
+        process.stderr.write(`${name}: no TypeScript fences\n`);
+        fs.rmSync(dest, { force: true });
+        return false;
+    }
+    fs.writeFileSync(dest, out);
+    return true;
+};
+
+/** Generate the doc-test for one source; return whether a file was written. */
+const generateFor = (root: string, source: string): boolean => {
+    const abs = path.join(root, source);
+    if (!fs.existsSync(abs)) {
+        process.stderr.write(`skipped missing ${source}\n`);
+        return false;
+    }
+    const name = path.basename(source);
+    const out = buildTestFile(name, fs.readFileSync(abs, "utf8"));
+    return writeIfAny(path.join(root, outputFor(source)), name, out);
+};
+
+const main = (): void => {
+    const root = process.cwd();
+    fs.mkdirSync(path.join(root, OUTPUT_DIR), { recursive: true });
+    let written = INITIAL_COUNT;
+    for (const source of SOURCES) {
+        if (generateFor(root, source)) {
+            written += WRITE_INCREMENT;
+        }
+    }
+    process.stdout.write(
+        `created doc-tests for ${written} file(s) under ${OUTPUT_DIR}/\n`,
+    );
+};
+
+try {
+    main();
+} catch (error) {
+    if (error instanceof DocTestError) {
+        process.stderr.write(`${error.message}\n`);
+        process.exitCode = FAILURE_EXIT_CODE;
+    } else {
+        throw error;
+    }
+}
@@ -0,0 +1,12 @@
+# Generated doc-tests
+
+`__generated__/` is the output of [`scripts/create-doc-tests.ts`](../../scripts/create-doc-tests.ts):
+each `*.test.ts` is compiled from the ```ts fences in a prose doc and is
+**gitignored**, not edited by hand. `npm run create:doc-tests` regenerates them,
+formats them with `oxfmt`, then type-checks them with
+[`tsconfig.json`](./tsconfig.json) — the scoped config that relaxes
+`noUnusedLocals` so example-only locals (and the injected `assert`) compile.
+
+CI regenerates them in an explicit step before `test:ci`; the fast local `test` /
+`verify` tiers do not. See [development/docs.md](../../development/docs.md) for why the
+generator is shaped this way.
@@ -0,0 +1,10 @@
+{
+    "extends": "../../tsconfig.json",
+    "compilerOptions": {
+        "noEmit": true,
+        "noUnusedLocals": false,
+        "noUnusedParameters": false
+    },
+    "include": ["__generated__/**/*.test.ts"],
+    "exclude": []
+}
@@ -0,0 +1,27 @@
+import { strict as assert } from "node:assert";
+import { test } from "node:test";
+
+import { expectTypeOf } from "expect-type";
+
+import {
+    getPrimitiveUnionMatcher,
+    getPrimitiveUnionMatcherW,
+    getTaggedUnionMatcher,
+    getTaggedUnionMatcherW,
+} from "./index.ts";
+
+// The published entry point is the barrel (`package.json` exports
+// `./dist/index.js`), so every factory must be reachable from here. Importing it
+// also loads the module, which is what lets c8's `--all` measure it — see
+// development/ci.md § Coverage threshold.
+test("index: the public entry point re-exports every matcher factory", () => {
+    // Assert
+    expectTypeOf(getPrimitiveUnionMatcher).toBeFunction();
+    assert.equal(typeof getPrimitiveUnionMatcher, "function");
+    expectTypeOf(getPrimitiveUnionMatcherW).toBeFunction();
+    assert.equal(typeof getPrimitiveUnionMatcherW, "function");
+    expectTypeOf(getTaggedUnionMatcher).toBeFunction();
+    assert.equal(typeof getTaggedUnionMatcher, "function");
+    expectTypeOf(getTaggedUnionMatcherW).toBeFunction();
+    assert.equal(typeof getTaggedUnionMatcherW, "function");
+});
@@ -1 +1,16 @@
-export { getMatcher, getMatcherW } from "./primitive.ts";
+/**
+ * The public entry point of `tiny-pattern-ts`.
+ *
+ * Exports the four matcher factories and nothing else; the builder types they
+ * return and the rest of `src/` are implementation detail.
+ *
+ * @module
+ */
+export {
+    getPrimitiveUnionMatcher,
+    getPrimitiveUnionMatcherW,
+} from "./primitive-union.ts";
+export {
+    getTaggedUnionMatcher,
+    getTaggedUnionMatcherW,
+} from "./tagged-union.ts";
@@ -0,0 +1,102 @@
+/* c8 ignore start -- types only: the module has no runtime image to cover */
+import type { IsLiteral, IsNever, ValueOf } from "type-fest";
+
+// The primitive-union and tagged-union matchers differ in their universe, but the
+// handler/fallback plumbing is identical; these are the shared pieces. The
+// boundary is deliberate: `Handlers`, `Fallback` and `MustBePartial` stay with
+// each matcher because they are built from its universe. See development/library.md.
+
+// A handler: one universe member in, one return value out.
+export type UnaryFn<T, R> = (shape: T) => R;
+
+// The primitive universe a matcher can discriminate. The primitive-union matcher
+// uses it directly; the tagged-union matcher uses it as the set of allowed
+// discriminant (`Tag`) values. `boolean` is admitted as the pair `true | false`;
+// see README § Caveats for the unsupported members.
+export type Matchable = string | number | boolean | null | undefined;
+
+// `boolean`, `null` and `undefined` cannot be property keys, so a mapped type
+// over a universe that includes one keys each such member by its
+// stringification. `Member` inverts that projection against the universe, so a
+// handler callback still receives the *real* member (`true`, not `"true"`).
+// Both matchers use the projection: the primitive-union matcher over its
+// universe, the tagged-union matcher over a discriminant property's values. See
+// README § Caveats for the limits.
+export type PatternKey<T> = T extends boolean
+    ? T extends true
+        ? "true"
+        : "false"
+    : T extends null
+      ? "null"
+      : T extends undefined
+        ? "undefined"
+        : T;
+
+// The member(s) of `T` whose `PatternKey` is `K`: the universe-keyed inverse of
+// `PatternKey`. The key alone cannot recover the member (`"true"` and `true`
+// share it), so the handler parameter is derived from `T` instead. For a
+// supported (injective) universe the result is a single member.
+export type Member<
+    T extends Matchable,
+    K extends PropertyKey,
+> = T extends Matchable ? (PatternKey<T> extends K ? T : never) : never;
+
+// The property key a `Matchable` member takes at runtime: booleans, `null` and
+// `undefined` stringify, and a numeric literal becomes its decimal string.
+export type Stringified<T> = T extends boolean
+    ? T extends true
+        ? "true"
+        : "false"
+    : T extends null
+      ? "null"
+      : T extends undefined
+        ? "undefined"
+        : T extends number
+          ? `${T}`
+          : never;
+
+// The members of `T` that are also the stringification of another member, so
+// `PatternKey` cannot invert them. `never` means the universe is injective.
+export type Collisions<T> = Extract<T, Stringified<T>>;
+
+// Why `T` is not a supported universe, or `never` when it is. A supported
+// universe is a finite union of literals with no value/stringification
+// collision: only then can `PatternKey` be inverted unambiguously.
+export type UnsupportedReason<T extends Matchable> =
+    IsLiteral<PatternKey<T>> extends true
+        ? [Collisions<T>] extends [never]
+            ? never
+            : `a value and its stringification collide. Value is "${Collisions<T> & string}"`
+        : "broad types like string, number and template literals are not supported";
+
+// The diagnostic for an unsupported universe. Extends `HandlerMap` so the
+// implementation's `handlers: HandlerMap` stays assignable when the gate is
+// intersected into a parameter; the property name is the message.
+export type UnsupportedUniverse<Reason extends string> = HandlerMap &
+    Readonly<Record<`unsupported universe: ${Reason}`, never>>;
+
+// `unknown` for a supported universe (an intersection no-op), the diagnostic
+// otherwise. Intersecting rather than branching keeps `R` inference intact.
+export type UniverseGate<T extends Matchable> =
+    IsNever<UnsupportedReason<T>> extends true
+        ? unknown
+        : UnsupportedUniverse<UnsupportedReason<T>>;
+
+// `Extract` drops optional handlers (`undefined`) so `PatternReturns` also works
+// when `P`'s constraint has optional keys.
+export type PatternReturns<P> = ReturnType<
+    Extract<ValueOf<P>, (...args: never[]) => unknown>
+>;
+
+// The diagnostic raised when a fallback is supplied for an already-exhaustive
+// handler map. Each matcher's `MustBePartial` folds it into `Handled`'s
+// constraint so the guard is checked after inference.
+export interface RedundantFallback {
+    readonly "every case is already handled, so the fallback is redundant": never;
+}
+
+// The runtime dispatch map the handler maps and fallback erase to.
+export type HandlerMap = Record<
+    string | number,
+    UnaryFn<never, unknown> | undefined
+>;
@@ -0,0 +1,145 @@
+import type { Exact } from "type-fest";
+
+import type {
+    HandlerMap,
+    Matchable,
+    Member,
+    PatternKey,
+    PatternReturns,
+    RedundantFallback,
+    UnaryFn,
+    UniverseGate,
+} from "./matcher-shared.ts";
+
+type Handlers<T extends Matchable, R> = {
+    [K in PatternKey<T>]: UnaryFn<Member<T, K>, R>;
+};
+
+// The fallback is a *second argument*, not a property of the handler map,
+// because its parameter is the remainder `Exclude<T, Member<T, keyof Handled>>` and TypeScript
+// fixes a property's contextual type before it infers its sibling keys. A later
+// argument, by contrast, is contextually typed from inference on an earlier
+// one, so the split is what makes the remainder expressible at all.
+// See development/library.md.
+type Fallback<T extends Matchable, Handled, R> = UnaryFn<
+    Exclude<T, Member<T, keyof Handled>>,
+    R
+>;
+
+// A fallback is redundant once the handler map covers `T`. The guard is folded
+// into `Handled`'s own (self-referential) constraint so it is checked *after*
+// inference; a conditional in the fallback's parameter type is evaluated while
+// `Handled` is still its constraint and would reject context-sensitive partial
+// maps. That placement also fixes where the diagnostic lands: the constraint
+// failure is reported on the argument that inferred `Handled` (the handler
+// map), so the required property is spelled as the message instead of relying
+// on its position. See development/library.md.
+type MustBePartial<T extends Matchable, Handled> =
+    PatternKey<T> extends keyof Handled ? RedundantFallback : unknown;
+
+// TypeScript does not apply the excess-property check to a generic constraint,
+// so `Exact` restores it for the generic forms: a handler map can otherwise
+// carry keys outside `T`.
+
+// Strict returns: one common `R`. Overload order is load-bearing:
+//   #1 Handlers (first) -> the exhaustive form and the autocomplete popup
+//   #2 Fallback (last)  -> accepts a partial handler map plus a fallback
+interface PrimitiveUnionMatcherStrict<T extends Matchable> {
+    <R>(handlers: Handlers<T, R> & UniverseGate<T>): UnaryFn<T, R>;
+    <
+        R,
+        Handled extends Exact<Partial<Handlers<T, R>>, Handled> &
+            MustBePartial<T, Handled>,
+    >(
+        handlers: Handled & Partial<Handlers<T, R>> & UniverseGate<T>,
+        fallback: Fallback<T, Handled, R> & UniverseGate<T>,
+    ): UnaryFn<T, R>;
+}
+
+// Widened returns: the union of every handler's return type. `P` is inferred
+// from the whole handler map, whose closed constraint supplies the
+// contextual/autocomplete type.
+interface PrimitiveUnionMatcherWidening<T extends Matchable> {
+    <P extends Exact<Handlers<T, unknown>, P>>(
+        handlers: P & UniverseGate<T>,
+    ): UnaryFn<T, PatternReturns<P>>;
+    <
+        R,
+        Handled extends Exact<Partial<Handlers<T, unknown>>, Handled> &
+            MustBePartial<T, Handled>,
+    >(
+        handlers: Handled & UniverseGate<T>,
+        fallback: Fallback<T, Handled, R> & UniverseGate<T>,
+    ): UnaryFn<T, PatternReturns<Handled> | R>;
+}
+
+const dispatch =
+    (handlers: HandlerMap, fallback?: UnaryFn<never, unknown>) =>
+    (shape: Matchable): unknown =>
+        // `handlers[true]` already coerces to the `"true"` property at runtime,
+        // identical to `handlers[String(shape)]`, so indexing with `shape`
+        // directly is sound: `shape` is a facade-checked universe member and
+        // `PatternKey` only ever produces valid property keys. The assertion is
+        // needed solely because TypeScript forbids indexing with
+        // `boolean`/`null`/`undefined` (TS2538); it buys the number fast path.
+        (
+            handlers[
+                // oxlint-disable-next-line typescript/no-unsafe-type-assertion
+                shape as string | number
+            ] ??
+            fallback ??
+            (() => {
+                throw new Error(`Unhandled shape: ${String(shape)}`);
+            })
+        )(
+            // oxlint-disable-next-line typescript/no-unsafe-type-assertion
+            shape as never,
+        );
+
+/**
+ * Create a matcher for a finite primitive universe, with one common return
+ * type.
+ *
+ * Use it when the value itself is the union (`"yes" | "no"`) and every handler
+ * returns the same type.
+ *
+ * The returned builder takes a handler map keyed by the members; supplying a
+ * second fallback argument allows a partial map and receives the unhandled
+ * remainder.
+ *
+ * @typeParam T - The finite universe of primitive members to match.
+ * @returns A builder for the handler map, or the handler map plus a fallback.
+ * @example
+ *     const matchAnswer = getPrimitiveUnionMatcher<"yes" | "no">();
+ *     const describe = matchAnswer({
+ *         yes: () => "agreed",
+ *         no: () => "declined",
+ *     });
+ *     describe("yes"); // "agreed"
+ */
+export const getPrimitiveUnionMatcher = <
+    T extends Matchable,
+>(): PrimitiveUnionMatcherStrict<T> => dispatch;
+
+/**
+ * Create a matcher for a finite primitive universe whose return type is the
+ * union of every handler's return type.
+ *
+ * Use it when the value itself is the union (`"yes" | "no"`) and the handlers
+ * return different types. The `W` (widening) counterpart of
+ * {@link getPrimitiveUnionMatcher}; the universe constraint and the optional
+ * fallback are identical.
+ *
+ * @typeParam T - The finite universe of primitive members to match.
+ * @returns A builder for the handler map, or the handler map plus a fallback.
+ * @example
+ *     const matchReply = getPrimitiveUnionMatcherW<"yes" | "no">();
+ *     const reply = matchReply({
+ *         yes: () => 1,
+ *         no: () => "declined",
+ *     });
+ *     // reply: (shape: "yes" | "no") => number | string
+ */
+export const getPrimitiveUnionMatcherW = <
+    T extends Matchable,
+>(): PrimitiveUnionMatcherWidening<T> => dispatch;
@@ -1,491 +0,0 @@
-/* oxlint-disable typescript/no-floating-promises -- expectTypeOf() is a sync type-assertion library that the type-aware linter misidentifies as a promise */
-import { strict as assert } from "node:assert";
-import path from "node:path";
-import { test } from "node:test";
-
-import { expectTypeOf } from "expect-type";
-
-import {
-    type CompletionTarget,
-    LspSession,
-} from "#test-utils/lsp-completion.ts";
-
-import { getMatcher, getMatcherW } from "./primitive.ts";
-
-// ============================================================================
-// API: getMatcher — ✔️ Exhaustive / ✔️ ReturnsStrict
-// ============================================================================
-
-test("getMatcher: exhaustive pattern infers one common return type", () => {
-    // Arrange
-    const factory = getMatcher<"a" | "b">();
-
-    // Act
-    const matcher = factory({
-        a: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"a">();
-            return 1;
-        },
-        b: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"b">();
-            return 2;
-        },
-    });
-
-    // Assert
-    // ✔️ ReturnsStrict: R is the best common return type, not a widening union.
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b") => number>();
-    // ❌ a value outside T must not be accepted by the matcher
-    expectTypeOf<"c">().not.toExtend<Parameters<typeof matcher>[0]>();
-    assert.equal(matcher("a"), 1);
-    assert.equal(matcher("b"), 2);
-});
-
-test("getMatcher: dispatches on numeric literal keys", () => {
-    // Arrange
-    const factory = getMatcher<1 | 2>();
-
-    // Act
-    const matcher = factory({
-        1: (n) => {
-            expectTypeOf(n).toEqualTypeOf<1>();
-            return n + 1;
-        },
-        2: (n) => {
-            expectTypeOf(n).toEqualTypeOf<2>();
-            return n * 10;
-        },
-    });
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<(shape: 1 | 2) => number>();
-    assert.equal(matcher(1), 2);
-    assert.equal(matcher(2), 20);
-});
-
-test("getMatcher: dispatches on mixed string and numeric keys", () => {
-    // Arrange
-    const factory = getMatcher<"a" | "b" | 1 | 2>();
-
-    // Act
-    const matcher = factory({
-        a: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"a">();
-            return 1;
-        },
-        b: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"b">();
-            return 2;
-        },
-        1: (n) => {
-            expectTypeOf(n).toEqualTypeOf<1>();
-            return 10;
-        },
-        2: (n) => {
-            expectTypeOf(n).toEqualTypeOf<2>();
-            return 20;
-        },
-    });
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b" | 1 | 2) => number>();
-    assert.equal(matcher("a"), 1);
-    assert.equal(matcher("b"), 2);
-    assert.equal(matcher(1), 10);
-    assert.equal(matcher(2), 20);
-});
-
-// ============================================================================
-// API: getMatcher — ❌ Exhaustive (fallback) / ✔️ ReturnsStrict
-// ============================================================================
-
-test("getMatcher: a `_` fallback receives the unhandled keys", () => {
-    // Arrange
-    const factory = getMatcher<"a" | "b" | "c">();
-
-    // Act
-    const matcher = factory(
-        {
-            a: (s) => {
-                expectTypeOf(s).toEqualTypeOf<"a">();
-                return 1 as const;
-            },
-        },
-        (s) => {
-            // The fallback sees only the keys `a` did not handle.
-            expectTypeOf(s).toEqualTypeOf<"b" | "c">();
-            return 2 as const;
-        },
-    );
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b" | "c") => 1 | 2>();
-    // ❌ a value outside T must not be accepted by the matcher
-    expectTypeOf<"d">().not.toExtend<Parameters<typeof matcher>[0]>();
-    assert.equal(matcher("a"), 1);
-    assert.equal(matcher("b"), 2);
-    assert.equal(matcher("c"), 2);
-});
-
-test("getMatcher: a `_` fallback routes mixed string and numeric gaps", () => {
-    // Arrange
-    const factory = getMatcher<"a" | "b" | 1 | 2>();
-
-    // Act
-    const matcher = factory(
-        {
-            a: (s) => {
-                expectTypeOf(s).toEqualTypeOf<"a">();
-                return "A";
-            },
-            1: (n) => {
-                expectTypeOf(n).toEqualTypeOf<1>();
-                return "one";
-            },
-        },
-        (s) => {
-            expectTypeOf(s).toEqualTypeOf<"b" | 2>();
-            return "fallback";
-        },
-    );
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "a" | "b" | 1 | 2) => string>();
-    assert.equal(matcher("a"), "A");
-    assert.equal(matcher("b"), "fallback");
-    assert.equal(matcher(1), "one");
-    assert.equal(matcher(2), "fallback");
-});
-
-// ============================================================================
-// API: getMatcherW — ✔️ Exhaustive / ❌ ReturnsStrict
-// ============================================================================
-
-test("getMatcherW: exhaustive pattern widens to the union of handler returns", () => {
-    // Arrange
-    const factory = getMatcherW<"x" | "y">();
-
-    // Act
-    const matcher = factory({
-        x: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"x">();
-            return 1 as const;
-        },
-        y: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"y">();
-            return "two" as const;
-        },
-    });
-
-    // Assert
-    // ❌ ReturnsStrict: mixed handler returns widen to their union.
-    expectTypeOf(matcher).toEqualTypeOf<(shape: "x" | "y") => 1 | "two">();
-    // ❌ a value outside T must not be accepted by the matcher
-    expectTypeOf<"z">().not.toExtend<Parameters<typeof matcher>[0]>();
-    assert.equal(matcher("x"), 1);
-    assert.equal(matcher("y"), "two");
-});
-
-test("getMatcherW: dispatches on mixed string and numeric keys", () => {
-    // Arrange
-    const factory = getMatcherW<"a" | "b" | 1 | 2>();
-
-    // Act
-    const matcher = factory({
-        a: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"a">();
-            return "A" as const;
-        },
-        b: (s) => {
-            expectTypeOf(s).toEqualTypeOf<"b">();
-            return "B" as const;
-        },
-        1: (n) => {
-            expectTypeOf(n).toEqualTypeOf<1>();
-            return 10 as const;
-        },
-        2: (n) => {
-            expectTypeOf(n).toEqualTypeOf<2>();
-            return 20 as const;
-        },
-    });
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<
-        (shape: "a" | "b" | 1 | 2) => "A" | "B" | 10 | 20
-    >();
-    assert.equal(matcher("a"), "A");
-    assert.equal(matcher("b"), "B");
-    assert.equal(matcher(1), 10);
-    assert.equal(matcher(2), 20);
-});
-
-// ============================================================================
-// API: getMatcherW — ❌ Exhaustive (fallback) / ❌ ReturnsStrict
-// ============================================================================
-
-test("getMatcherW: a `_` fallback widens gaps into the union", () => {
-    // Arrange
-    const factory = getMatcherW<"x" | "y" | "z">();
-
-    // Act
-    const matcher = factory(
-        {
-            x: (s) => {
-                expectTypeOf(s).toEqualTypeOf<"x">();
-                return 1 as const;
-            },
-        },
-        (s) => {
-            // The fallback sees only the keys `x` did not handle.
-            expectTypeOf(s).toEqualTypeOf<"y" | "z">();
-            return "fallback" as const;
-        },
-    );
-
-    // Assert
-    expectTypeOf(matcher).toEqualTypeOf<
-        (shape: "x" | "y" | "z") => 1 | "fallback"
-    >();
-    // ❌ a value outside T must not be accepted by the matcher
-    expectTypeOf<"w">().not.toExtend<Parameters<typeof matcher>[0]>();
-    assert.equal(matcher("x"), 1);
-    assert.equal(matcher("y"), "fallback");
-    assert.equal(matcher("z"), "fallback");
-});
-
-test("getMatcherW: a `_` fallback widens mixed string and numeric returns", () => {
-    // Arrange
-    const factory = getMatcherW<"a" | "b" | 1 | 2>();
-
-    // Act
-    const matcher = factory(
-        {
-            a: (s) => {
-                expectTypeOf(s).toEqualTypeOf<"a">();
-                return "A" as const;
-            },
-            1: (n) => {
-                expectTypeOf(n).toEqualTypeOf<1>();
-                return 10 as const;
-            },
-        },
-        (s) => {
-            expectTypeOf(s).toEqualTypeOf<"b" | 2>();
-            return "fallback" as const;
-        },
-    );
-
-    // Assert
-    // ❌ ReturnsStrict: mixed handler returns widen to their union.
-    expectTypeOf(matcher).toEqualTypeOf<
-        (shape: "a" | "b" | 1 | 2) => "A" | 10 | "fallback"
-    >();
-    assert.equal(matcher("a"), "A");
-    assert.equal(matcher("b"), "fallback");
-    assert.equal(matcher(1), 10);
-    assert.equal(matcher(2), "fallback");
-});
-
-// ============================================================================
-// Factory contracts — calls that must not compile
-// ============================================================================
-
-test("getMatcher factory rejects patterns outside its contract", () => {
-    // Arrange
-    const factory = getMatcher<"a" | "b">();
-
-    // Act / Assert — the calls below must not compile.
-    // `Parameters<typeof factory>[0]` resolves only the *last* overload, so it
-    // is not a sound "rejected" oracle (an accepted partial pattern does not
-    // extend it either); the call sites are the oracle instead.
-    factory({
-        a: () => 1,
-        b: () => 2,
-        // @ts-expect-error `c` is not part of the universe `"a" | "b"`
-        c: () => 3,
-    });
-    // @ts-expect-error a gap without a `_` fallback is not exhaustive
-    factory({ a: () => 1 });
-    // @ts-expect-error a `string` fallback does not fit the `number` handler
-    factory({ a: () => 1 }, () => "x");
-});
-
-test("getMatcherW factory rejects patterns outside its contract", () => {
-    // Arrange
-    const factory = getMatcherW<"x" | "y">();
-
-    // Act / Assert — the calls below must not compile
-    factory({
-        x: () => 1 as const,
-        y: () => 2 as const,
-        // @ts-expect-error `z` is not part of the universe `"x" | "y"`
-        z: () => 3 as const,
-    });
-    // @ts-expect-error a gap without a `_` fallback is not exhaustive
-    factory({ x: () => 1 as const });
-});
-
-// ============================================================================
-// Dispatch — runtime behavior
-// ============================================================================
-
-test("getMatcher: an unhandled shape throws without a fallback", () => {
-    // Arrange — an open universe types its handler map as an index signature,
-    // so the type system cannot prove the runtime map is exhaustive.
-    const handlers: Record<string, () => number> = { a: () => 1 };
-    const matcher = getMatcher<string>()(handlers);
-
-    // Act / Assert
-    assert.equal(matcher("a"), 1);
-    assert.throws(() => matcher("b"), /Unhandled shape: b/);
-});
-
-// ============================================================================
-// Autocomplete — the language server is the oracle, not the type system
-// ============================================================================
-
-// Completion is a contextual-type property that the type system cannot observe,
-// so the cases below read the popup from the repo's language server (via the
-// test helper) rather than pairing `expectTypeOf` with `assert` — see
-// development/testing.md § Autocomplete. They probe the *matcher's* overloads,
-// which is why they live with the matcher and not with the helper.
-const REPO_ROOT = path.resolve(import.meta.dirname, "..");
-const UNIVERSE = `"a" | "b" | "c"`;
-
-// One session per probe: the tests share no language-server state (open
-// documents, project membership), so they pass in any order.
-interface LabelsProbe {
-    readonly name: string;
-    readonly factory: "getMatcher" | "getMatcherW";
-    readonly body: string;
-    readonly tail?: string;
-}
-
-const labelsFor = ({
-    name,
-    factory,
-    body,
-    tail = "",
-}: LabelsProbe): Promise<readonly string[]> => {
-    const session = new LspSession(REPO_ROOT);
-    const target: CompletionTarget = {
-        file: `src/__autocomplete_${name}.ts`,
-        source: [
-            `import { ${factory} } from "./index.ts";`,
-            `const m = ${factory}<${UNIVERSE}>()({`,
-            body,
-            `}${tail});`,
-            "",
-        ].join("\n"),
-    };
-    return session
-        .completionLabelsAt(target)
-        .then((result) => result.labels)
-        .finally(() => session.close());
-};
-
-test("autocomplete: an exhaustive pattern requires the universe", () => {
-    // Arrange
-    const name = "getMatcher_fresh";
-
-    // Act
-    const labels = labelsFor({
-        name,
-        factory: "getMatcher",
-        body: "    /*COMPLETE*/",
-    });
-
-    // Assert
-    return labels.then((result) => {
-        assert.deepEqual([...result], ["a", "b", "c"]);
-    });
-});
-
-test("autocomplete: handled keys drop out of the popup", () => {
-    // Arrange
-    const name = "getMatcher_after_key";
-
-    // Act
-    const labels = labelsFor({
-        name,
-        factory: "getMatcher",
-        body: "    a: () => 1,\n    /*COMPLETE*/",
-    });
-
-    // Assert
-    return labels.then((result) => {
-        assert.deepEqual([...result], ["b", "c"]);
-    });
-});
-
-test("autocomplete: a fallback makes the remaining keys optional", () => {
-    // Arrange
-    const name = "getMatcher_with_fallback";
-
-    // Act
-    const labels = labelsFor({
-        name,
-        factory: "getMatcher",
-        body: "    /*COMPLETE*/",
-        tail: ", () => 0",
-    });
-
-    // Assert
-    return labels.then((result) => {
-        assert.deepEqual([...result], ["a?", "b?", "c?"]);
-    });
-});
-
-test("autocomplete: with a fallback, handled keys stay optional", () => {
-    // Arrange
-    const name = "getMatcher_with_fallback_after_key";
-
-    // Act
-    const labels = labelsFor({
-        name,
-        factory: "getMatcher",
-        body: "    a: () => 1,\n    /*COMPLETE*/",
-        tail: ", () => 0",
-    });
-
-    // Assert
-    return labels.then((result) => {
-        assert.deepEqual([...result], ["b?", "c?"]);
-    });
-});
-
-test("autocomplete: `getMatcherW` offers the same popup as `getMatcher`", () => {
-    // Arrange
-    const name = "getMatcherW_fresh";
-
-    // Act
-    const labels = labelsFor({
-        name,
-        factory: "getMatcherW",
-        body: "    /*COMPLETE*/",
-    });
-
-    // Assert
-    return labels.then((result) => {
-        assert.deepEqual([...result], ["a", "b", "c"]);
-    });
-});
-
-test("autocomplete: `getMatcherW` also offers optional keys with a fallback", () => {
-    // Arrange
-    const name = "getMatcherW_with_fallback";
-
-    // Act
-    const labels = labelsFor({
-        name,
-        factory: "getMatcherW",
-        body: "    /*COMPLETE*/",
-        tail: ", () => 0",
-    });
-
-    // Assert
-    return labels.then((result) => {
-        assert.deepEqual([...result], ["a?", "b?", "c?"]);
-    });
-});
@@ -1,75 +0,0 @@
-import type { Exact, ValueOf } from "type-fest";
-
-type UnaryFn<T, R> = (shape: T) => R;
-
-// `Extract` drops optional handlers (`undefined`) so `PatternReturns` also works
-// when `P`'s constraint has optional keys.
-type PatternReturns<P> = ReturnType<
-    Extract<ValueOf<P>, (...args: never[]) => unknown>
->;
-
-type Handlers<T extends string | number, R> = { [K in T]: UnaryFn<K, R> };
-
-// The fallback is a *second argument*, not a property of the handler map,
-// because its parameter is the remainder `Exclude<T, keyof Handled>` and TypeScript
-// fixes a property's contextual type before it infers its sibling keys. A later
-// argument, by contrast, is contextually typed from inference on an earlier
-// one, so the split is what makes the remainder expressible at all.
-// See development/library.md.
-type Fallback<T extends string | number, Handled, R> = UnaryFn<
-    Exclude<T, keyof Handled>,
-    R
->;
-
-// TypeScript does not apply the excess-property check to a generic constraint,
-// so `Exact` restores it for the generic forms: a handler map can otherwise
-// carry keys outside `T`.
-
-// oxlint-disable typescript/unified-signatures
-// Strict returns: one common `R`. Overload order is load-bearing:
-//   #1 Handlers (first) -> the exhaustive form and the autocomplete popup
-//   #2 Fallback (last)  -> accepts a partial handler map plus a fallback
-interface MatcherStrict<T extends string | number> {
-    <R>(handlers: Handlers<T, R>): UnaryFn<T, R>;
-    <R, Handled extends Exact<Partial<Handlers<T, R>>, Handled>>(
-        handlers: Handled & Partial<Handlers<T, R>>,
-        fallback: Fallback<T, Handled, R>,
-    ): UnaryFn<T, R>;
-}
-// oxlint-enable typescript/unified-signatures
-
-// Widened returns: the union of every handler's return type. `P` is inferred
-// from the whole handler map, whose closed constraint supplies the
-// contextual/autocomplete type.
-// oxlint-disable typescript/unified-signatures
-interface MatcherWidening<T extends string | number> {
-    <P extends Exact<Handlers<T, unknown>, P>>(
-        handlers: P,
-    ): UnaryFn<T, PatternReturns<P>>;
-    <R, Handled extends Exact<Partial<Handlers<T, unknown>>, Handled>>(
-        handlers: Handled,
-        fallback: Fallback<T, Handled, R>,
-    ): UnaryFn<T, PatternReturns<Handled> | R>;
-}
-// oxlint-enable typescript/unified-signatures
-
-type HandlerMap = Record<string | number, UnaryFn<never, unknown> | undefined>;
-
-const dispatch =
-    (handlers: HandlerMap, fallback?: UnaryFn<never, unknown>) =>
-    (shape: string | number): unknown =>
-        (
-            handlers[shape] ??
-            fallback ??
-            (() => {
-                throw new Error(`Unhandled shape: ${shape}`);
-            })
-        )(
-            // oxlint-disable-next-line typescript/no-unsafe-type-assertion
-            shape as never,
-        );
-
-export const getMatcher = <T extends string | number>(): MatcherStrict<T> =>
-    dispatch;
-export const getMatcherW = <T extends string | number>(): MatcherWidening<T> =>
-    dispatch;
@@ -0,0 +1,207 @@
+import type { Exact, SetRequired, UnknownRecord } from "type-fest";
+
+import type {
+    HandlerMap,
+    Matchable,
+    Member,
+    PatternKey,
+    PatternReturns,
+    RedundantFallback,
+    UnaryFn,
+    UniverseGate,
+} from "./matcher-shared.ts";
+
+// A tagged union is discriminated by one property whose values are the tags (a
+// `Matchable`). `string` and `number` tags key a handler map directly;
+// `boolean`, `null` and `undefined` are admitted too but are not property keys,
+// so they go through the `PatternKey` projection. `symbol` has no literal syntax
+// to write a handler under, and `bigint` is not a property key.
+
+// The discriminant values of `T` under `K`. `Extract` keeps the finite literal
+// tags and leaves a widened `string`/`number` as itself; a broad tag is then
+// rejected by the universe gate.
+type Tags<T extends object, K extends keyof T> = Extract<T[K], Matchable>;
+
+// The keys of `T` that can act as a discriminant. `getTaggedUnionMatcher<T>()`
+// accepts only these, so the factory rejects a key whose values are not tags.
+type Discriminated<T extends object> = {
+    [K in keyof T]: T[K] extends Matchable ? K : never;
+}[keyof T];
+
+// The member(s) of `T` narrowed to the tag(s) `V`. Distributes over `T`, so a
+// duplicated tag maps to a union of members rather than silently dropping one.
+// A member whose `K` cannot take `V` drops out; the rest have `K` narrowed to
+// `Extract<T[K], V>`. `T[K] extends V` keeps an exact member (a discriminated
+// union's declared interface) untouched, so only a property that is itself a
+// union — or optional — goes through the mapped form. `SetRequired` makes `K`
+// required unless `V` admits `undefined`: a defined tag yields `{ type: "x" }`,
+// while the `undefined` tag keeps `{ type?: never }` (absence) or
+// `{ type?: undefined }` when the property admits an explicit `undefined`.
+// Keyed by `PatternKey`, so `boolean`/`null`/`undefined` tags can key a mapped
+// type; `Member` inverts the projection against the tag set to recover the tag.
+type Narrowed<T extends object, K extends keyof T, V> = T extends object
+    ? [Extract<T[K], V>] extends [never]
+        ? never
+        : T[K] extends V
+          ? T
+          : undefined extends V
+            ? { [P in keyof T]: P extends K ? Extract<T[P], V> : T[P] }
+            : SetRequired<
+                  { [P in keyof T]: P extends K ? Extract<T[P], V> : T[P] },
+                  K
+              >
+    : never;
+
+type MapTaggedUnion<T extends object, K extends keyof T> = {
+    [P in PatternKey<Tags<T, K>>]: Narrowed<T, K, Member<Tags<T, K>, P>>;
+};
+
+type Handlers<T extends object, K extends keyof T, R> = {
+    [P in PatternKey<Tags<T, K>>]: UnaryFn<MapTaggedUnion<T, K>[P], R>;
+};
+
+// The tag values whose `PatternKey` is handled.
+type HandledTags<T extends object, K extends keyof T, Handled> = Member<
+    Tags<T, K>,
+    keyof Handled
+>;
+
+// The fallback is a *second argument*, not a property of the handler map, so
+// its parameter can be the remainder the map left uncovered: the members
+// narrowed to the tags the map did not handle. See development/library.md.
+type Fallback<T extends object, K extends keyof T, Handled, R> = UnaryFn<
+    Narrowed<T, K, Exclude<Tags<T, K>, HandledTags<T, K, Handled>>>,
+    R
+>;
+
+// A fallback is redundant once the handler map covers every tag of `T`. Folded
+// into `Handled`'s own (self-referential) constraint so it is checked *after*
+// inference; see the primitive-union matcher for why a conditional in the fallback's
+// parameter is evaluated too early.
+type MustBePartial<T extends object, K extends keyof T, Handled> =
+    PatternKey<Tags<T, K>> extends keyof Handled ? RedundantFallback : unknown;
+
+// TypeScript does not apply the excess-property check to a generic constraint,
+// so `Exact` restores it for the generic forms.
+
+// Strict returns: one common `R`. Overload order is load-bearing:
+//   #1 Handlers (first) -> the exhaustive form and the autocomplete popup
+//   #2 Fallback (last)  -> accepts a partial handler map plus a fallback
+interface TaggedUnionMatcherStrict<T extends object, K extends keyof T> {
+    <R>(handlers: Handlers<T, K, R> & UniverseGate<Tags<T, K>>): UnaryFn<T, R>;
+    <
+        R,
+        Handled extends Exact<Partial<Handlers<T, K, R>>, Handled> &
+            MustBePartial<T, K, Handled>,
+    >(
+        handlers: Handled &
+            Partial<Handlers<T, K, R>> &
+            UniverseGate<Tags<T, K>>,
+        fallback: Fallback<T, K, Handled, R> & UniverseGate<Tags<T, K>>,
+    ): UnaryFn<T, R>;
+}
+
+// Widened returns: the union of every handler's return type. `P` is inferred
+// from the whole handler map, whose closed constraint supplies the
+// contextual/autocomplete type.
+interface TaggedUnionMatcherWidening<T extends object, K extends keyof T> {
+    <P extends Exact<Handlers<T, K, unknown>, P>>(
+        handlers: P & UniverseGate<Tags<T, K>>,
+    ): UnaryFn<T, PatternReturns<P>>;
+    <
+        R,
+        Handled extends Exact<Partial<Handlers<T, K, unknown>>, Handled> &
+            MustBePartial<T, K, Handled>,
+    >(
+        handlers: Handled & UniverseGate<Tags<T, K>>,
+        fallback: Fallback<T, K, Handled, R> & UniverseGate<Tags<T, K>>,
+    ): UnaryFn<T, PatternReturns<Handled> | R>;
+}
+
+// The key-taking step of the curried factory. Naming it lets the factory return
+// `dispatch` directly, the tacit twin of the primitive-union factory's bare
+// `=> dispatch`.
+type TaggedUnionMatcherFactory<T extends object> = <K extends Discriminated<T>>(
+    k: K,
+) => TaggedUnionMatcherStrict<T, K>;
+
+type TaggedUnionMatcherWideningFactory<T extends object> = <
+    K extends Discriminated<T>,
+>(
+    k: K,
+) => TaggedUnionMatcherWidening<T, K>;
+
+const dispatch =
+    (k: PropertyKey) =>
+    (handlers: HandlerMap, fallback?: UnaryFn<never, unknown>) =>
+    (shape: object): unknown => {
+        // `object` carries no index signature, so the read needs the assertion;
+        // the factory admits only keys whose values are tags, and the map keys
+        // them by `PatternKey`, so the result is narrowed to the map's key space.
+        // oxlint-disable-next-line typescript/no-unsafe-type-assertion
+        const tag = (shape as UnknownRecord)[k] as string | number;
+        return (
+            handlers[tag] ??
+            fallback ??
+            (() => {
+                throw new Error(`Unhandled tag: ${String(tag)}`);
+            })
+        )(
+            // oxlint-disable-next-line typescript/no-unsafe-type-assertion
+            shape as never,
+        );
+    };
+
+/**
+ * Create a matcher for a discriminated union, with one common return type.
+ *
+ * Use it when the value is an object discriminated by a property
+ * (`{ kind: "circle" } | { kind: "square" }`) and every handler returns the
+ * same type.
+ *
+ * The first call fixes the union `T`; the returned function takes the
+ * discriminant property's name (`K`, restricted to properties whose values are
+ * tags), and that returns the handler-map builder. Supplying a second fallback
+ * argument to the builder allows a partial map and receives the members whose
+ * tag was not handled. A `boolean`, `null` or `undefined` tag is keyed by its
+ * stringified form (`true` -> `"true"`); see README § Caveats.
+ *
+ * @typeParam T - The discriminated-union type to match.
+ * @returns A function that takes the discriminant property's name.
+ * @example
+ *     type Shape =
+ *         | { kind: "circle"; radius: number }
+ *         | { kind: "square"; side: number };
+ *
+ *     const matchShape = getTaggedUnionMatcher<Shape>()("kind");
+ *     const area = matchShape({
+ *         circle: (s) => Math.PI * s.radius ** 2,
+ *         square: (s) => s.side ** 2,
+ *     });
+ */
+export const getTaggedUnionMatcher = <
+    T extends object,
+>(): TaggedUnionMatcherFactory<T> => dispatch;
+
+/**
+ * Create a matcher for a discriminated union whose return type is the union of
+ * every handler's return type.
+ *
+ * Use it when the value is a discriminated object and the handlers return
+ * different types. The `W` (widening) counterpart of
+ * {@link getTaggedUnionMatcher}; the curried key step and the optional fallback
+ * are identical.
+ *
+ * @typeParam T - The discriminated-union type to match.
+ * @returns A function that takes the discriminant property's name.
+ * @example
+ *     const matchShape = getTaggedUnionMatcherW<Shape>()("kind");
+ *     const describe = matchShape({
+ *         circle: () => "round",
+ *         square: () => 4,
+ *     });
+ *     // describe: (shape: Shape) => string | number
+ */
+export const getTaggedUnionMatcherW = <
+    T extends object,
+>(): TaggedUnionMatcherWideningFactory<T> => dispatch;
@@ -1,4 +1,3 @@
-/* oxlint-disable typescript/no-floating-promises -- expectTypeOf() is a sync type-assertion library that the type-aware linter misidentifies as a promise */
 import { strict as assert } from "node:assert";
 import path from "node:path";
 import { test } from "node:test";
@@ -8,5 +8,6 @@
        "allowImportingTsExtensions": true,
        "verbatimModuleSyntax": true
    },
-    "include": ["src", "scripts"]
+    "include": ["src", "scripts"],
+    "exclude": ["src/doc-test"]
 }