🔀 Merge chore/cleanup-readme into main

The README collapsed to API-only prose once the old match/P surface was
dropped. Rebuild its entry-point shape: a quick-start Synopsis walked through
a Contact union, a dedicated Installation section, and a real-world Examples
section (primitive-union dispatch, a fallback, a property union narrowed by
the tagged-union matcher, and the widening variant).

Set the tagline and Description to the library's identity — exhaustive,
type-safe pattern matching for TypeScript — and state the goal Moose-style:
type-safe pattern matching with a lean syntax, accomplished by exhaustive
branches and typed per-branch handler parameters, backed by autocomplete and
a tiny footprint. Drop the F#-style framing and the stale "matches method is
a type guard" and "(not regex)" copy from the README, AGENTS.md and the
package.json description. Every fence is a doc-test, so the examples cannot
drift from the API.

.gitea/workflows/ci.yml

@@ -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

.gitignore

@@ -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
@@ -10,5 +14,6 @@ coverage
 !.vscode/extensions.json
 !.vscode/settings.json
 !.vscode/tasks.json
+!.vscode/launch.json
 .idea
-.DS_Store
+.DS_Store

.oxlintrc.json

@@ -13,6 +13,8 @@
         "eslint/no-undefined": "off",
         "eslint/sort-keys": "off",
         "eslint/id-length": "off",
+        "eslint/capitalized-comments": "off",
+        "eslint/no-ternary": "off",
         "import/no-named-export": "off",
         "eslint/one-var": "off",
         "import/group-exports": "off",
@@ -20,7 +22,8 @@
         "eslint/sort-imports": "off",
         "import/consistent-type-specifier-style": "off",
         "unicorn/prefer-export-from": "off",
-        "typescript/method-signature-style": "off"
+        "typescript/method-signature-style": "off",
+        "typescript/promise-function-async": "off"
     },
     "options": { "typeAware": true },
     "env": { "builtin": true, "es2024": true, "node": true },
@@ -31,7 +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"
             }
         },
         {
@@ -39,6 +44,12 @@
             "rules": {
                 "import/no-nodejs-modules": "off"
             }
+        },
+        {
+            "files": ["src/util/__tests__/**"],
+            "rules": {
+                "import/no-nodejs-modules": "off"
+            }
         }
     ],
     "ignorePatterns": ["dist", "node_modules", "coverage"]

.vscode/extensions.json

@@ -1,5 +1,6 @@
 {
     "recommendations": [
+        "connor4312.nodejs-testing",
         "oxc.oxc-vscode",
         "streetsidesoftware.code-spell-checker",
         "typescriptteam.native-preview",

.vscode/launch.json

@@ -0,0 +1,15 @@
+{
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "type": "node",
+            "request": "launch",
+            "name": "Debug current test file",
+            "runtimeExecutable": "node",
+            "runtimeArgs": ["--test", "--strip-types"],
+            "args": ["${file}"],
+            "cwd": "${workspaceFolder}",
+            "console": "integratedTerminal"
+        }
+    ]
+}

.vscode/settings.json

@@ -1,4 +1,14 @@
 {
+    "nodejs-testing.extensions": [
+        {
+            "extensions": ["mjs", "cjs", "js"],
+            "parameters": []
+        },
+        {
+            "extensions": ["ts"],
+            "parameters": ["--strip-types"]
+        }
+    ],
     "[typescript]": {
         "editor.defaultFormatter": "oxc.oxc-vscode",
         "editor.formatOnSave": true

AGENTS.md

@@ -7,10 +7,11 @@ first-action facts. Do not restate evolving prose here — it will drift.
 
 ## First action
 
-- Project: F#-style pattern matching for TypeScript/ESM. Node `>=26` (pinned via `.node-version`), ESM-only (no CommonJS shim).
+- Project: 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.
@@ -26,9 +27,10 @@ Don't silence the type system to force a green run. As an agent these are forbid
 
 - `// @ts-nocheck`, `// @ts-ignore`, `// @ts-expect-error`
 - `// 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. 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>`.
 
@@ -42,7 +44,9 @@ Never start a long-lived / blocking process such as `npm run watch`. It runs unt
 
 ### Working on tasks
 
-- **Task with subtasks** (a task that has indented children): create the branch with `npm run create:branch -- <prefix>/<desc>`, inferring the prefix from the task content (`feature/…` / `fix/…` / `chore/…`) — do not hand-write `git switch -c`, the command enforces the clean-tree / current-`main` / green-baseline precondition. Work on each subtask with commits, then present a concise handover for the user to review. Use this fixed shape:
+Every task — with or without subtasks — goes through the complete branching model:
+
+- Create the branch with `npm run create:branch -- <prefix>/<desc>`, inferring the prefix from the task content (`feature/…` / `fix/…` / `chore/…`) — do not hand-write `git switch -c`, the command enforces the clean-tree / current-`main` / green-baseline precondition. Work with commits (each subtask gets one or more), then present a concise handover for the user to review. Use this fixed shape:
 
     ```md
     ## Handover — <branch>
@@ -54,9 +58,7 @@ Never start a long-lived / blocking process such as `npm run watch`. It runs unt
 
     Once the user has no further objections, merge back: `npm run create:finish` (on the branch — it merges `--no-ff`, runs `npm run verify`, and deletes the branch). The branching model is documented in [CONTRIBUTING.md § Branching model](./CONTRIBUTING.md#branching-model).
 
-- **Leaf task** (no indented children): implement on the current branch and commit.
+Follow [CONTRIBUTING.md § Testing discipline (type-driven)](./CONTRIBUTING.md#testing-discipline-type-driven) throughout.
-
-In both cases, follow [CONTRIBUTING.md § Testing discipline (type-driven)](./CONTRIBUTING.md#testing-discipline-type-driven). Each subtask gets one or more commits.
 
 ## Read these
 

CHANGELOG.md

@@ -7,6 +7,73 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+- restore the README's Synopsis and Examples sections: a quick-start recipe plus
+  real-world examples (primitive-union dispatch, a fallback, a property union
+  narrowed by the tagged-union matcher, and a widening variant)
+- 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
+
+## [0.3.0] - 2026-09-18
+
+- condensed primitive matcher factories down to 2 from formerly 4
+- house shared test helpers under `src/util/__tests__/`
+
+## [0.2.0] - 2026-09-16
+
+- allow ternaries and lowercase comments in oxlint
+- switch `src/primitive.ts` prose from block comments to line comments
+- remove the template's `match`/`P` example modules and their documentation, and point `src/index.ts` at the primitive matchers
+
+## [0.1.8] - 2026-09-16
+
+- upgrade dependencies
+- ignore `@types/node` in `maintain:outdated` (misleading `latest` dist-tag)
+- require extremely concise prose in `development/`
+
 ## [0.1.7] - 2026-09-16
 
 - require a short summary under `[Unreleased]` in the changelog before a branch is finished
@@ -42,7 +109,17 @@ 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/0.1.7...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
+[0.1.8]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.1.7...0.1.8
 [0.1.7]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.1.6...0.1.7
 [0.1.6]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.1.5...0.1.6
 [0.1.5]: https://gitea.e1nsnull.de/tmu/tiny-pattern-ts/compare/0.1.4...0.1.5

CONTRIBUTING.md

@@ -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  |
 
@@ -66,13 +68,38 @@ 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-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)).
+Each test body follows **AAA (Arrange–Act–Assert)** with labeled blocks
+separated by a blank line: `// Arrange` sets up the inputs (e.g. the matcher
+factory), `// Act` exercises the subject once from them (not a second
+throwaway call), `// Assert` holds every check — type expectations first,
+runtime assertions last; an empty block drops its label (see
+[development/testing.md § AAA ordering](./development/testing.md#aaa-ordering)).
 Type-first is enforced structurally: `npm test` runs `check:tsc` before the
 test runner, so a wrong type can never be papered over by a passing assertion.
 Per [AGENTS.md § Never do](./AGENTS.md#never-do), reach green honestly — fix the
 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).
@@ -101,17 +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
+- `create:*` — front doors of the repo's own workflow; these produce or mutate
-  rather than the source. `create:branch` opens a unit of work, `create:finish`
+  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
@@ -144,7 +175,8 @@ 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
+  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.**
@@ -173,6 +205,9 @@ reaches for by default:
   alternatives and the known issues. When you change a rule, update its category
   file in the same commit and cross-link the two. (why:
   [development/README.md](./development/README.md))
+- **Prose in `development/` is extremely concise.** When adding or changing a
+  decision, write fragments if needed — sacrifice grammar for concision. (why:
+  [development/README.md § Decision blocks](./development/README.md#decision-blocks))
 
 ## Branching model
 

LICENSE

@@ -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

README.md

@@ -1,35 +1,53 @@
 # tiny-pattern-ts
 
-Pattern matching for TypeScript/ESM environments (F#-style, not regex).
+Exhaustive, type-safe pattern matching for TypeScript.
 
 ## Synopsis
 
 ```ts
-import { match, P } from "tiny-pattern-ts";
+import { getTaggedUnionMatcher } from "tiny-pattern-ts";
 
-const reply = (answer: "yes" | "no") =>
+// 1. We have a union type
-    match(answer)
+type Contact =
-        .with(P.literal("yes"), (): "agreed" => "agreed")
+    | { kind: "email"; address: string }
-        .with(P.literal("no"), (): "declined" => "declined")
+    | { kind: "phone"; number: string }
-        .exhaustive();
+    | { kind: "messenger"; username: string };
 
-reply("yes"); // "agreed"
+// 2. Create a matcher providing the discriminant property
+const matchContact = getTaggedUnionMatcher<Contact>()("kind");
+
+// 3. Define handlers for each branch of the union
+const formatContact = matchContact({
+    email: (e) => `MAIL: ${e.address}`,
+    phone: (p) => `PHONE: ${p.number}`,
+    messenger: (m) => `MESSENGER: @${m.username}`,
+});
+
+// 4. Call the matcher with a value
+const mailOutput = formatContact({ kind: "email", address: "ada@example.com" });
+assert.equal(mailOutput, "MAIL: ada@example.com");
+
+const phoneOutput = formatContact({ kind: "phone", number: "+1 555 0100" });
+assert.equal(phoneOutput, "PHONE: +1 555 0100");
 ```
 
 ## Description
 
-`tiny-pattern-ts` gives TypeScript the shape of F#-style pattern matching:
+`tiny-pattern-ts` is a pattern-matching library for TypeScript.
-a value flows through a chain of patterns, the first one that matches runs its
-handler, and the handler receives the value narrowed to that pattern's type. The
-"patterns" are 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
+The main goal of `tiny-pattern-ts` is to make pattern matching type-safe with a
-and no DSL to learn: `match(value)` returns a builder, `.with(pattern, handler)`
+lean syntax. This is accomplished by being exhaustive and passing typed
-adds a case, and the chain ends in either `.exhaustive()` or `.otherwise(...)`.
+parameters per branch to the handlers — supported by an outstanding
-The type-level contract is the feature — see
+autocomplete and a tiny footprint.
-[development/library.md](./development/library.md) for the design decisions and
+
-the known limitations.
+See [development/library.md](./development/library.md) for the design decisions
+and [Caveats](#caveats) for the limits.
+
+## Installation
+
+```sh
+npm install tiny-pattern-ts
+```
 
 ## Requirements
 
@@ -41,129 +59,234 @@ the known limitations.
 
 ## Examples
 
-### Literal matching and `exhaustive()`
+A few real-world recipes. Each binds the handler-map function once and reuses
+it, so the matcher is allocated a single time.
 
-`.exhaustive()` returns the union of the handler return types and throws if no
+### Dispatch on a primitive union
-case matched. Annotate handler returns when you want literal types rather than
+
-`string`:
+A result code is itself a finite union, so `getPrimitiveUnionMatcher` keys a
+handler on each member:
 
 ```ts
-type Answer = "yes" | "no";
+import { getPrimitiveUnionMatcher } from "tiny-pattern-ts";
 
-const reply = (answer: Answer): "agreed" | "declined" =>
+type ResultCode = "ok" | "created" | "no-content";
-    match(answer)
-        .with(P.literal("yes"), (): "agreed" => "agreed")
-        .with(P.literal("no"), (): "declined" => "declined")
-        .exhaustive();
 
-reply("yes"); // "agreed"
+const toStatus = getPrimitiveUnionMatcher<ResultCode>()({
+    ok: () => 200,
+    created: () => 201,
+    "no-content": () => 204,
+});
+
+assert.equal(toStatus("ok"), 200);
+assert.equal(toStatus("created"), 201);
+assert.equal(toStatus("no-content"), 204);
 ```
 
-`exhaustive()` checks at runtime, not at compile time — TypeScript does not force
+### Leave cases to a fallback
-every union member to have a case (see
+
-[development/library.md](./development/library.md#exhaustive-is-a-runtime-check)).
+Pass a fallback as the second argument to handle only part of the universe; it
-Use `.otherwise(...)` when a fallback is wanted:
+receives the members the map leaves uncovered — here the parameter is
+`"deprecated" | "gateway-timeout"`:
 
 ```ts
-const label = (answer: Answer): string =>
+import { getPrimitiveUnionMatcher } from "tiny-pattern-ts";
-    match(answer)
+
-        .with(P.literal("yes"), () => "agreed")
+type Status = "active" | "beta" | "deprecated" | "gateway-timeout";
-        .otherwise(() => "not agreed");
+
+const rollout = getPrimitiveUnionMatcher<Status>()(
+    { active: () => "enabled", beta: () => "enabled" },
+    (status) => `blocked (${status})`,
+);
+
+assert.equal(rollout("active"), "enabled");
+assert.equal(rollout("deprecated"), "blocked (deprecated)");
 ```
 
-### Matching by `typeof`
+### Dispatch on a property union
 
-`P.type<T>(name)` pairs an explicit type `T` with the runtime `typeof` name it
+The value does not have to be the union itself. When a single property carries a
-should test for:
+finite union, the tagged-union matcher keys on it and narrows the whole record to
+the selected value:
 
 ```ts
-const describe = (value: unknown): string =>
+import { getTaggedUnionMatcher } from "tiny-pattern-ts";
-    match(value)
-        .with(P.type<string>("string"), (s) => `string of length ${s.length}`)
-        .with(P.type<number>("number"), (n) => `number ${n.toFixed(2)}`)
-        .otherwise(() => "something else");
-```
 
-The supported names are `string`, `number`, `boolean`, `bigint`, `symbol`,
+interface Invoice {
-`undefined`, `object`, and `function`. `"object"` matches non-null objects and
+    readonly currency: "eur" | "usd" | "jpy";
-functions; `"undefined"` compares against `undefined` directly.
+    readonly amount: number;
-
-### Structural matching and discriminated unions
-
-`P.shape(shape, refine?)` checks that every key in `shape` exists on the value.
-A value that is itself a matcher is applied, otherwise it is compared with
-strict equality. To narrow to a concrete type, pass a `refine` type guard:
-
-```ts
-interface Circle {
-    readonly kind: "circle";
-    readonly radius: number;
 }
 
-interface Square {
+const matchCurrency = getTaggedUnionMatcher<Invoice>()("currency");
-    readonly kind: "square";
-    readonly side: number;
-}
 
-type Shape = Circle | Square;
+const symbolOf = matchCurrency({
+    eur: (i) => `€${i.amount.toFixed(2)}`,
+    usd: (i) => `$${i.amount.toFixed(2)}`,
+    jpy: (i) => `¥${i.amount.toFixed(0)}`,
+});
 
-const area = (shape: Shape): number =>
+assert.equal(symbolOf({ currency: "usd", amount: 12.5 }), "$12.50");
-    match(shape)
+assert.equal(symbolOf({ currency: "jpy", amount: 900 }), "¥900");
-        .with(
-            P.shape({ kind: "circle" }, (v): v is Circle => "radius" in v),
-            (c) => Math.PI * c.radius ** 2,
-        )
-        .with(
-            P.shape({ kind: "square" }, (v): v is Square => "side" in v),
-            (s) => s.side ** 2,
-        )
-        .exhaustive();
 ```
 
-Without `refine`, `P.shape` returns a matcher for the shape's own type, not the
+### Widen the return type
-narrowed one. Nested matchers can be used in the shape object, for example
-`P.shape({ name: P.type<string>("string") })`.
 
-### Custom guards with `when`
+When the handlers return different types, reach for the widening `W` variant: the
-
+matcher's return is their union rather than one common type — here
-`P.when` takes a type guard and infers the narrowed type from it:
+`string | string[] | undefined`:
 
 ```ts
-const toNumber = (value: unknown): number =>
+import { getPrimitiveUnionMatcherW } from "tiny-pattern-ts";
-    match(value)
-        .with(
-            P.when((v): v is string => typeof v === "string"),
-            (s) => Number.parseInt(s, 10),
-        )
-        .otherwise(() => 0);
-```
 
-### Widening with `any`
+type Field = "name" | "tags" | "note";
 
-`P.any<T>(predicate)` takes a plain boolean predicate and a declared type `T`,
+const parse = getPrimitiveUnionMatcherW<Field>()({
-for cases where the predicate cannot be written as a type guard:
+    name: () => "Ada",
+    tags: () => ["admin", "beta"],
+    note: () => undefined,
+});
 
-```ts
+assert.equal(parse("name"), "Ada");
-const firstNumber = (items: readonly unknown[]): number | undefined =>
+assert.deepEqual(parse("tags"), ["admin", "beta"]);
-    match(items)
+assert.equal(parse("note"), undefined);
-        .with(
-            P.any<readonly number[]>(
-                (v) =>
-                    Array.isArray(v) &&
-                    v.every((item) => typeof item === "number"),
-            ),
-            (xs) => xs[0],
-        )
-        .otherwise(() => undefined);
 ```
 
 ## 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](#examples) 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
 

backlog.tasks

@@ -5,31 +5,34 @@ 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 @high
+☐ Split off template into separate package => pi --session 01a07dde-7050-7054-bb36-1606d7eb2bc3 @low
 
 v1.0:
-☐ API surface is stable and fully typed
+✔ API surface is stable and fully typed @done
-    ☐ Finalize public exports in `src/index.ts`
+    ✔ Finalize public exports in `src/index.ts` @done
-    ☐ Document all exported types and functions
+    ✔ Document all exported types and functions @done
-    ☐ Add JSDoc for public APIs
+    ✔ Add JSDoc for public APIs @done
-☐ Test coverage meets threshold
+✔ Test coverage meets threshold @done
-    ☐ Achieve 100% branch coverage on `src/pattern.ts`
+    ✔ Achieve 100% branch coverage on `src/primitive-union.ts` @done
-    ☐ Achieve 100% branch coverage on `src/match.ts`
+    ✔ Achieve 100% branch coverage on `src/index.ts` @done
-    ☐ Achieve 100% branch coverage on `src/index.ts`
 
-Bugs:
+Matcher:
-
+✔ when using a union type as a property, the current behavior of tagged union matcher is @done
-Enhancements:
+    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 @done
+    → previous section order: title, tagline, Synopsis, Description, Requirements, Examples, API, License, Contributing
+    → 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
-☐ Write migration guide for users coming from discriminated unions
+    ☐ Why do we do this? => exhaustiveness encoded type safe
-☐ Create backlog tasks for implementation
+    ☐ Why this form? little syntax, data last, very small, autocomplete, strict typing in the handler; for more features use ts-pattern
-☐ Validate code fences in Markdown (start with README.md) — compile the TypeScript examples against `src/` so the docs cannot drift from the API
+✔ Write migration guide for users coming from discriminated unions @done (9/24/2026, 10:21:17 PM)
-
+✔ Create backlog tasks for implementation @done (9/24/2026, 10:21:16 PM)
-Workflow:
+✔ Validate code fences in Markdown (start with README.md) — compile the TypeScript examples against `src/` so the docs cannot drift from the API @done
-✔ 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
 
 Maintenance:
 ☐ Serve CI coverage over a tiny self-hosted webserver (replace the zip artifact) @low
@@ -43,3 +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

cspell.json

@@ -42,7 +42,10 @@
         "bestikk",
         "silverwind",
         "idris",
-        "todotasks"
+        "todotasks",
+        "connor",
+        "injective",
+        "injectivity"
     ],
     "ignorePaths": ["dist", "node_modules", "coverage", "*.svg", ".gitignore"]
 }

development/README.md

@@ -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                                              |
 
@@ -66,6 +67,9 @@ of downloading per job.
   goes under a `## Known issues` section at the end of the file.
 - Replace a superseded decision in place rather than archiving it; git history
   is the archive.
+- Terse is the point: humans skim and agents imitate the style already in the
+  file, so verbosity compounds edit over edit. Grammar loses to density here on
+  purpose.
 
 ## Adding to these docs
 

development/ci.md

@@ -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)

development/docs.md

@@ -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.

development/library.md

@@ -3,4 +3,309 @@
 The type-level design of the public API and the limitations it carries. The
 user-facing reference is [README § API](../README.md#api).
 
-Currently 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
+
+#### Decision (2026-09)
+
+A factory takes the universe and returns a builder; the builder takes a handler
+map and an optional fallback:
+
+```ts
+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
+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:
+
+- `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>` 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
+
+- **The fallback is an argument, not a property.** TypeScript fixes a property's
+  contextual type before it infers its sibling keys, so `_: (s) => …` in the
+  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.
+- **`R` needs an inference site.** `R` inside the `Exact<…>` constraint is not
+  one, so `handlers: Handled & Partial<Handlers<T, R>>` re-adds it; without that
+  `R` collapses to `unknown` when the handler params are inferred.
+- **`Exact` restores the excess-property check.** TypeScript skips it for a
+  generic constraint, so without `Exact` the handler map accepts keys outside
+  `T`.
+- Two factories, not four: the fallback is an argument, not a separate API.
+
+#### Rejected
+
+- **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)`. 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.
+- **Variance / `const` type parameters / `NoInfer` / `unique symbol` brands /
+  defaulted type-param guards.** None change inference or evaluation order;
+  `in`/`out` on the handler map broke contextual typing outright. `NoInfer`
+  specifically leaks into the emitted `.d.ts`, raising the consumer floor to
+  TypeScript 5.4 (README promises `>= 5.0`).
+- **Union merge**, **overload merge with only the exhaustive arm last**,
+  **inferred universe**, **conditional `RequireKeys`**, **cases-first curried** —
+  decided against while the API was single-object; their reasons (reported
+  near-miss member, no `_` in the exhaustive popup, `NoInfer`/floor, `keyof P`
+  counts optional keys, not pipe-friendly) hold where they still apply.
+
+#### Known issue
+
+- `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).

development/testing.md

@@ -27,19 +27,188 @@ 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 wiring, `exhaustive()`
+- Testing the type only: it would not catch handler dispatch or the `_`
-  throwing, or the `otherwise` fallback (see `src/index.test.ts`).
+  fallback (see `src/primitive-union.test.ts`).
 
-The runner is `node --test --strip-types "src/**/*.test.ts"` and the tiers are in
+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
+
+The rule is in
+[CONTRIBUTING.md § Testing discipline (type-driven)](../CONTRIBUTING.md#testing-discipline-type-driven).
+
+#### Decision (2026-09)
+
+Test bodies read arrange → act → assert: inputs (the factory) set up first, the
+subject exercised once from them, all checks last — types then runtime. The
+blocks are labeled with `// Arrange` / `// Act` / `// Assert` comments and
+separated by a blank line; an empty block drops its label.
+
+#### Why
+
+- Interleaved setup/checks hide what runs vs. what is observed; the eye
+  re-reads the block to find the seams.
+- A factory built mid-test invites a second throwaway call of the subject;
+  arranging it once makes the positive construction and the negative
+  `Parameters<…>` check share one source of truth.
+- Labels make the seams explicit, not inferred — grep-able and reviewable
+  without reading the statements.
+
+#### Rejected
+
+- Unlabeled ordering (bare blank lines): the seams still have to be found by
+  reading; the labels cost nothing.
+
+## Test helpers
+
+The rule is enforced by `import/no-relative-parent-imports`; this section records
+why the mechanism is shaped like this.
+
+#### Decision (2026-09)
+
+A shared test helper — code that scattered `*.test.ts` files import to do their
+testing — lives under `src/util/__tests__/` and is addressed by the
+`#test-utils/…` self-reference (`package.json#imports`:
+`"#test-utils/*": "./src/util/__tests__/*"`), never by a relative path:
+
+```ts
+import { LspSession } from "#test-utils/lsp-completion.ts";
+```
+
+#### Why
+
+- The lint rule bans upward (`../`) imports, and a cross-cutting helper can
+  always be placed above _some_ scattered consumer, wherever it goes. Name
+  beats path: a `#test-utils/…` specifier has no direction, so the rule never
+  fires and file moves only touch the one mapping in `package.json`.
+- `#…` is Node's reserved prefix for _private_ subpath imports: publishing
+  `package.json` leaks nothing and resolves nothing for consumers.
+- `__tests__` as the folder name is not about tests living there; it is the
+  directory pattern `tsconfig.build.json` already excludes, so a helper can
+  never be emitted into `dist/` and shipped by accident.
+- Node's own resolver handles `#…` under `--strip-types`, and `tsc` resolves it
+  via the same `imports` field — one mechanism for runtime and type gate, no
+  loader needed.
+- The helper uses `node:` builtins (it drives a language server), so
+  `import/no-nodejs-modules` is off for `src/util/__tests__/**` in
+  `.oxlintrc.json` — the same exception the `scripts/**` scope carries.
+- Helpers carry no library coupling: their tests probe in-memory documents
+  whose contextual types are written inline, so only the helper's own contract
+  (marker handling, position, label extraction) is under test. A test that
+  asserts a _matcher's_ popup belongs with the matcher.
+
+#### Rejected
+
+- A helper beside the tests (`src/util/test.ts`, flat `src/`): composes only
+  while `src/` stays flat; the first nested test reaching it reintroduces the
+  banned upward import.
+- Bare `~/…` specifier: not valid in `imports` (keys must start with `#`) —
+  resolution fails at runtime with `ERR_MODULE_NOT_FOUND`. A `#~/…` “home”
+  shorthand was dropped in review; `#test-utils/…` states what it is.
+- tsconfig `paths` alias: resolves for `tsc` but not for plain
+  `node --test --strip-types` (no loader hook), breaking the fast tier.
+- Turning `import/no-relative-parent-imports` off for test files: the rule
+  still guards non-test helpers importing each other, and the exemption is
+  only needed for the one specifier the mapping already solves cleanly.
+
+## Autocomplete
+
+#### Decision (2026-09)
+
+Completion is verified by driving the repo's own language server
+(`tsc --lsp --stdio`, the same server pi's LSP extension talks to) through
+the test helper `#test-utils/lsp-completion.ts`
+(`src/util/__tests__/lsp-completion.ts`), not through the type system:
+
+```sh
+node --strip-types src/util/__tests__/lsp-completion.ts <file> [<marker>]
+```
+
+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-union.test.ts`, where the same probe asserts the matcher's popup;
+the CLI is for manual inspection.
+
+#### Why
+
+- Completion is a contextual-type property: it depends on which overload
+  signature TypeScript picks for the object literal, and no type-level assertion
+  observes that.
+- `Parameters<typeof factory>[0]` resolves only the _last_ overload, so it is
+  not the popup's contextual type either — see
+  [library.md § Matcher shape](./library.md#matcher-shape).
+- The server is the only ground truth; the script reproduces what the editor
+  shows.
+
+#### Rejected
+
+- **`expect-type` would not work**: there is no operator for “the popup offers
+  these labels”. `toExtend` / `toEqualTypeOf` test assignability and cannot say
+  which overload supplied the contextual type.
+- **Checking by hand in the editor**: not reproducible in review or by an agent.
+- **`@ts-expect-error` at a completion position**: it asserts the absence of a
+  compile error, not the presence of specific labels.
+
+#### Known issue
+
+- Each test spawns its own `tsc` server so the tests share no state and pass in
+  any order; the file is an integration test (~1.6 s) that needs `node_modules`.
+  `didOpen` is handled in order before the completion request, so no settle
+  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
+- The type-aware linter misidentifies `expectTypeOf()` as a floating promise.
-  `src/index.test.ts` carries a file-level `oxlint-disable
+  It is a known false positive, so `typescript/no-floating-promises` is off for
-typescript/no-floating-promises` with an explanatory comment. It is a known
+  `**/*.test.ts` in the `.oxlintrc.json` override rather than repeated as a
-  false positive, not a rule worth disabling project-wide (see
+  file-level header (see
   [tooling.md § oxlint-disable directives live next to the code](./tooling.md#oxlint-disable-directives-live-next-to-the-code)).

development/tooling.md

@@ -24,6 +24,8 @@ in [package.json](../package.json).
 - **@spences10/pi-lsp** — read-only LSP code intelligence for AI agents
   (project-local `.pi/settings.json`); talks to this repo's TypeScript 7 via
   `tsc --lsp --stdio`.
+- **vscode-languageserver-protocol** — LSP client and protocol types for the
+  autocomplete test helper (`src/util/__tests__/lsp-completion.ts`).
 
 When each runs is in
 [CONTRIBUTING.md § Feedback tiers](../CONTRIBUTING.md#feedback-tiers).
@@ -104,24 +106,55 @@ Type-aware oxlint is enabled via `options.typeAware: true` in `.oxlintrc.json`
 
 #### Decision (2026-09)
 
-Known type-aware false positives are silenced with source-level `oxlint-disable`
+A type-aware rule that false-positives **at one site** is silenced with a
-directives (see `src/pattern.ts`, `src/match.ts`, `src/index.test.ts`), not with
+source-level `oxlint-disable` directive (see `src/primitive-union.ts`). A rule that is
-rules disabled in `.oxlintrc.json`.
+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
+- A one-site disable sits next to the code it silences, visible to anyone
-  source.
+  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`: it hides the suppression from the
+- A project-wide disable in `.oxlintrc.json` for a one-site false positive: it
-  reader of the affected code.
+  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;
+- Both placements are _human_ last resorts. AI agents must neither add a source
-  they fix the type at its root (see [AGENTS.md § Never do](../AGENTS.md#never-do)).
+  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
+
+#### Decision (2026-09)
+
+A stylistic rule the project rejects is `"off"` in the `.oxlintrc.json` `rules`
+map, not silenced at a use site. Current entries: `eslint/capitalized-comments`
+(comments may start lowercase) and `eslint/no-ternary` (ternaries are allowed),
+joining the oxfmt-superseded rules already off.
+
+#### Why
+
+- The rule is wrong for the whole project, not mis-firing at one site, so there
+  is no line to annotate.
+- Keeping the two mechanisms separate keeps a source-level `oxlint-disable`
+  meaningful: it marks a lone exception.
+
+#### Rejected
+
+- A source-level `oxlint-disable` per use: the same exemption repeated at every
+  site, and oxfmt can move the site.
 
 ### `check:tsc` runs first
 
@@ -163,6 +196,50 @@ rules disabled in `.oxlintrc.json`.
   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)
+
+Pass `--ignore-packages @types/node`.
+
+#### Why
+
+- DT pins `@types/node`'s `latest` dist-tag to LTS (22.x); current-line types
+  ride other tags. Scan sees latest < installed — permanent "reverted", exit
+  1, zero signal. `--ignore-pre-releases` no help: 22.20.3 is stable.
+
+#### Rejected
+
+- `--types major,minor,patch`: hides real reverted reports elsewhere.
+- `@types/node@26.*`: tag stays wrong across majors; un-pin per bump = ritual.
+
+#### Known issue
+
+- A genuinely behind `@types/node` goes unreported; match it to
+  `.node-version` by hand.
+
 ### `attw` targets ESM-only
 
 #### Decision (2026-09)
@@ -202,6 +279,38 @@ project.
 - The same script works by hand (whole project) and staged (scoped), so there is
   no second command to maintain.
 
+## Language server tooling
+
+### `vscode-languageserver-protocol` backs the autocomplete helper
+
+#### Decision (2026-09)
+
+The autocomplete helper (`src/util/__tests__/lsp-completion.ts`) drives
+`tsc --lsp --stdio` through `vscode-languageserver-protocol`'s
+`createMessageConnection` and its typed request / notification objects, instead
+of a hand-rolled JSON-RPC client.
+
+#### Why
+
+- Framing, `Content-Length` parsing, the pending-request map and server-request
+  dispatch are protocol plumbing the helper only reimplemented; the official
+  client owns them and tolerates the server's `string | number` ids.
+- `InitializeRequest`, `CompletionRequest`, `DidOpenTextDocumentNotification`,
+  … carry their parameter and result types, so `CompletionList` / `CompletionItem`
+  replace the helper's ad-hoc shape guards.
+- The `./node` entry re-exports `vscode-jsonrpc/node`, so one devDependency
+  supplies both the transport and the protocol types. It is test-only and never
+  ships (`files` publishes `dist/` only).
+
+#### Rejected
+
+- `vscode-languageclient`: the editor-side client with a full feature registry
+  — far more than a test helper needs.
+- Generic JSON-RPC (`jsonrpc-lite`, `jayson`): still no LSP types, so they
+  replace framing only and leave the typed protocol surface unimplemented.
+- Keeping the hand-rolled client: the low-level shape is the maintenance cost
+  the helper exists to remove, and it must be re-audited against the server.
+
 ## Editor and agent tooling
 
 ### VSCode integration

development/workflow.md

@@ -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.

knip.json

@@ -1,5 +1,5 @@
 {
     "$schema": "./node_modules/knip/schema.json",
-    "entry": ["scripts/*.ts"],
+    "entry": ["src/index.ts", "scripts/*.ts"],
     "ignoreDependencies": ["@runwisp/pubv"]
 }

package.json

@@ -1,7 +1,7 @@
 {
     "name": "tiny-pattern-ts",
-    "version": "0.1.7",
+    "version": "0.8.1",
-    "description": "Pattern matching for TypeScript/ESM environments (F#-style, not regex)",
+    "description": "Exhaustive, type-safe pattern matching for TypeScript",
     "keywords": [
         "adt",
         "algebraic-data-types",
@@ -27,6 +27,10 @@
     ],
     "type": "module",
     "sideEffects": false,
+    "imports": {
+        "#test-utils/*": "./src/util/__tests__/*",
+        "#test-tiny-pattern-ts": "./src/index.ts"
+    },
     "exports": {
         ".": {
             "types": "./dist/index.d.ts",
@@ -44,7 +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",
@@ -53,9 +58,11 @@
         "create:release": "./scripts/release.sh",
         "maintain": "npm run maintain:knip; npm run maintain:outdated",
         "maintain:knip": "knip --include dependencies,exports,files",
-        "maintain:outdated": "check-outdated --ignore-pre-releases",
+        "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",
@@ -65,23 +72,28 @@
         "setup": "npm run setup:git-commit-message",
         "setup:git-commit-message": "git config commit.template commit-message-template"
     },
+    "dependencies": {
+        "type-fest": "^5.9.0"
+    },
     "devDependencies": {
         "@arethetypeswrong/cli": "^0.18.5",
         "@runwisp/pubv": "^1.5.1",
         "@tsconfig/node26": "^26.0.1",
         "@tsconfig/strictest": "^2.0.8",
-        "@types/node": "^26.4.1",
+        "@types/node": "^26.6.1",
         "c8": "^12.0.0",
         "check-outdated": "^3.0.0",
-        "cspell": "^10.2.2",
+        "cspell": "^10.3.2",
         "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",
-        "typescript": "^7.0.2"
+        "typescript": "^7.0.2",
+        "vscode-languageserver-protocol": "^3.18.3"
     },
     "engines": {
         "node": ">=26"

scripts/branch.sh

@@ -73,9 +73,9 @@ git show-ref --verify --quiet "refs/heads/${BASE}" || {
     exit 1
 }
 
-# Derive the remote rather than hardcoding it: this repo has `origin` (ssh) and
+# Derive the remote rather than hardcoding it: `main` tracks `origin` (ssh)
-# `origin_https`, and `main` tracks the latter — `git fetch origin main` would
+# here; a hardcoded name would check currency against a ref that may not
-# check currency against a ref that is never updated here.
+# exist on a differently configured clone.
 # `--quiet` echoes the unresolved `main@{upstream}` literal to stdout on
 # failure, so it cannot be paired with a `$(...) || fallback`; the non-quiet
 # form prints nothing on failure and the fallback runs.

scripts/create-doc-tests.ts

@@ -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;
+    }
+}

src/doc-test/README.md

@@ -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.

src/doc-test/tsconfig.json

@@ -0,0 +1,10 @@
+{
+    "extends": "../../tsconfig.json",
+    "compilerOptions": {
+        "noEmit": true,
+        "noUnusedLocals": false,
+        "noUnusedParameters": false
+    },
+    "include": ["__generated__/**/*.test.ts"],
+    "exclude": []
+}

src/index.test.ts

@@ -1,56 +1,27 @@
-/* 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 { test } from "node:test";
 
 import { expectTypeOf } from "expect-type";
 
-import { type Matcher, P, match } from "./index.ts";
+import {
+    getPrimitiveUnionMatcher,
+    getPrimitiveUnionMatcherW,
+    getTaggedUnionMatcher,
+    getTaggedUnionMatcherW,
+} from "./index.ts";
 
-test("match returns a builder", () => {
+// The published entry point is the barrel (`package.json` exports
-    const builder = match("x");
+// `./dist/index.js`), so every factory must be reachable from here. Importing it
-    expectTypeOf(builder).toHaveProperty("with");
+// also loads the module, which is what lets c8's `--all` measure it — see
-    expectTypeOf(builder).toHaveProperty("exhaustive");
+// development/ci.md § Coverage threshold.
-    expectTypeOf(builder).toHaveProperty("otherwise");
+test("index: the public entry point re-exports every matcher factory", () => {
-});
+    // Assert
-
+    expectTypeOf(getPrimitiveUnionMatcher).toBeFunction();
-test("P.literal narrows to its literal type", () => {
+    assert.equal(typeof getPrimitiveUnionMatcher, "function");
-    const matcher = P.literal("yes");
+    expectTypeOf(getPrimitiveUnionMatcherW).toBeFunction();
-    expectTypeOf(matcher).toMatchTypeOf<Matcher<"yes">>();
+    assert.equal(typeof getPrimitiveUnionMatcherW, "function");
-    assert.equal(matcher.matches("yes"), true);
+    expectTypeOf(getTaggedUnionMatcher).toBeFunction();
-    assert.equal(matcher.matches("no"), false);
+    assert.equal(typeof getTaggedUnionMatcher, "function");
-});
+    expectTypeOf(getTaggedUnionMatcherW).toBeFunction();
-
+    assert.equal(typeof getTaggedUnionMatcherW, "function");
-test("P.type narrows to the typeof target", () => {
-    const matcher = P.type<string>("string");
-    expectTypeOf(matcher).toMatchTypeOf<Matcher<string>>();
-    assert.equal(matcher.matches("hi"), true);
-    assert.equal(matcher.matches(42), false);
-});
-
-test("exhaustive() returns the union of handler return types", () => {
-    const result = match<"a" | "b">("a")
-        .with(P.literal("a"), () => 1 as const)
-        .with(P.literal("b"), () => "two" as const)
-        .exhaustive();
-
-    expectTypeOf(result).toEqualTypeOf<1 | "two">();
-    assert.equal(result, 1);
-});
-
-test("otherwise() falls back when no case matches", () => {
-    const result = match<"x" | "y" | "z">("z")
-        .with(P.literal("x"), (v): string => `got ${v}`)
-        .otherwise((v): string => `fallback ${v}`);
-    assert.equal(result, "fallback z");
-});
-
-test("exhaustive throws when no case matches", () => {
-    assert.throws(
-        () =>
-            match<"a" | "b" | "c">("c")
-                .with(P.literal("a"), () => "A")
-                .with(P.literal("b"), () => "B")
-                .exhaustive(),
-        /no matching case/,
-    );
 });

src/index.ts

@@ -1,2 +1,16 @@
-export { match, P } from "./match.ts";
+/**
-export type { Matcher, Pattern } from "./pattern.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";

src/match.ts

@@ -1,62 +0,0 @@
-import { P, type Matcher, type Pattern } from "./pattern.ts";
-
-type Cases<R> = readonly (readonly [Matcher<unknown>, (value: unknown) => R])[];
-
-interface MatchBuilder<T, R> {
-    with<U extends T, V>(
-        pattern: Matcher<U>,
-        handler: (value: U) => V,
-    ): MatchBuilder<T, R | V>;
-    exhaustive(): R;
-    otherwise(handler: (value: T) => R): R;
-}
-
-const buildMatch = <T, R>(value: T, cases: Cases<R>): MatchBuilder<T, R> => {
-    const apply = (): R | undefined => {
-        for (const [matcher, handler] of cases) {
-            if (matcher.matches(value)) {
-                return handler(value);
-            }
-        }
-        return undefined;
-    };
-
-    const builder = {
-        with<U extends T, V>(
-            pattern: Matcher<U>,
-            handler: (value: U) => V,
-        ): MatchBuilder<T, R | V> {
-            const nextCases: Cases<R | V> = [
-                ...cases,
-                // oxlint-disable-next-line typescript/no-unsafe-type-assertion
-                [pattern, handler as (value: unknown) => R | V],
-            ];
-            return buildMatch(value, nextCases);
-        },
-        exhaustive(): R {
-            const result = apply();
-            if (result === undefined) {
-                throw new Error(
-                    "tiny-pattern-ts: match.exhaustive() called with no matching case",
-                );
-            }
-            return result;
-        },
-        otherwise(handler: (value: T) => R): R {
-            for (const [matcher, run] of cases) {
-                if (matcher.matches(value)) {
-                    return run(value);
-                }
-            }
-            return handler(value);
-        },
-    };
-
-    return builder;
-};
-
-export const match = <T>(value: T): MatchBuilder<T, never> =>
-    buildMatch<T, never>(value, []);
-
-export type { Matcher, Pattern };
-export { P };

src/matcher-shared.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
+>;

src/pattern.ts

@@ -1,105 +0,0 @@
-/**
- * Pattern matching primitives. Each constructor returns a lightweight
- * matcher object whose `matches` method returns a type guard.
- */
-
-export interface Matcher<T> {
-    readonly matches: (value: unknown) => value is T;
-}
-
-const literalMatcher = <
-    const L extends string | number | boolean | null | undefined,
->(
-    value: L,
-): Matcher<L> => ({
-    matches: (candidate): candidate is L => candidate === value,
-});
-const typeMatcher = <T>(
-        type:
-            | "string"
-            | "number"
-            | "boolean"
-            | "bigint"
-            | "symbol"
-            | "undefined"
-            | "object"
-            | "function",
-    ): Matcher<T> => {
-        const matches = (value: unknown): value is T => {
-            if (type === "undefined") {
-                return value === undefined;
-            }
-            if (type === "object") {
-                return (
-                    (typeof value === "object" && value !== null) ||
-                    typeof value === "function"
-                );
-            }
-            return typeof value === type;
-        };
-        return { matches };
-    },
-    whenMatcher = <T>(
-        predicate: (value: unknown) => value is T,
-    ): Matcher<T> => ({
-        matches: predicate,
-    }),
-    whenMatcherAny = <T>(
-        predicate: (value: unknown) => boolean,
-    ): Matcher<T> => ({
-        matches: (value: unknown): value is T => predicate(value),
-    }),
-    isNestedMatcher = (expected: unknown): expected is Matcher<unknown> =>
-        typeof expected === "object" &&
-        expected !== null &&
-        "matches" in expected,
-    // oxlint-disable-next-line typescript/no-unnecessary-type-parameters
-    keysMatch = <S extends object>(shape: S, candidate: object): boolean => {
-        // oxlint-disable-next-line typescript/no-unsafe-type-assertion
-        for (const key of Object.keys(shape) as (keyof S)[]) {
-            if (!(key in candidate)) {
-                return false;
-            }
-            // oxlint-disable-next-line typescript/no-unsafe-type-assertion
-            const expected = shape[key],
-                // oxlint-disable-next-line typescript/no-unsafe-type-assertion
-                actual = candidate[key as keyof object];
-            if (isNestedMatcher(expected)) {
-                if (!expected.matches(actual)) {
-                    return false;
-                }
-            } else if (actual !== expected) {
-                return false;
-            }
-        }
-        return true;
-    },
-    structuralMatcher = <S extends object, T extends S>(
-        shape: S,
-        refine?: (value: S) => value is T,
-    ): Matcher<T> => ({
-        matches: (value: unknown): value is T => {
-            if (typeof value !== "object" || value === null) {
-                return false;
-            }
-            // oxlint-disable-next-line typescript/no-unsafe-type-assertion
-            const candidate = value as S;
-            if (!keysMatch(shape, candidate)) {
-                return false;
-            }
-            if (refine && !refine(candidate)) {
-                return false;
-            }
-            return true;
-        },
-    });
-
-export const P = {
-    literal: literalMatcher,
-    type: typeMatcher,
-    when: whenMatcher,
-    any: whenMatcherAny,
-    shape: structuralMatcher,
-} as const;
-
-export type Pattern<T> = Matcher<T>;

src/primitive-union.ts

@@ -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;

src/tagged-union.ts

@@ -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;

src/util/__tests__/lsp-completion.test.ts

@@ -0,0 +1,147 @@
+import { strict as assert } from "node:assert";
+import path from "node:path";
+import { test } from "node:test";
+
+import { expectTypeOf } from "expect-type";
+
+import {
+    type CompletionResult,
+    type CompletionTarget,
+    LspSession,
+} from "#test-utils/lsp-completion.ts";
+
+const REPO_ROOT = path.resolve(import.meta.dirname, "../../..");
+
+// The `#test-utils/*` self-reference (package.json#imports) is the one route
+// scattered test files take to reach test helpers; this pins that it resolves
+// and types without starting a language server
+// (development/testing.md § Test helpers).
+test("test-helper home: `#test-utils/…` resolves to the helper and types it", () => {
+    // Arrange
+    const target: CompletionTarget = {
+        file: "src/util/__tests__/lsp-completion.ts",
+        source: "",
+    };
+
+    // Assert — no session is constructed: the constructor spawns `tsc --lsp`,
+    // so only the resolved values and their types are checked here.
+    expectTypeOf(LspSession).toBeConstructibleWith("repo-root");
+    expectTypeOf<CompletionResult>().toMatchTypeOf<{
+        labels: readonly string[];
+    }>();
+    assert.equal(typeof LspSession, "function");
+    assert.equal(target.file, "src/util/__tests__/lsp-completion.ts");
+});
+
+// The helper drives a language server, so its contract is asserted against the
+// server. Every document below is probed from memory and carries its
+// contextual type inline, so the helper is tested without the library's code.
+const probe = (source: string, marker?: string): Promise<CompletionResult> => {
+    const session = new LspSession(REPO_ROOT);
+    const target: CompletionTarget =
+        marker === undefined
+            ? { file: "src/__probe.ts", source }
+            : { file: "src/__probe.ts", source, marker };
+    return session.completionLabelsAt(target).finally(() => session.close());
+};
+
+const HANDLERS = [
+    "type Handlers = { a: () => number; b: () => number; _?: () => number };",
+    "declare const apply: (handlers: Handlers) => Handlers;",
+];
+
+test("helper: a fresh object literal completes with its contextual keys", () => {
+    // Arrange
+    const source = [
+        ...HANDLERS,
+        "const done = apply({",
+        "    /*COMPLETE*/",
+        "});",
+        "export { done };",
+    ].join("\n");
+
+    // Act
+    const probed = probe(source);
+
+    // Assert — the position is the marker's, which the helper strips
+    expectTypeOf(probed).toEqualTypeOf<Promise<CompletionResult>>();
+    return probed.then((result) => {
+        assert.deepEqual(result.position, { line: 3, character: 4 });
+        assert.deepEqual([...result.labels], ["_?", "a", "b"]);
+    });
+});
+
+test("helper: a handled key drops out of the popup", () => {
+    // Arrange
+    const source = [
+        ...HANDLERS,
+        "const done = apply({",
+        "    b: () => 1,",
+        "    /*COMPLETE*/",
+        "});",
+        "export { done };",
+    ].join("\n");
+
+    // Act
+    const probed = probe(source);
+
+    // Assert
+    expectTypeOf<CompletionResult["labels"]>().toEqualTypeOf<
+        readonly string[]
+    >();
+    return probed.then((result) => {
+        assert.deepEqual([...result.labels], ["_?", "a"]);
+    });
+});
+
+test("helper: a custom marker is located at the line start", () => {
+    // Arrange
+    const source = [
+        "type Handlers = { a: () => number; _?: () => number };",
+        "declare const apply: (handlers: Handlers) => Handlers;",
+        "const done = apply({",
+        "@@@",
+        "});",
+        "export { done };",
+    ].join("\n");
+
+    // Act
+    const probed = probe(source, "@@@");
+
+    // Assert
+    expectTypeOf(probed).resolves.toEqualTypeOf<CompletionResult>();
+    return probed.then((result) => {
+        assert.deepEqual(result.position, { line: 3, character: 0 });
+        assert.deepEqual([...result.labels], ["_?", "a"]);
+    });
+});
+
+test("helper: labels are read from the server, not from a pattern literal", () => {
+    // Arrange
+    const source = [
+        'const text = "x";',
+        "const upper = text./*COMPLETE*/;",
+        "export { upper };",
+    ].join("\n");
+
+    // Act
+    const probed = probe(source);
+
+    // Assert
+    expectTypeOf(probed).resolves.toHaveProperty("labels");
+    return probed.then((result) => {
+        assert.ok(result.labels.includes("toUpperCase"));
+    });
+});
+
+test("helper: a source without the marker rejects", () => {
+    // Arrange
+    const source = "const text = 1;\nexport { text };\n";
+
+    // Act
+    const probed = probe(source);
+
+    // Assert
+    expectTypeOf(probed).resolves.toEqualTypeOf<CompletionResult>();
+    return assert.rejects(probed, /marker not found/);
+});

src/util/__tests__/lsp-completion.ts

@@ -0,0 +1,254 @@
+// oxlint-disable no-magic-numbers unicorn/no-null - tolerable here, this is a helper
+import { spawn, type ChildProcessWithoutNullStreams } from "node:child_process";
+import fs from "node:fs";
+import path from "node:path";
+import { setTimeout as delay } from "node:timers/promises";
+import { pathToFileURL } from "node:url";
+
+import {
+    type CompletionItem,
+    type CompletionList,
+    CompletionRequest,
+    ConfigurationRequest,
+    createMessageConnection,
+    DidOpenTextDocumentNotification,
+    InitializedNotification,
+    InitializeRequest,
+    type MessageConnection,
+    ShutdownRequest,
+    StreamMessageReader,
+    StreamMessageWriter,
+} from "vscode-languageserver-protocol/node";
+
+/**
+ * Print the completion labels TypeScript's language server offers at a marker
+ * inside a source file.
+ *
+ * Usage: node --strip-types src/util/__tests__/lsp-completion.ts <file> [<marker>]
+ *
+ * The marker (default `DEFAULT_MARKER`, a `COMPLETE` block comment) is stripped
+ * from the source before it is sent; the request position is where the marker
+ * stood. Point it at a scratch file whose expected type is a matcher pattern to
+ * inspect the popup.
+ *
+ * `LspSession` exposes the same probe to the test suite, so completion is
+ * asserted against the server rather than the type system.
+ *
+ * Why a script and not a type assertion: completion is a contextual-type
+ * property that `expect-type` cannot observe, and `Parameters<…>` resolves only
+ * a single overload. The server is the only ground truth. See
+ * development/testing.md § Autocomplete.
+ */
+
+const DEFAULT_MARKER = "/*COMPLETE*/";
+const ARGV_PREFIX_LENGTH = 2;
+const EXIT_DELAY_MS = 200;
+const FAILURE_EXIT_CODE = 1;
+const USAGE = "usage: lsp-completion.ts <file> [<marker>]";
+
+export interface Position {
+    readonly line: number;
+    readonly character: number;
+}
+
+/** A document to probe: `source` carries `marker`, which is stripped. */
+export interface CompletionTarget {
+    /** Path relative to the session's repo root; drives URI and resolution. */
+    readonly file: string;
+    readonly source: string;
+    readonly marker?: string;
+}
+
+export interface CompletionResult {
+    readonly position: Position;
+    readonly labels: readonly string[];
+}
+
+const completionLabels = (
+    result: CompletionItem[] | CompletionList | null,
+): readonly string[] => {
+    if (result === null) {
+        return [];
+    }
+    const items = Array.isArray(result) ? result : result.items;
+    return items
+        .map((item) => item.label)
+        .toSorted((left, right) => left.localeCompare(right));
+};
+
+const markerPosition = (source: string, marker: string): Position => {
+    const index = source.indexOf(marker);
+    if (index === -1) {
+        throw new Error(`marker not found: ${marker}`);
+    }
+    const lines = source.slice(0, index).split("\n");
+    const lineIndex = lines.length - 1;
+    const line = lines[lineIndex];
+    return { line: lineIndex, character: line === undefined ? 0 : line.length };
+};
+
+/**
+ * One language server, initialized on first use, shared across probes. Callers
+ * own the lifecycle and must `close()` it.
+ */
+export class LspSession {
+    readonly #child: ChildProcessWithoutNullStreams;
+    readonly #connection: MessageConnection;
+    readonly #repoRoot: string;
+    #ready: Promise<void> | undefined;
+
+    public constructor(repoRoot: string) {
+        this.#repoRoot = repoRoot;
+        this.#child = spawn(
+            path.join(repoRoot, "node_modules", ".bin", "tsc"),
+            ["--lsp", "--stdio"],
+            { cwd: repoRoot },
+        );
+        this.#child.stderr.on("data", (chunk: Buffer) => {
+            process.stderr.write(chunk);
+        });
+        this.#connection = createMessageConnection(
+            new StreamMessageReader(this.#child.stdout),
+            new StreamMessageWriter(this.#child.stdin),
+        );
+        // Server -> client requests the server waits on: answer so it proceeds.
+        // `workspace/configuration` wants one reply per requested item; a
+        // catch-all covers the rest (e.g. `client/registerCapability`).
+        this.#connection.onRequest(ConfigurationRequest.type, ({ items }) =>
+            items.map(() => null),
+        );
+        this.#connection.onRequest(() => null);
+        this.#connection.listen();
+    }
+
+    public completionLabelsAt(
+        target: CompletionTarget,
+    ): Promise<CompletionResult> {
+        return this.#ensureInitialized()
+            .then(() => this.#open(target))
+            .then(({ uri, position }) =>
+                this.#connection
+                    .sendRequest(CompletionRequest.type, {
+                        textDocument: { uri },
+                        position,
+                        context: { triggerKind: 1 },
+                    })
+                    .then((result) => ({
+                        position,
+                        labels: completionLabels(result),
+                    })),
+            );
+    }
+
+    /** `shutdown` + close stdin, then kill the server; safe after a failed probe. */
+    public close(): Promise<void> {
+        return (
+            this.#connection
+                .sendRequest(ShutdownRequest.type)
+                // The TS 7 Go server logs a bare `context canceled` to stderr
+                // when it handles `exit`; EOF on stdin shuts it down cleanly
+                // (exit 0, no output) instead.
+                .then(() => {
+                    this.#child.stdin.end();
+                })
+                .then(() => delay(EXIT_DELAY_MS))
+                .finally(() => {
+                    this.#connection.dispose();
+                    this.#child.kill();
+                })
+        );
+    }
+
+    #ensureInitialized(): Promise<void> {
+        this.#ready ??= this.#initialize();
+        return this.#ready;
+    }
+
+    #initialize(): Promise<void> {
+        return this.#connection
+            .sendRequest(InitializeRequest.type, {
+                processId: process.pid,
+                rootUri: pathToFileURL(this.#repoRoot).href,
+                workspaceFolders: [
+                    { uri: pathToFileURL(this.#repoRoot).href, name: "repo" },
+                ],
+                capabilities: {
+                    textDocument: {
+                        completion: {
+                            completionItem: { snippetSupport: false },
+                        },
+                        publishDiagnostics: {},
+                    },
+                },
+            })
+            .then(() =>
+                this.#connection.sendNotification(
+                    InitializedNotification.type,
+                    {},
+                ),
+            );
+    }
+
+    #open(target: CompletionTarget): Promise<{
+        readonly uri: string;
+        readonly position: Position;
+    }> {
+        const absolute = path.resolve(this.#repoRoot, target.file);
+        const marker = target.marker ?? DEFAULT_MARKER;
+        const position = markerPosition(target.source, marker);
+        const text = target.source.replace(marker, "");
+        const uri = pathToFileURL(absolute).href;
+        // The server handles `didOpen` in order before the completion request,
+        // so no settle delay is needed.
+        return this.#connection
+            .sendNotification(DidOpenTextDocumentNotification.type, {
+                textDocument: {
+                    uri,
+                    languageId: "typescript",
+                    version: 1,
+                    text,
+                },
+            })
+            .then(() => ({ uri, position }));
+    }
+}
+
+const main = (args: readonly string[]): Promise<void> => {
+    const [file, markerArgument] = args;
+    if (file === undefined) {
+        process.stderr.write(`${USAGE}\n`);
+        process.exitCode = FAILURE_EXIT_CODE;
+        return Promise.resolve();
+    }
+
+    const repoRoot = path.resolve(import.meta.dirname, "../../..");
+    const absolute = path.resolve(repoRoot, file);
+    const source = fs.readFileSync(absolute, "utf8");
+    const session = new LspSession(repoRoot);
+    return session
+        .completionLabelsAt({
+            file,
+            source,
+            marker: markerArgument ?? DEFAULT_MARKER,
+        })
+        .then(({ position, labels }) => {
+            process.stdout.write(
+                `\n[${file}] completions @ ${position.line}:${position.character}:\n${labels.join(", ")}\n`,
+            );
+        })
+        .finally(() => session.close());
+};
+
+const isEntryPoint = (): boolean => {
+    const [entry] = process.argv.slice(1, 2);
+    return entry !== undefined && import.meta.url === pathToFileURL(entry).href;
+};
+
+if (isEntryPoint()) {
+    main(process.argv.slice(ARGV_PREFIX_LENGTH)).catch((error: unknown) => {
+        process.stderr.write(
+            `${error instanceof Error ? error.message : String(error)}\n`,
+        );
+        process.exitCode = FAILURE_EXIT_CODE;
+    });
+}

tsconfig.json

@@ -8,5 +8,6 @@
         "allowImportingTsExtensions": true,
         "verbatimModuleSyntax": true
     },
-    "include": ["src", "scripts"]
+    "include": ["src", "scripts"],
+    "exclude": ["src/doc-test"]
 }