# Library design

The type-level design of the public API and the limitations it carries. The
user-facing reference is [README § API](../README.md#api).

The matchers are implemented in `src/primitive.ts` (`getMatcher` /
`getMatcherW`) and `src/tagged-union.ts` (`getTaggedUnionMatcher` /
`getTaggedUnionMatcherW`), re-exported from `src/index.ts`; the rest of the
library is placeholder code.

## 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 = getMatcher<"a" | "b">()({ a: (s) => …, b: (s) => … });
const fallback = getMatcher<"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:

- `getMatcher` — one common `R`; the fallback must fit it;
- `getMatcherW` — 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 four universe-agnostic pieces both matchers
use: `UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`.

#### Why

- `RedundantFallback`'s property name is the diagnostic, so one definition
  keeps the two matchers' message from drifting; the other three appear verbatim
  in both public signatures.

#### Rejected

- **A generic `Matcher<Universe>` over the interface pair, `Handlers`,
  `Fallback` and `MustBePartial`.** Each is built from its own universe
  (`PatternKey`/`PatternParam` vs `Tags`/`MapTaggedUnion`); abstracting over the
  F-bounded `Handled` constraint that makes the remainder work risks the
  contextual typing it exists to preserve.

## Tagged-union matcher

#### Decision (2026-09)

`getTaggedUnionMatcher` / `getTaggedUnionMatcherW` mirror the primitive 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 matcher.** `HandledMembers`
  maps the handled tags to their members and `Exclude<T, …>` 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 dispatch's `shape as string | number`.
- **`MapTaggedUnion` distributes with `Extract`.** A duplicated tag yields a
  union of members instead of dropping one.

#### Known issue

- Tags are `string | number` only. A `boolean` / `null` / `undefined`
  discriminant (`{ ok: true } | { ok: false }`) is rejected by `Discriminated`,
  because those values are not property keys; supporting them needs the
  primitive matcher's `PatternKey` / `PatternParam` projection.
- A member's tag must be unique across the union; two members with the same tag
  collapse to a union under one handler.

## 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 `PatternParam` inverts
it, so callbacks receive the real member (`true`, not `"true"`). The popup
offers `true`, `false`, `null`, `undefined` by name (verified over LSP).

#### 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 unguarded: user-facing, stated once in
  [README § Caveats](../README.md#caveats).