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type UnaryFn<T, R> = (shape: T) => R;
// `Extract` drops optional handlers (`undefined`) so `PatternReturns` also works
// when `P`'s constraint has optional keys.
type PatternReturns<P> = ReturnType<
Extract<ValueOf<P>, (...args: never[]) => unknown>
>;
type Handlers<T extends string | number, R> = { [K in T]: UnaryFn<K, R> };
// The fallback is a *second argument*, not a property of the handler map,
// because its parameter is the remainder `Exclude<T, keyof Handled>` and TypeScript
// fixes a property's contextual type before it infers its sibling keys. A later
// argument, by contrast, is contextually typed from inference on an earlier
// one, so the split is what makes the remainder expressible at all.
// See development/library.md.
type Fallback<T extends string | number, Handled, R> = UnaryFn<
Exclude<T, keyof Handled>,
R
>;
// TypeScript does not apply the excess-property check to a generic constraint,
// so `Exact` restores it for the generic forms: a handler map can otherwise
// carry keys outside `T`.
// oxlint-disable typescript/unified-signatures
// Strict returns: one common `R`. Overload order is load-bearing:
// #1 Handlers (first) -> the exhaustive form and the autocomplete popup
// #2 Fallback (last) -> accepts a partial handler map plus a fallback
interface MatcherStrict<T extends string | number> {
<R>(handlers: Handlers<T, R>): UnaryFn<T, R>;
<R, Handled extends Exact<Partial<Handlers<T, R>>, Handled>>(
handlers: Handled & Partial<Handlers<T, R>>,
fallback: Fallback<T, Handled, R>,
): UnaryFn<T, R>;
}
// oxlint-enable typescript/unified-signatures
// Widened returns: the union of every handler's return type. `P` is inferred
// from the whole handler map, whose closed constraint supplies the
// contextual/autocomplete type.
// oxlint-disable typescript/unified-signatures
interface MatcherWidening<T extends string | number> {
<P extends Exact<Handlers<T, unknown>, P>>(
handlers: P,
): UnaryFn<T, PatternReturns<P>>;
<R, Handled extends Exact<Partial<Handlers<T, unknown>>, Handled>>(
handlers: Handled,
fallback: Fallback<T, Handled, R>,
): UnaryFn<T, PatternReturns<Handled> | R>;
}
// oxlint-enable typescript/unified-signatures
type HandlerMap = Record<string | number, UnaryFn<never, unknown> | undefined>;
const dispatch =
(handlers: HandlerMap, fallback?: UnaryFn<never, unknown>) =>
(shape: string | number): unknown =>
(
handlers[shape] ??
fallback ??
(() => {
throw new Error(`Unhandled shape: ${shape}`);
})
)(
// oxlint-disable-next-line typescript/no-unsafe-type-assertion
shape as never,
);
export const getMatcher = <T extends string | number>(): MatcherStrict<T> =>
dispatch;
export const getMatcherW = <T extends string | number>(): MatcherWidening<T> =>
dispatch;
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