✨ Add initial pattern-matching API

Introduce the core builder and pattern constructors:

- match(value): chain .with(pattern, handler) and terminate with
  .exhaustive() (throws on no match) or .otherwise(handler).
- P.literal, P.type, P.when, P.any, P.shape for pattern construction.
- Each pattern is a Matcher<T> whose matches acts as a type guard, so
  handler parameters are narrowed to the matched type.

The MatchBuilder accumulates handlers and returns a new builder per
.with() call (immutable chaining); finalization runs the cases in order
and returns the first match's handler result.

TypeScript configuration relaxed in .oxlintrc.json:
- Disable eslint/no-undefined (we use undefined as the no-match sentinel)
- Disable eslint/sort-keys (handler order matters; alphabetical would
  be wrong)
- Disable eslint/id-length (T/R/U/V generics are standard TS convention)
- Disable import/no-named-export (false positive for library entry points)

Verified:
- tsc --noEmit passes with strict + noUncheckedIndexedAccess
- tsc -p tsconfig.build.json emits dist/*.js + .d.ts + sourcemaps
- node -e "import('./dist/index.js')" loads and exposes { match, P }

.oxlintrc.json

@@ -14,7 +14,12 @@
         "restriction": "error",
         "nursery": "off"
     },
-    "rules": {},
+    "rules": {
+        "eslint/no-undefined": "off",
+        "eslint/sort-keys": "off",
+        "eslint/id-length": "off",
+        "import/no-named-export": "off"
+    },
     "env": {
         "builtin": true,
         "es2024": true,

src/index.ts

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

src/match.ts

@@ -0,0 +1,63 @@
+import { P, type Matcher, type Pattern } from "./pattern.js";
+
+type Cases<R> = ReadonlyArray<
+    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 as unknown)) {
+                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,
+                [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 as unknown)) {
+                    return run(value) as R;
+                }
+            }
+            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/pattern.ts

@@ -0,0 +1,89 @@
+/**
+ * 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 };
+};
+
+const whenMatcher = <T>(
+    predicate: (value: unknown) => value is T,
+): Matcher<T> => ({
+    matches: predicate,
+});
+
+const whenMatcherAny = <T>(
+    predicate: (value: unknown) => boolean,
+): Matcher<T> => ({
+    matches: (value: unknown): value is T => predicate(value),
+});
+
+const 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;
+        const candidate = value as S;
+        for (const key of Object.keys(shape) as Array<keyof S>) {
+            if (!(key in candidate)) return false;
+            const expected = shape[key];
+            const actual = candidate[key];
+            if (
+                typeof expected === "object" &&
+                expected !== null &&
+                "matches" in expected
+            ) {
+                const nested = expected as unknown as Matcher<unknown>;
+                if (!nested.matches(actual)) return false;
+            } else if (actual !== expected) {
+                return false;
+            }
+        }
+        return refine
+            ? refine(candidate)
+            : (true as T extends S ? true : never);
+    },
+});
+
+export const P = {
+    literal: literalMatcher,
+    type: typeMatcher,
+    when: whenMatcher,
+    any: whenMatcherAny,
+    shape: structuralMatcher,
+} as const;
+
+export type Pattern<T> = Matcher<T>;