tiny-pattern-ts/compat/fixture.ts

// Consumer smoke test for the minimum supported TypeScript (see README
// § Requirements). It imports the package by name, so it resolves through the
// `exports` map to the emitted `dist/*.d.ts` — including the relative `.ts`
// specifiers they keep — rather than to the source. Run by `npm run test:compat`
// and the CI `compat` job only; never by `check` / `verify`.
//
// Why the `expectTypeOf` assertions: a compile that merely succeeds is a weak
// oracle. An `any`-typed declaration would compile, but `expect-type`'s exact
// equality rejects `any`, so the assertions prove the emitted types are real.
// Every handler *parameter* is asserted too, not just the matcher: the handler's
// member type comes from the `Member` inversion in the declarations, and a wrong
// or `any` parameter would otherwise slip through, since the matcher's own
// `(shape: T) => R` type is built from `T` and the handler returns. See
// development/ci.md § TypeScript compatibility.

import { expectTypeOf } from "expect-type";
import {
    getPrimitiveUnionMatcher,
    getPrimitiveUnionMatcherW,
    getTaggedUnionMatcher,
    getTaggedUnionMatcherW,
} from "tiny-pattern-ts";

type ResultCode = "ok" | "created";

const toStatus = getPrimitiveUnionMatcher<ResultCode>()({
    ok: (s) => {
        expectTypeOf(s).toEqualTypeOf<"ok">();
        return "OK";
    },
    created: (s) => {
        expectTypeOf(s).toEqualTypeOf<"created">();
        return "CREATED";
    },
});

expectTypeOf(toStatus).toEqualTypeOf<(shape: ResultCode) => string>();

// The widening twin keeps each handler's own return type in the union.
const toStatusW = getPrimitiveUnionMatcherW<ResultCode>()(
    {
        ok: (s) => {
            expectTypeOf(s).toEqualTypeOf<"ok">();
            return "OK" as const;
        },
    },
    (rest) => {
        expectTypeOf(rest).toEqualTypeOf<"created">();
        return "CREATED" as const;
    },
);

expectTypeOf(toStatusW).toEqualTypeOf<
    (shape: ResultCode) => "OK" | "CREATED"
>();

type Contact =
    | { kind: "email"; address: string }
    | { kind: "phone"; number: string };

const format = getTaggedUnionMatcher<Contact>()("kind")({
    email: (e) => {
        expectTypeOf(e).toEqualTypeOf<{ kind: "email"; address: string }>();
        return e.address;
    },
    phone: (p) => {
        expectTypeOf(p).toEqualTypeOf<{ kind: "phone"; number: string }>();
        return p.number;
    },
});

expectTypeOf(format).toEqualTypeOf<(shape: Contact) => string>();

const formatW = getTaggedUnionMatcherW<Contact>()("kind")(
    {
        email: (e) => {
            expectTypeOf(e).toEqualTypeOf<{ kind: "email"; address: string }>();
            return e.address;
        },
    },
    (rest) => {
        expectTypeOf(rest).toEqualTypeOf<{ kind: "phone"; number: string }>();
        return rest.number;
    },
);

expectTypeOf(formatW).toEqualTypeOf<(shape: Contact) => string>();

export { format, formatW, toStatus, toStatusW };