Wrapping fast-check as a DSL for topic-specific tests

[ornamentist]

I'm really enjoying fast-check for property based testing in JavaScript and TypeScript.

I'd like to take things further by wrapping property tests in a topic-specific testing DSL in TypeScript:

type Swans = ...

theorem("all swans are white", swans(100), (s: Swan) => {
  ...
});

Where theorem would wrap say a Mocha test and fast-check properties via function overloads:

import * as Mocha from "mocha";
import * as Check from "fast-check";

function theorem<T>(a1: Check.Arbitrary<T>, c: (x: T) => boolean | void): void;
function theorem<T, U>(a1: Check.Arbitrary<T>, a2: Check.Arbitrary<U>, c: (x: T, y: U) => boolean | void): void;
// more function overloads

function theorem<T, U = T,...>(?): void {
  Mocha.it("foo", () => Check.assert(Check.property(?)));
}

I'm not sure how to type the theorem function body. Should I be using the user-facing fast-check types based on <T extends [unknown, ...unknown[]]> or the same approach that test runners use in wrapping fast-check?

Any advice much appreciated.

[dubzzz]

You can probably have a look to the typing declared for test.prop in @fast-check/{vitest,jest} (in this repository). They seem pretty close to your needs.

[ornamentist]

Thanks -- fast-check is a seriously useful project.

[ornamentist]

This looks useful:

// ...

/**
 * prop has just been declared for typing reasons, ideally TestProp should be enough
 * and should be used to replace `{ prop: typeof prop }` by `{ prop: TestProp<???> }`
 */
declare const prop: <Ts, TsParameters extends Ts = Ts>(
  arbitraries: Ts extends [any] | any[] ? ArbitraryTuple<Ts> : ArbitraryRecord<Ts>,
  params?: FcParameters<TsParameters>
) => (
  testName: string,
  prop: Ts extends [any] | any[] ? Prop<Ts> : PropRecord<Ts>,
  timeout?: number | undefined
) => void;