Add fc.assertProperty function

🚀 Feature Request

Add a function fc.assertProperty(...args) that’s equivalent to fc.assert(fc.property(...args)). For consistency, there should perhaps also be fc.checkProperty, equivalent to fc.check(fc.property(...args)).

Motivation

Pretty much all of my tests look like fc.assert(fc.property(...args)). It’d be nice to avoid the extra layer of nesting and just have an assertProperty function for these common cases. I migrated from js-verify to fast-check, and love it so far, but the lack of an analog to js-verify’s assertforall has been a bit of a nuisance.

While I could easily define this function myself, it’s actually a little messy to define its type, since fc.property has so many overloaded signatures, and TS offers no easy way to have my wrapper function inherit all those overloads, so I’d have to copy and paste them.

Example

it('should satisfy some property', () => {
  fc.assertProperty(fc.boolean(), (bool) => true);
}

[jdmarshall]

So first of all, Mocha suggests you not use arrow functions if you want to be able to utilize mocha internals at all, which I expect you might.

I think the interface could be even simpler than suggested above. If I work from the project description toward an implementation instead of vice versa, then I think tests would look something like this:

let fc = require("fast-check");
let all = fc.all;

describe("something()", function() {
    all("should satisfy some constraint", function(a = fc.boolean(), b = fc.boolean()) {
        let reply = something(a, b);    
        expect(reply).to.be.true;
    });
});

That’s it. All the magic is on one line. Everything else is bog-standard mocha, jest, or whatever. Nothing new to learn, no weird stack traces or unusual breakpoint stepping.

[dubzzz]

Thanks a lot for the suggestion. This is indeed a recurring question or suggestion for the lib. And I really think it will be useful to come with it quickly.

IMO one cool thing would be to directly provide test wrappers, such as it or test, with property based capabilities.

My target will be to provide some kind of itProp that could be used as follow:

import { itProp, fc } from "jest-fast-check"; // not yet published
itProp('should satisfy some property', [fc.boolean()], () => true);

It would directly wrap it, fc.assert and fc.property behind a single function.

I initially drafted this suggestion for ava but I’ll soon implement it for others (see https://github.com/dubzzz/ava-fast-check/).

@ethanresnick Would it fit your need? If so I’ll come back to you as soon as I get an implementation for other test runners.