Custom examples for property(..., context(), ...)?

💬 Question and Help

It is unclear to me how to provide a custom example for the following:

fc.assert(
  fc.property(fc.string(), fc.context(), (str, ctx) => /* ... */),
  { examples: [["x", /* what here? */]] }
);

context() seems to instantiate an class which is not exported. I would prefer not to make my own implementation of ContextValue.

It would be the most convenient not to have to include a value corresponding to the context in the examples at all – maybe the arbitraries provided to the property call could be used if there are more of them than elements in the example tuples – but any solution that gets this to work is good enough. Thanks!

[dubzzz]

Hello,

There is today no way to build partial or incomplete examples. A proposal to do so was suggested in the issue https://github.com/dubzzz/fast-check/issues/710#issuecomment-657102743 but for the moment it has not been implemented.

Regarding ContextValue for the moment, the implementation is not exposed to the user. I should probably either expose the class or add something like fc.contextFor() as I did for fc.scheduler() (see fc.schedulerFor()).

Given what has been released so far, the only trick I’d have is:

const buildContext = () => fc.sample(fc.context(), {numRuns:1})[0];
fc.assert(
  fc.property(fc.string(), fc.context(), (str, ctx) => /* ... */),
  { examples: [["x", buildContext()]] }
);

Not perfect but it should work well.

[tskj]

Sure thing!

I have submitted a proper bug report here, and also two suggestions for improving the docs if fixing the bug will never be on the roadmap here and here!