Proposal: Negate/Invert properties

I am currently in the situation where I wrote a complex property and want to test that a specific value does not match it. I’m unsure if there is currently a way to do this. Here is an example of what I would have in mind:

let prop1 thing = ...

let prop2 thing = ...

let bothProps thing = (prop1 thing) .&. (prop2 thing) // True if both prop1 and prop2 are true

let notBothProps thing = Prop.not (bothProps thing) // Invert the property so it is true if either prop1 or prop2 fail

My suggestion is in the last line. I would love a Prop.not, Prop.negate or Prop.invert to flip properties. Could also add an operator such as .!.

Edit: If there is already a way to do this, I could not find it in the documentation. In that case, I request that it be documented somewhere.

Thanks

[jzabroski]

A related concern to then Some (Age 0) with “testing your tests” is Gettier Problems in Justified-True-Belief theory. Justified-True-Belief theory is a theory of how we know things, dating back to Plato and Socrates. A Gettier Problem was a counter-example that showed how our justifications might be true but erroneous in the context of proving what we believe is knowledge. This can happen in programming - imagine you test a “concurrent design” using linear, serial, deterministic inputs only. The tests always pass, and so you believe you have correctly designed a concurrent system. Yet, you overlook the fact that you mocked a key detail that controls non-deterministic choice to always return a certain value without delay. Your system works correctly, but not for the reasons you’ve justified to yourself and codified in tests.

[ploeh]

Would you also write properties that test that the properties that test your generators work correctly?