I don't think there is anything theoretical that prevents you from doing that. In fact, it has been tried: https://en.wikipedia.org/wiki/N-version_programming h

I don't think there is anything theoretical that prevents you from doing that. In fact, it has been tried: https://en.wikipedia.org/wiki/N-version_programming https://en.wikipedia.org/wiki/N-version_programming The problem in practice is though, for larger programs, it's really difficult to delineate what are the inputs and outputs (so they could be recorded and compared), especially since they take so many different forms. But on small scale (like functions or modules), it should be possible, but the tooling is not widespread. IMHO it would save lots of time on doing unit tests for refactoring, and would be a real progress. And I will go on a limb and claim, what people (in OOP world) mean by testability, is really just referential transparency (in FP parlance), in other words, our ability to delineate what are all the inputs and outputs of a module. Thus, adopt more FP and this will become increasingly possible.