If you're just testing pieces of code in isolation, then you're either writing lots of tests or making assumptions about the kinds of input that your function c

If you're just testing pieces of code in isolation, then you're either writing lots of tests or making assumptions about the kinds of input that your function could reasonably receive or return. If you're testing end-to-end, you're either writing lots of (much more expensive) tests or you're missing corner cases. I don't think that's problematic per se. Testing is just a bit of a mismatch for the problems that type systems solve.