draft a new post about testing

content/posts/moving-from-example-based-to-generative-tests.md

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+title = "Moving From Example-Based to Generative Tests"
+date = 2021-01-19
+draft = true
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+
+(TODO: this title is not great. Figure out a better one.)
+- Trading Specificity for Coverage (?)
+
+When I think of "testing" as a whole, unit tests always show up first in my mind.
+Their core characteristic is that they're based on individual examples.
+In other words:
+
+- They each take a specific input and some process, and assert that you get a specific output.
+- I probably have a lot of them, and I can run a small group of examples to zoom in on something I'm implementing.
+- Hopefully, they're exhaustive: if I run all of them, I want to find out if I broke anything, so I run them all regularly (for example, in CI.)
+
+The specificity here is really useful!
+If I have some specific thing I want to do, all I have to do is write down the specific input and output I want.
+The test framework then makes sure that the code does what I intend.
+Great!
+
+This specificity leads to practices like test-driven development, where you write the smallest possible examples that move towards the thing you intend.
+That's way harder to do if you can't express yourself with this kind of specificity!
+
+But example-based testing has drawbacks too: that amazing level of control means that you have to specify 100% of the cases that you want to be tested.
+If you forget one, it's just not verified.
+And that's before even thinking about the fact that to test all the edge cases in your integrations between different things (classes, data structures, whatever) you've got to *multiply* the number of edge case tests in each!
+That's a lot of examples!
+
+This is where a lot of people learn about generative tests, usually through property tests.
+The basic idea here is that you trade specificity for coverage: you don't have to write every single case yourself, but tell the computer how to write them and it can test hundreds or thousands.
+But to do that, you have to come up with some pretty specific properties.
+This, in a word, is hard!