alt Hacker NewsThe biggest flaw I've seen with TDD is the fact that correctness does not compose upward. Every time two units come into contact, you've got an entirely new kind of unit. The tests from constituents do not cover emergent properties of the new things. You will repeat this same exercise the entire way up to the top, and the moment you come into contact with the customer (they want to change everything), the house of cards comes crumbling down and you have to start your agonizingly-slow process all over from the bottom again.
The only thing that the business seems to care about is top-down UI testing. This is also convenient because you can leave it until the very end after the customer has already seen several prototypes.
I do think TDD makes sense in isolated scopes (prove this specific custom parser works at the edges), but as the general policy for the entire product it's definitely not a viable practice. Much of the time if comes off as an ego trip to see just how cleverly we can mock something so that we can say we technically tested it.
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I tell people you should be testing at the level where a change would be so hard you wouldn't do it anyway. Internal helper functions - they are tested only because the code that calls them passes. Interfaces that are used thousands of places - you better test them well because you wouldn't dare change that anyway: it would break too many others.
Or to put it differently: a test is an assertion that no matter what, for all time this should never change again. Even if customer requirements change in the future they won't change in such a way as to break this test (this isn't always true, but you should believe it is true).
A test is most valuable when it alerts you to a real problem when it fails. If the test fails but there isn't a real problem (either because customer requirements have changed, or it is flaky) it was needless cost to investigate it. If the test passes that gives some hope of correctness, but you can never be sure it is really correct vs a bug in the test (even if you use TDD and so the test failed when you wrote it that doesn't mean a refactoring since didn't make this an always pass test).
Part of the problem is if I tell you to write sort() or your new toy language's list type you have an intuitive idea of what it should look like and probably will get them right the first time (other than bugs you want the tests so you catch). These should have tiny micro tests. These things also are really easy to use as examples of how to do TDD - which they are, but they are not representative: this type of code is generally in your standard library already and you are not writing it.
Instead you are writing code that isn't well defined with lots of industry experience. It is not clear what the exact interface should be (or more likely it is clear customer requirements will change but you don't know how yet). You have no idea what the best implementation is. You don't know if this will be used in this one place, or if it will become a useful key part that many future projects depend on. You have to make guesses.
That is a flaw with unit tests written at far too low a level, not with TDD.
You would have the same problem if you wrote tests like that after the code.
TDD has no opinion about the level at which you wrote your test, it just assumes it's the correct one.
This is the number one biggest misconception about TDD which I keep seeing repeated on hacker news.
Exactly, the whole system thinking and large scale architecture also fails apart, when writing everything from little working tests.
TDD is perfect for bugs; codify a replication first, then fix it.
I feel this, especially with the crazy lengths people go to mock things sometimes. A couple years back I was having a discussion with a friend/former coworker about testing (I was griping about unnecessary mocks I had to deal with for something at a job causing unnecessary extra work), and he asked how I would approach trying to get full unit test coverage instead. I was taken aback and said that I wouldn't try to get literally everything covered by unit tests in the first place. Most of the teams I've worked on have had the approach that test coverage is good, but it isn't necessarily going to be 100% even when considering all tests; I can't even imagine trying to get 100% coverage for unit tests alone being anywhere close to worth the extra effort, let alone the contortions that the code would need to take to support it.