This is why designing the interface in a language with a powerful type system provides the same quoted benefit of TDD. It helps you think about the interface before you move to implementation.

In my experience, using a type system to do this requires much less effort and refactoring.

> This is why designing the interface in a language with a powerful type system provides the same quoted benefit of TDD.

Yes. TDD came from developers who worked in weak/dynamic typed languages.

It is mandatory to discover errors, that are otherwise discovered instantly for free by compilers in the stronger typed languages.

Strangely, the TDD folks never outline this point.

I am a big fan of "testing via types".

Unit tests can demonstrate that my code is correct; it can say nothing about how my code is used by others.

In contrast, a type system can extend those protections downstream.

(Yes, yes, unless you go way off the deep end of dependent types, you still need tests. But a powerful type system can systemically prevent a huge number of very common bugs.)

My take it on it is that it can remove most of the boring bugs you would get in a less strongly-typed language and leaves you open to the interesting bugs (eg, logic bugs).

As for dependent types, I'm not sure that even then you would be able to ensure that, say, your cache is correctly invalidated.

Additionally, strict TDD teaches you lessons. Once you've used it enough, chances are that you'll be making better interfaces even if you code first.

I've found implementing test as I go along writing a new API help code quality and speed in two ways: I catch my own logic bugs early and the self-feedback I get from writing the tests helps me to write a more usable interface.

I never catch all the design errors with non-coding design work - this gives me the high level view which is critical - but there are always some logical errors and bugs I've failed to model correctly which early tests catch.

But yeah, the optimal design-implememt-test probably depends probably very much on the implememter and on the problem to be solved.

The third meta-advantage is that if I'm writing hobby code for myself and the profject is large compared to the weekly time allocations available in my sparetime I get something to help me remember where I was (say a month back) and gives me something to compile and run immediately (which is a huge motivational booster for me).

> 80% of software development is designing correct abstractions/interfaces/APIs

Not sure if that claim is wrong or not, but I will state it anyway. If you write in TDD manner and you decide to change the API/abstraction a little bit because of something you didn't think of, you will have to correct both tests and the code. When you do test-after-code it's much easier (IMO) to hit correct abstractions and interfaces because you are working on a living thing, not some artificial test cases you have to figure out. Then you write tests to ensure those abstractions are respected and in working order.

TDD people, please correct me if I'm wrong.

EDIT: disclaimer: I'm a front-end developer working mostly in React and Redux