A great example here: https://news.ycombinator.com/item?id=18776215

I am quite familiar with Spec. In my few years of professional Clojure experience, working with people who write Clojure every day, and enjoy it, and advocate using things like Spec, I have never actually seen anyone do it successfully in practice.

I understand the idealist world view is seductive — we'll be discipined, we'll write the tests, we'll make good use of Spec, etc etc.

In reality, I have never seen this happen. Humans are lazy and undisciplined, so that's what I optimise for.

We use spec a lot, using orchestra’s defn-spec combined with expound for improved error reporting.

It’s not a type system, but it’s very concise and easy to add validations to important functions. This removes the “lazy and undisciplined” part of the equation.

Basically you says if a type system is optional, people tend not to use it, because most of the times they are good without it.

I agree.

But we do use spec very successfully for complex data structure validations and example data generation.