React is not "functional". You extend the Component type just to make a view, you can encapsulate local state, you can inherit functions, yadda yadda. Calling it a "component" doesnt make it any less OO
Yes, but thats not how the render function is supposed to work. Its a function from state/props to a VDOM tree. In contrast, direct DOM manipulation is meant to be stateful.
For some reason, everyone always likes dealing with absolutes. Thats not how things work
* "Tests wont exhaustively check everything!"
Yes but they exercise the common cases, and the cases where the code will trip-up in real use are rare if the code is decently covered with tests.
Yes, its still possible to encounter edge cases. If it looks like that might be the case, add fuzz testing and property testing too. Pick the tool.
* "Types wont detect all errors!" -
Yes but detecting some errors is still useful. They mostly track dependencies between parts of the codebase: they determine how modules talk to eachother. This is useful, especially when I'm reorganizing code.
Also, its exactly where unit test fail to help, due to mocks: if I change module M, i don't know whether its dependencies are affected or not without looking at all of them and checking their mocks of module M. The situation gets even worse for second-level dependencies, and so on.
* "Unsound type systems are useless!" - No they are not. Not if its possible to isolate unsound usage and "hand-prove" it by carefully checking it. There is still value there: you can write things that the type system doesn't know are sound and hand-check them (by paying with more effort), or you can write things that it does know are sound, and enjoy the type system's ability to help you avoid errors.
* "Soundness isn't very important" - It depends. Is this unsoundness pervasive, like the null/undefined unsoundness? Maybe its useful to fix it then.
Yeah I suppose I am talking to myself. I'll try to stay on topic.
With the browser DOM model, end-users perform the DOM mutations. This generally means that they must keep the application state in sync with the DOM state, manually.
React's VDOM model encourages pure render functions, since a new VDOM is constructed every time and mutations are performed by React itself.
Also, nothing stops you from writing impure functions in PureScript either. You could define FFI functions that claim to be pure but aren't. I guess PureScript isn't pure then? Oh, and Haskell isn't pure due to unsafePerformIO. So nothing is absolutely pure; the question is what style is generally encouraged by the design of the framework/language, and to what degree. (there are no absolutes)
And PureRenderMixin isn't default because it does a shallow reference comparison on props/state, which only works reliably for immutable data structures. If all JS datastructures were immutable, it would've been used everywhere. However thats unrelated to whether `render` is pure.
