[rifung]

> wouldn't a really smart compiler be able to do the same thing?

Well compilers are tied to the language they need to compile so language choice does still matters. As an extreme example, GHC can be much more aggressive with inlining because it's pure, whereas most other languages don't provide the same guarantees.

> And if you have to be able to write 'some parts' in C for speed ... then why not just use C? And have nice libs for whatever you are doing?

Yes I think they could do that, but then they'd need people to write and maintain those libraries. My understanding is they want to find something which strikes a better balance between productivity and speed. Having to write your own libraries to do everything is likely considered a productivity loss.