> Thanks for the honest assessment. What about correctness/ composability of compiler transforms like AD, reliability of GPU acceleration and predictability of optimizations? (basically what you've discussed in some of your compiler talks).

I don't think we really have a good answer yet, but it's actively being worked on. That said, I don't think we can be faulted for that one, because I don't think anybody really has a good answer to this particular design problem. There's a lot of new ground being broken, so some experimentation will be required.

> TPUs, kernel fusion, branching (Lux relies on generated functions) and copy elision (though this last part is being worked on IIUC).

We have demonstrated that we can target TPUs. Kernel fusion is a bit of an interesting case, because julia doesn't really use "kernels" in the same way that the big C++ packages do. If you broadcast something, we'll just compile the "fused" kernel on the GPU, no magic required. There is still something remaining, which is that when you're working on the array level, you want to be able to do array-level optimization, which we currently don't really do (though again, the TPU work showed that we could), but is broadly being planned.

> Edit: re : bandwidth issue Jan Vitek's group is thinking a lot about the verification vs flexibility tradeoff and some people are working on a trait/ static typing system. Maybe something can be done to help them along?

We work closely with them of course, so I think there'll be some discussions there, but it's a very tough design problem.