Fwiw, I took the approach of having a hard separation between stateful and functional code partly so that I could dodge most of these hard questions around muta

Fwiw, I took the approach of having a hard separation between stateful and functional code partly so that I could dodge most of these hard questions around mutation tracking ;) The stateless code can be really aggressively inferred, and even re-ordered, inlined, detected as dead and thrown away, memoized, whatever else is useful. And then the (hopefully small) stateful code can just be typed at roughly the same level TypeScript already does