> it's not unsound, but it's inelegant. A runtime system designed this way only understands your program in a statistical sense (based on concrete execution pro

> it's not unsound, but it's inelegant. A runtime system designed this way only understands your program in a statistical sense (based on concrete execution profiles, which may vary from one run to another), never with the full certainty that static analyses (type checking, abstract interpretation) can give you. This is a false dichotomy. You can always build a JIT that uses runtime statistics to speed things up, even in languages that are quite amenable to static analysis, for the simple reason that type systems cannot capture all relevant runtime context. Java is a good example here, since it's strongly statically typed. By your reckoning, all optimistic heuristics that CPUs do like branch prediction and prefetching are also similarly 'inelegant', because they can be wrong and require rolling back. Optimistic heuristics have a long history in computer science, and IMO it seems strange to single one particular use case as being particularly evil.