TCO is classically a compiler/runtime optimization, but that's only because people didn't commonly implement fundamental control flow as libraries until relativ

TCO is classically a compiler/runtime optimization, but that's only because people didn't commonly implement fundamental control flow as libraries until relatively recently. Whether it is the runtime introspecting function calls to decide whether it can elide the current stack frame, or a futures library introspecting return values to decide whether the current future can delegate to the returned one, it's the same concept. As for "weird universes", it is typically only functional programming languages that guarantee TCO as part of their standard. Most programmers live in a world where they can't rely on their implementation to optimize tail-calls, which is to say they don't make unbounded recursive calls. If they do, they end up with an insidious form of non-portable code.