[sabauma]

The notion that not having proper tail calls aids debugging always seemed like a post-hoc justification. The stack trace of an iterative function will lack exactly the same intermediate evaluation frames as a tail-recursive implementation.

[za_creature]

The thing is, tail calls aren't _just_ about emulating iteration via recursion:

  def foo():
      raise ValueError

  def bar():
      return foo()

  bar()

With TCO, the stack trace would contain `main` and `foo`, as `bar`'s frame would be overwritten by `foo`. This example is simple, but `bar` could be a 50 line long if-else chain of tail calls and when debugging you won't necessarily know which condition was evaluated.

[sabauma]

> The thing is, tail calls aren't _just_ about emulating iteration via recursion:

I completely agree, but there is also no need to perform TCO to make code like this safely runnable. TCO only becomes necessary/useful when implementing an iterative process where we can't statically know that the call stack won't be exhausted. That said, TCO is usually an all or nothing transformation, and it would be difficult to accurately avoid eliminating trivial tail calls like in your example.

A reasonable compromise might be for the Python VM to implement a TAIL_CALL bytecode op and require the programmer to decorate functions which rely on TCO. This wouldn't be any more onerous than manually trampolining large portions of code, which is the current method of getting around the lack of TCO.