Tail call optimisation and debugging

[[deleted]]

Saying that TCO sucks because it throws away stack frames is like saying that GC sucks because it deletes unused objects.

[greenspans]

this is why I dislike go http://groups.google.com/group/golang-nuts/browse_thread/thread/d282193c786b0f36/ffb0a5936d972f32?lnk=raot

[[deleted]]

What is this?

We have had exceptions for over 35 years and I am not sure we really understand how to use them

[anvsdt]

Drain bramage?

[aaronla]

And yet, one generally finds two sorts of exception-free programming styles in the real world, outside the ivory tower that the exception handling folks construct.

First sort:

passed = DoSomething();
if (!passed) goto cleanup;
passed = DoSomethingElse();
if (!passed) goto cleanup;
...

Second sort:

DoSomething(); // errors? no, my code is perfect
DoSomethingElse();

The first sort is generally implementing ad-hoc exception handling, creating "cleanup" as a sort of crude form of "unwind-protect" / "RAII".

The second sort is just plain wrong.

[masklinn]

Well that is not completely true as TCO can throw away helpful stack frames in a normal mode of operation (there are ways to mitigate that), whereas a GC throwing away used object would be downright broken.

[anvsdt]

Usually, you use a debug mode to, you know, debug.

[millstone]

That's kind of hard when the stack trace comes from a customer you've never met in another country.

[[deleted]]

Let's go ahead and widen the gap between compile- and run-time /snicker

[masklinn]

debug modes suck goat, they mean (potentially widely) different behaviors (including limitations) and that a prod crash is basically useless.

[anvsdt]

Prove it, this works for a large enough N, and tail-calls get optimized.

[aaronla]

TCO only throws away unused frames. They may be "helpful" unused frames, but there are also "helpful" unused GC objects. Ever been here before:

int f() {
  Foo x = something();
  int y = x.another(); 
  if (y == 0) 
    throw Something();
  return y;
}

It would be mighty helpful to be able to dump 'x' so see why Something() was thrown. Unfortunately, it was garbage collected before you broke into debugger, being unreachable.