Re^2: No Pause on Elsif in Debugger

[QM]

It "pauses" on the beginning of a statement. elsif is not the beginning of a statement.

Is that DWIM? What behavior do we want?

I expect that it will at least pause on the evaluation of interesting conditionals [elsif (0) being an example of an uninteresting one]

[merlyn]

It doesn't really matter if that's exactly what you want. It would take a major overhaul (major) of the interface between the execution engine and the debugger Perl code to get it to do anything else. It's a very fundamental design choice. It's just like you can't set a breakpoint to see the "middle" of a map operation, other than at a statement boundary in a called subroutine.

The debugger gets notified only as each statement is entered. That's it.

[QM]

Any chance of getting a nice little discussion about this decision, or a point to the design discussion? The historian in me is looking for new material.

[tye]

My somewhat wild conjecture is that this is related to how line numbers are reported incorrectly for elsif code (the number of the line of the if is reported instead).

The broken line numbering is clearly a bug and justifies more effort be marshaled to fix it than the debugger issue.

Further, I conjecture that both are somewhat affected by the Perl4 optimization where a suitable if/elsif/else chain is converted into an efficient 'switch statement' (computed goto).

[merlyn]

/me points at "Larry's Head".

[tye]

I'm no expert on this but I goaded a few experts on this and in the end I don't think they came up with any good reasons why 'elsif' shouldn't just be declared "a statement" as far as opcode generation is concerned so that run-time line numbers can be correct and the debugger can stop on such lines.

I expected that this would find its way to p5p and someone would post some evidence of that to this thread. Maybe this goading will make that happen. If not, you might want to escalate to p5p yourself (after first searching the archive to see if it has already been discussed).

[pemungkah]

Of course, the Right Place to put the breakpoints is on the code *inside* the if/elsif/else - "right" being "you will actually stop somewhere near where you want to be". I've noticed similar annoying behavior with for loops at times. Just set your breakpoints inside and the problem goes away. This is the least intrusive solution; obviously, you can code to the debugger's limitations by doing things like

if ($cond1) {
  ...
}
else {
  if ($cond2) {
    ...
  }
  else {
    if ($cond3) {
      ..
    }
  }
}

and now you can break at every if. (That looks scarily like some of my old PL/1 programs...)

[QM]

I started to say

If I have a long if/elsif/elsif/... chain, then setting breakpoints inside every block is painful (especially when repeating the same breakpoints while doing the debug/edit loop).

However, more than 3 is a good indicator I'm doing something wrong, and should rethink my attack.

[pemungkah]

Yes indeed. After more than two or three, it's time to consider a hash and coderefs.