Until such a day comes, `printf()` debugging will still be useful. Heck, even after that day comes, it may still find uses.

The other day I was writing LPEG [1]. I had a rule that wasn't firing for some reason and I wanted to know why LPEG was skipping that part of the input. It was not a simple matter to fire up a debugger and put a breakpoint on the rule:

    -- look for 1 or more ASCII characters minus the colon
    local hdr_name = (R"\0\127" - P":")^1 -- [2]

I mean, I could put a breakpoint there, but that would trigger when the expression is being compiled, not run. LPEG has its own VM geared for parsing (it really is its own language) so yes, it is sometimes difficult to debug (especially when you end up with a large parsing expression to parse a document---I only found the bug when I was creating another document similar to my initial testing document that was just different enough).

Fortunately, there is a way to hook into the parsing done by LPEG and I was able to insert literal print statements during parsing that showed what exactly was going on (my rule was a bit too promiscuous).

[1] Lua Parsing Expression Grammar http://www.inf.puc-rio.br/~roberto/lpeg/

[2] Yes, a regular expression for that is smaller than what I typed, but with LPEG, I can reuse that rule in other expressions. Also, that wasn't the actual rule used, but the effective rule I was using.