I've spent a horribly long time tonight staring at nested stack manipulation code now to get algebraic expression parsing (mostly?) working. I hate unary minus; life is pretty good except for that ugly little weiner with its binary operator twin, and then BAM weird compromises in your code.
For most errors, I rely on testing (even if just firing a test script through the language, as I'm doing with tbasic) and debug mode with verbose stderr logging. But this is C, where the slightest mistake can be EXC_BAD_ACCESS with no clue where. So then I need a debugger…
% make && lldb -o run -- tbasic -d test1.bas
[update: Forgot the -- before the program, which prevents lldb from reading those params. Command lines without parens are easy to get wrong!]
I don't really do much serious with lldb, I just need to see where an error occurred, backtrace (bt), and sometimes print some variables, to usually be able to solve a crash. It's a little frustrating that the lldb environment is so primitive, though, doesn't even have stdout, stderr (weirdly \<stdio.h> is callable), so how do I call utility functions? Had to rewrite some functions to take default NULL values.
Anyway, let and print (and error, my idiosyncratic stderr print) work, there's not a lot left in BASIC then I can get back to more serious things.