Lua has a function to capture stack traces. You can pass in a thread to capture the stack trace from, a message to prepend, and how many levels of the stack to skip. It returns a string.

The signature from the Lua 5.1 reference manual is:

debug.traceback ([thread,] [message [, level]])

We’ve got optional argument soup here, which is a recipe for disaster. Let’s open my trusty Lua 5.1 REPL and see what happens!

Lua 5.1.4  Copyright (C) 1994-2008 Lua.org, PUC-Rio
> =debug.traceback()
stack traceback:
        stdin:1: in main chunk
        [C]: ?

So far so good.

> =debug.traceback(1)
1
stack traceback:
        stdin:1: in main chunk
        [C]: ?

Okay, that’s kind of annoying – a numeric argument gets coerced into a string as message field. No big deal, we’ll just pass nil in for message.

> =debug.traceback(nil, 1)
nil

That’s interesting. Maybe nil is being interpreted as the thread argument. Let’s pass in the empty string instead.

> =debug.traceback("", 1)

stack traceback:
        stdin:1: in main chunk
        [C]: ?

It inserted a leading newline. Alright, that’s fine. Let’s put in nil for both thread and message.

> =debug.traceback(nil, nil, 1)
1

Wait, hold on, what?

> =type(debug.traceback(nil, nil, 1))
number
> =type(debug.traceback(nil, nil, {}))
table

Oof.