How does debug.traceback get its information? - lua

Question

With the below code from the Lua demo page, I was trying to get the name of the function being pcalled.
function test()
local info = debug.getinfo(1);
for k, v in pairs(info) do
print(k, v);
end;
end;
pcall(function()
test();
end);
This was a success, as I received the following output, containing the name:
source =input
func function: 0x25a1830
nparams 0
short_src input
isvararg false
name test
namewhat global
istailcall false
linedefined 1
lastlinedefined 7
nups 1
currentline 2
what Lua
If I change the code to the following, I no longer get that information:
function test()
local info = debug.getinfo(1);
for k, v in pairs(info) do
print(k, v);
end;
end;
pcall(test);
The output is as follows:
func function: 0x21ee790
linedefined 1
nups 1
short_src input
namewhat
lastlinedefined 7
isvararg false
istailcall false
what Lua
source =input
currentline 2
nparams 0
If, however, I change the code to the following, I can obtain the name of the function passed to pcall:
function test()
local traceback = debug.traceback();
print(traceback);
end;
pcall(test);
With the output being as follows:
stack traceback:
input:2: in function 'test'
[C]: in function 'pcall'
input:7: in main chunk
[C]: in function 'pcall'
demo.lua:49: in main chunk
[C]: in ?
How does debug.traceback get this extra information, and using solely Lua is there a way to get it without extracting it from debug.traceback's return value?

Answer 1

debug.getinfo and debug.traceback get their info from various sources, some of them hacky. For instance, the function name is generally extracted from the source code of the calling function: whatever name the code used to look up the thing it called, that's what gets used as the name. (That's why your second code snippet didn't give you the name: pcall doesn't have Lua bytecode backing it up, so it can't tell you "test" unless there's a function in the middle to call it "test".) Lua functions do not have inherent names, any more than Lua integers do; a function is just another kind of value, which can be automatically assigned to a variable via some syntactic sugar.

Answer 2

Functions are values. Values don't have names.
Functions are not "declared" as in some other languages. A function value is created when a function definition is evaluated.
Debugging output is just trying to be helpful in simple cases by giving the name of a variable associated with calling the function value.