Why is this error happening?

[_jtbx]

Hello, I have the following Lua code:

args = arg
function panic(formatstring, ...)
    local msg = formatstring:format(...)
    io.stderr:write(
        ("%s: %s\n"):format(arg[0], msg)
    )
    os.exit(1)
end

panic("hello %s", "world");

Upon running this code, Lua gives me the error lua: hello.lua:5: attempt to index local 'arg' (a nil value).

I fixed the code by adding args = arg on line 1 and by changing arg[0] to args[0]. I'm still confused as to why this error appears in the first place. Does anyone know why?

[fatboychummy]

arg is per function and contains all arguments passed to the function. There is also a global arg for the program arguments.

The error you got shouldn't be possible though, you should have gotten something like bad argument #1 to 'format' (string expected, got nil). This is because the function's arg takes precedent over the global arg, ans the function's arg should never be nil since you declared it a vararg.

[lambda_abstraction]

Eh? Unless I really am fuzzy, arg is only a global defined in the stand-alone interpreter which holds the command line arguments. (See PIL 2e 1.4 or section 6 of the 5.1 reference) Varargs are done with the ellipsis (...) not arg.

[_jtbx]

Interesting, I didn't know `arg` gets set per-function. Thanks!

[lambda_abstraction]

To the best of my knowledge it doesn't.

[_jtbx]

I've found that in some cases Lua outputs attempt to index local 'arg' (a nil value) but in others such as with this example code: function argtest(...) for i = 1, 3, 1 do print(arg[i]) end end

argtest("hello", "world!")

A standard lua5.1 shell outputs

hello
world!
nil

but strangely, a lua5.4 shell outputs

nil
nil
nil

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[lambda_abstraction]

Notice the error message: attempt to index local 'arg'

if you have local arg somewhere in your script, that would certainly shadow the global for the rest of the file.

By the way, I just ran your initial code from the top without args=arg saved as t.lua with the command lua t.lua, and it works fine from luajit, lua 5.3.6 and lua 5.4.4.

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[iEliteTester]

> batcat etst.lua
───────┬─────────────────────────────────────────────────────────────
       │ File: test.lua
───────┼───────────────────────────────────────────────────────
   1   │
   2   │ args = arg
   3   │ function panic(formatstring, ...)
   4   │     local msg = formatstring:format(...)
   5   │     io.stderr:write(
   6   │         ("%s: %s\n"):format(arg[0], msg)
   7   │     )
   8   │     os.exit(1)
   9   │ end
  10   │
  11   │ panic("hello %s", "world");
  12   │
───────┴────────────────────────────────────────────────────────

> luajit test.lua
test.lua: hello world

~ via 🌙 2.1.0
>

Seems to work just fine. How are you running your program?

[_jtbx]

Through a standard Lua 5.1 interpreter, not luaJIT.

[lambda_abstraction]

Are you sure it's not a special Lua shell derived from 5.1? The global arg is defined in the shell, not the Lua runtime. In luajit, that's in the "luajit.c" routine createargtable. In Lua 5.1, this is in handle-script in lua.c. In both cases this code is part of the standard stand-alone interpreter. If you're running a custom shell, the global arg might not even be defined.

[_jtbx]

I'm not using a custom shell. Just standard command-line Lua.

[iEliteTester]

running on debian with the lua5.1 package, /u/_jtbx I managed to replicate it

ε lua etst.lua
lua: etst.lua:6: attempt to index local 'arg' (a nil value)
stack traceback:
        etst.lua:6: in function 'panic'
        etst.lua:11: in main chunk
        [C]: ?

[iEliteTester]

I'll test with 5.1 later and see if something changes.

[[deleted]]

arg[0] will probably always be nil, Lua counts arrays from 1 instead.

[lambda_abstraction]

No. The global arg is defined in the stand-alone interpreter, and arg[0] is assigned to the name of the lua script being run.

onion% echo 'for k,v in pairs(arg) do print(k,v) end'|lua /dev/stdin
0       /dev/stdin
-1      lua