Re: table.unpack does not produce a line number on error

Between 5.2 the "Returns the elements from the given table." in
documentation at https://www.lua.org/manual/5.2/manual.html#pdf-table.unpack
changed to "Returns the elements from the given list." at
https://www.lua.org/manual/5.3/manual.html#pdf-table.unpack

The code at https://www.lua.org/source/5.3/ltablib.c.html#unpack indeed
changed (other than other integer types)
from https://www.lua.org/source/5.2/ltablib.c.html#unpack by removing
`luaL_checktype(L, 1, LUA_TTABLE);`

And you're right in 5.1 (plain `unpack` back then) and 5.2
`table.unpack(nil)` produces an error in line number but in 5.3 and up
don't.

Root cause seems to be that in 5.2 it ultimately goes to `luaL_argerror`
which takes debug info, but in
5.3 and up `luaL_len` ends up in `luaV_objlen` which doesn't.

OTOH the 5.3+ seemingly can handle not just tables, but anything with __len
and __index implemented.

To solve this for your application, if you have access to either C or Lua
debug library, you can make own reimplementation
or wrapper and replace `table.unpack` with that to add line number and file
name.


On Mon, 2 Mar 2026 at 18:14, 'Vadim (vadcx)' via lua-l <
[email protected]> wrote:

> Hi all,
>
> When table.unpack encounters an error (such as the above or "attempt to
> get length of a number value"), it doesn't add/push the source and line
> number of the error.
>
> Compare the output of the following two functions producing an error at
> runtime:
>
> > print( pcall(function() table.unpack(nil) end) )
> false   attempt to get length of a nil value
>
> Expected:
> > print( pcall(function() string.gsub(nil) end) )
> false   stdin:1: bad argument #1 to 'gsub' (string expected, got nil)
>
>
> Lua 5.2.4: unpack adds error source
> Lua 5.3.6 and above, including 5.5.0: table.unpack doesn't add error source
>
> A host application (proprietary, with Lua 5.3), that I'm working with,
> struggles to produce an error message at all in case of table.unpack(123)
> (no line number or error message). Yet other errors show up fine in the
> logs.
>
> If I read it correctly, 5.2's unpack has had an explicit
> luaL_checktype(L, 1, LUA_TTABLE);  ?
>
> https://github.com/lua/lua/blob/4324904b60db5243ede68d0922c1bf3c0dd05986/ltablib.c#L140
>
> --
> Cheers,
> Vadim
>
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>

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