Bizzare "attempt to call a table value" in Lua

Question

The following code snippet:

for weight, item in itemlist do
    weight_total=weight_total+weight
end

is causing the error "attempt to call table value" on the first line in that snippet. Why?

Itemlist is a table of tables of weights and strings, like such:

local itemlist = {
                        {4,"weapon_pistol"},
                        {2,"weapon_357"},
                        ...

Nothing is being called as far as I can tell; why is this error coming up?

Solution

The generic for expects 3 arguments: a callable value, some value which is repeatedly passed to it, and the key where the iteration shall start.
Stock lua does not call pairs on the first value passed to for if that's not callable, though some derivatives do.

Thus, you must use ipairs(itemlist), pairs(itemlist), next, itemlist or whatever you want (the last two have identical behavior, and are what most derivatives do).

As an example, an iterator unpacking the value sequence:

function awesome_next(t, k)
    k, t = next(t, k)
    if not t then return end
    return k, table.unpack(t)
end
for k, a, b, c, d in awesome_next, t do
end