Arguments to require'd function get lost/disappear in Lua?

Question

While I don't really know Lua, this is a rather unexpected and bizarre behavior.

Let's say I have my_module.lua:

local function dump(o) -- SO:9168058
  if type(o) == 'table' then
    local s = '{ '
    for k,v in pairs(o) do
       if type(k) ~= 'number' then k = '"'..k..'"' end
       s = s .. '['..k..'] = ' .. dump(v) .. ','
    end
    return s .. '} '
  else
    return tostring(o)
  end
end


local mymodule = {}

function mymodule.myfunction(indict)
  print(dump(indict))
end

return mymodule

Ok, now I run this:

lua5.3 -e "mm=require('my_module'); mm:myfunction({aa=12})"

This should not be complicated - I "import" the module, and call a function in it with an argument that is an object (the table/associative array/dictionary {aa=12}). Then I simply try to print this argument from within the function. However, I get this:

$ lua5.3 -e "mm=require('my_module'); mm:myfunction({aa=12})"
{ ["myfunction"] = function: 0x5619aeddf770,} 

So, the response to print(dump(indict)) inside myfunction, where indict is an argument passed to myfunction, Lua prints .... "myfunction" ????!

I cannot even wrap my head around this - how can this possibly happen ?

And how can I pass an object as an argument to a function, such that when I print the argument from within the function, the object which is the argument is printed - and not the function itself ??!

Btw, the same happens even if I just pass a number instead of an object, say:

lua5.3 -e "mm=require('my_module'); mm:myfunction(42)"

---

EDIT: did a bit of debugging - so with this:

function mymodule.myfunction(indict)
  print(indict)
  print(dump(indict))
end

... I get this printout when using a numeric argument:

$ lua5.3 -e "mm=require('my_module'); mm:myfunction(42)"
table: 0x55f15a7a07a0
{ ["myfunction"] = function: 0x55f15a7a07e0,} 

... so it simply doesn't see this number anywhere, but the function sees itself as the first argument.

This reminded me, how in a Python class, you have to write methods as functions with "self" as first argument, so I tried this:

function mymodule.myfunction(self, indict)
  print("self", self, "indict", indict)
  print(dump(indict))
end

... which prints:

$ lua5.3 -e "mm=require('my_module'); mm:myfunction(42)"
self    table: 0x560510b5a7d0   indict  42
42

... or in case of passing an obect:

$ lua5.3 -e "mm=require('my_module'); mm:myfunction({aa=12})"
self    table: 0x55d51c9d5800   indict  table: 0x55d51c9d5880
{ ["aa"] = 12,} 

... well, that's more like it ...

Can anyone explain where does this come from - why do I need to add a "self" argument in this case?

Answer 1

In lua, the call a:b(x) passes a reference to the object a as the first (self) parameter to the function b.

Since your module definition is:

function mymodule.myfunction(indict)

and the call statement is mm:myfunction, the object/table mm is passed as the first parameter (which here is indict).

Either change the function definition to

function mymodule:myfunction(indict)

if you want to keep call like mm:myfunction, or call the function as mm.myfunction.

---

The behaviour is discussed in detail in the PiL book on OOP concepts.

The effect of the colon is to add an extra hidden parameter in a method definition and to add an extra argument in a method call. The colon is only a syntactic facility, although a convenient one; there is nothing really new here.