Lua - calls only one operator in metatable

Question

I'm new to Lua, so maybe missed something on tutorials but the problem is:

I have original table and metatable with several operators that I'm applying to it:

original = { 1, 2, 3 }

test = setmetatable(original, {
  __add = function (lhs, rhs)
    print('adds')
  end,
  __mul = function (lhs, rhs)
    print('multiplies')
  end  
})

Unfortunately when I'm doing operations like:

test = test + 3
test = test * 3

I receive an error:

attempt to perform arithmetic on global 'test' (a table value)

Didn't find any descriptions on this problem. Also I noticed that if metatable is a separate variable and passed to setmetatable method then it works..

Answer 1

The error I get is

attempt to perform arithmetic on global 'test' (a nil value)

This means that test is nil in the last line. You need to return something in __add.

Answer 2

test = test + 3

Is loosely equivalent to:

test = getmetatable(test).__add(test, 3)

You're assigning the return value of __add to test.

_add returns nothing, so after the first line, test is nil. Then you do it again:

test = getmetatable(test).__add(test, 3)

You can't index or get the metatable of nil.

---

An easy what to have discovered this, probably the first thing I would have tried:

test = test + 3
print(test)
test = test * 3