Why does the doc say `__index` is looked up in the table?

Question

From the Lua 5.3 doc:

__index: The indexing access table[key]. ... The metamethod is looked up in table.

It says the same thing for __newindex, but not for any other metamethod.

If this were true (which it's not), it would be a major departure from previous versions of Lua. The following code outputs nil, as I would expect, but it's inconsistent with the doc.

#!/usr/bin/env lua5.3

local proto = {a = 54}
local t0 = {__index = proto}
print(t0.a)

To be clear: If the doc was correct, I would expect t0 in the above code to only require an __index field without an actual metatable for t0.a to be 54. So does anyone know what's going on with the doc?

Answer 1

You are incorrectly interpreting the meaning of the term. To say that it is "metamethod Y is looked up in X" does not mean that it searches the X table for an entry named Y. It means that it gets the metatable for X and looks up an entry named Y, as if by rawget(getmetatable(X) or {}, "Y"), as specified in the docs.

This terminology is repeatedly used in the metamethod descriptions. For example:

First, Lua will check the first operand (even if it is valid). If that operand does not define a metamethod for __add, then Lua will check the second operand.

It's not asking if the first (or second) operand have a method __add; it asks if they have a metamethod __add.

As you can see from the __add example, you have to specify which of the operands it tries to get metamethods from, and in which order. For table[key], the point of the text is that doesn't try to get the metamethod from key, only from table. That may seem a tad bit obvious, but completeness is better than incompleteness.