why can you set __index equal to a table

Question

The index metamethod can be set equal to tables. From what I can tell

foo.__index = function(self, k)
    return bar[k]
end

and

foo.__index = bar

are the same. Why is declaring functions this way allowed in this situation?

Corriveau

Have a look at the Lua string library for an example of when you would assign a table to __index – Adamantine

Answer 1

This isn't a function declaration - assigning a table to __index is just a shortcut for using the function that you described.

From Programming in Lua (for Lua 5.0, but this part of the language hasn't changed):

The use of the __index metamethod for inheritance is so common that Lua provides a shortcut. Despite the name, the __index metamethod does not need to be a function: It can be a table, instead. When it is a function, Lua calls it with the table and the absent key as its arguments. When it is a table, Lua redoes the access in that table.

It's not like the table that you assign magically becomes a function. type(foo.__index) will still return table, and you can still do things with it that you can do with other tables, like using pairs and next, etc.

Grandniece

Using function does not allow list of bar fields when you have access to foo table. – Sciolism

That's true, yes - I suppose that means that this feature isn't technically syntactic sugar. I'll tweak my answer a little. :) – Grandniece