__pairs() and __ipairs() metamethods does not work at all

Question

I don't know what I do wrong. Basically the code looks like this:

local t = setmetatable({}, {__pairs = function (self)
    print "Message from __pairs()"
    return function ()
        ...
    end
end})
for k, v in pairs(t) do ... end

The same for __ipairs(). Overloaded metamethods aren't called at all - no console output, no custom iteration at all. Instead I get the result as if I iterate through a table without metatable. What's wrong?

Answer 1

You most likely use Lua 5.1 (or its derivative), which doesn't have support for these metamethods, as these were introduced in Lua 5.2. I've tested in Lua 5.2-5.4 and confirmed that your code works there (the method is called).

Answer 2

for those who are not working on __pairs and __ipairs in older versions (for example, Solar2D is running on Lua 5.1), you can override the pairs and ipairs functions to add supports in main.lua using the following code:

local raw_pairs = pairs
pairs = function(t)
    local metatable = getmetatable(t)
    if metatable and metatable.__pairs then
        return metatable.__pairs(t)
    end
    return raw_pairs(t)
end

local raw_ipairs = ipairs
ipairs = function(t)
    local metatable = getmetatable(t)
    if metatable and metatable.__ipairs then
        return metatable.__ipairs(t)
    end
    return raw_ipairs(t)
end