[Quora] What is the call metamethod in Lua?

published 2020-04-09

From 2011 to 2014, I used to post answers on Quora [1]. I don't anymore, because I don't really like what the website has become. I have a copy of some of my answers here [2] but someone commented [3] on one of my answers that it should be available more prominently on the Web, so I decided to repost a few of my answers here, starting with this one.

The original question [4] was:

I'm really new to lua and relatively new to programming.,so kindly excuse if I say something stupid.

>

I have a table named x and its metatable named y. When I have a `__call` method defined for the metatable `y`, then I can call `x()` but if I have a `__call` for `x` then I can not call `x()`.

>

What is `__call` used for? How does it work and what are some examples of usage

I answered it on February 25, 2013.

`__call` is a metamethod, that means it is meant to be defined in a metatable. A `__call` field added to a regular table (`x` in your example) does nothing.

The role of `__call` is to make something that is not a function (usually a table) act like a function. There are a few reasons why you may want to do that. Here are two examples.

The first one is a memoizing factorial function. In Lua you could write a recursive factorial like this:

local function fact(n)
    if n == 0 then
        return 1
    else
        return n * fact(n - 1)
    end
end

Note: this is not a good way to write a recursive factorial because you are not taking advantage of tail calls, but it's enough for what I want to explain.

Now imagine your code uses that function to calculate the factorials of numbers from 1 to `N`. This would be very wasteful since you would calculate the factorial of `N` once, the factorial of `N-1` twice, and so on. You would end up computing approximately `N²/2` factorials.

Instead you could write that:

local fact
fact = setmetatable(
    {[0] = 1},
    {
        __call = function(t, n)
            if not t[n] then
                t[n] = n * fact(n - 1)
            end
            return t[n]
        end
    }
)

It is an implementation of factorial that memoizes the results it has already computed, which you can call like a function. You can use it exactly like the previous implementation of factorial and get linear complexity.

Another use case for `__call` is matrices. Imagine you have a matrix implementation that works like that:

local methods = {
    get = function(self, i, j)
        return self[i + 1][j + 1]
    end
}

local mt = {__index = methods}

local new_matrix = function(t)
    return setmetatable(t, mt)
end

You can use it like that:

local M = new_matrix({ {1, 2}, {3, 4} })
local v = M:get(0, 1)
assert(v == 2)

However scientists would probably expect something like this:

local v = M(0, 1)
assert(v == 2)

You can achieve that thanks to `__call`:

local mt = {
    __index = methods,
    __call = function(self, i, j)
        return self:get(i, j)
    end
}

I hope this gives you enough information to understand how you can use `__call`. A word of warning though: like most other metamethods, it is useful but it is important not to abuse it. Simple code is better :)