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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 :)