Closures

A function can create another function and return it, like any other value. The new function then remembers the local variables of the function that created it, even after that function has ended. Here's a function that makes counters:

local function makeCounter()
    local count = 0
    return function()
        count = count + 1
        return count
    end
end

local countEggs = makeCounter()
print(countEggs())
print(countEggs())
print(countEggs())

This prints:

1
2
3

Step by step: makeCounter() creates a local variable count with the value 0, then creates an anonymous function, returns it and ends. The returned function is stored in countEggs. Normally, the local variables of a function disappear when it ends, but count doesn't: the returned function uses it, so Lua keeps it for as long as that function exists. Each call of countEggs() increases this count and returns it.

A function together with the variables it remembers is called a closure. Every call of makeCounter creates a new count, so each counter has its own:

local function makeCounter()
    local count = 0
    return function()
        count = count + 1
        return count
    end
end

local eggs = makeCounter()
local milk = makeCounter()
eggs()
eggs()
milk()
print("Eggs: " .. eggs())
print("Milk: " .. milk())

This prints:

Eggs: 3
Milk: 2

eggs() on a line of its own calls the counter and throws away the number it returns; that's allowed. Why not simply use one variable outside the function? Then any part of the program could change it, and for two counters you'd need two variables and two functions. In a closure, count is hidden: only the counter itself can reach it.

The parameters of the outer function are its local variables too, so a closure remembers them as well:

local function makeSeller(price)
    return function(count)
        return count * price
    end
end

local sellCarrots = makeSeller(5)
local sellPumpkins = makeSeller(20)
print(sellCarrots(10), sellPumpkins(3), sellCarrots(2))

This prints:

50	60	10

sellCarrots remembers the price 5, and sellPumpkins remembers 20. Each returned function has its own price, set once when it was made.