Functions as Values

In Lua, a function is a value, just like a number, a text or a table. type says so: type(print) is "function". And like any value, a function can be stored in a variable. In fact, local function double(n) is just a shorter way of creating a function and storing it in a local variable, like this:

local double = function(n)
    return n * 2
end

local twice = double
print(double(4), twice(10))
print(type(double), type(double(4)))

This prints:

8	20
function	number

function(n) … end without a name creates a function, and local double = stores it in a variable. local twice = double doesn't call anything, because there are no parentheses: it stores the same function under a second name. type(double) asks about the function itself, while type(double(4)) asks about the number it returned.

The most useful part: a function can be passed to another function as an argument. This function goes through a list and changes every item with a function that it gets as a parameter:

local function applyToAll(list, change)
    local result = {}
    for _, value in ipairs(list) do
        table.insert(result, change(value))
    end
    return result
end

local function double(n)
    return n * 2
end

local prices = {2, 3, 5}
print(table.concat(applyToAll(prices, double), " "))
print(table.concat(applyToAll(prices, function(n) return n + 1 end), " "))

This prints:

4 6 10
3 4 6

Inside applyToAll, change is a parameter like any other, but it holds a function, so applyToAll calls it: change(value). applyToAll doesn't know what the change will be; the caller decides. The second call doesn't even give its function a name. It writes the function right in the parentheses of the call. A function without a name is called an anonymous function. It's handy for short functions that you need in one place only, and a short one can fit on a single line.

When you pass a function, write it without parentheses: double, not double(5). With parentheses, you call it right away and pass its result, the number 10:

local function applyToAll(list, change)
    local result = {}
    for _, value in ipairs(list) do
        table.insert(result, change(value))
    end
    return result
end

local function double(n)
    return n * 2
end

local prices = {2, 3, 5}
print(table.concat(applyToAll(prices, double(5)), " "))

The program ends with an error:

main.lua:4: attempt to call a number value (local 'change')

applyToAll got the number 10 in change and tried to call it. You'll see functions passed to other functions often: in the next exercise, you'll give one to table.sort.