Summary

In this lesson, you learned that:

• A function is a named piece of code that you write once and run as many times as you need. You define it with local function name() … end, and you call it with its name and parentheses: name(). A function exists only from the line where it's defined; calling it earlier gives attempt to call a nil value (global 'name').
• A parameter is a name in the definition, an argument is a value in the call: local function plant(crop, count), plant("carrots", 12). Arguments go into parameters in order. A missing argument is nil, and extra ones are ignored.
• return gives a value back to the place of the call and ends the function. print only shows a value; a function without return gives nil. A function that calculates something should return the result.
• return a, b returns several values: local boxes, left = packEggs(20). A call gives all its values only as the last item of a list.
• A default value is filled in at the start of the function: bags = bags or 1. For a parameter that can be false, check if x == nil then.
• A local variable exists from its line to the end of its block, and a function sees the local variables created above it. A variable without local is global: a typo quietly creates a new one, and any function can overwrite it. That's why every variable and every function is local.
• A function is a value: you can store it in a variable and pass it to another function, without parentheses. function(x) … end without a name is an anonymous function.
• table.sort(t, function(a, b) return a > b end) sorts by your rule: the function returns true when a comes before b, and false for equal items. Items that the rule considers equal can end up in any order, so add a second rule when it matters.
• A closure is a function that remembers the local variables of the function that created it, such as the count of a counter.
• error("message") stops the program with main.lua:N: message (error(message, 0) without the position), and assert(condition, message) does the same when the condition isn't true. pcall(f, …) catches the error: it returns true and the results, or false and the message.

In the next lesson, you'll use tables in a new way: as dictionaries, which store values under names instead of numbers, such as how many of each crop is in the barn.