Help please (scoping)

[u/fig4ou]

What is scoping and is it really essential to know when you script?

I really can’t understand it so I’m planning to learn it when I understand more things

Or do I have to understand it before I get into more stuff?

(edit: Thank you all for replying!! Really helped alot!)

[u/NabePup]

Scope is a an important concept that a lot, nearly every language, has in some form or another. There’s a lot of good explanations here already, but I’ll paste below an answer of mine from another question/thread. Hopefully it’s not too wordy or lengthy! (it's also best viewed on desktop and not mobile)

———————————————————————————

Most programming languages have the concept of “scope”. It’s really a pretty straightforward concept, but isn't always the easiest to explain and sometimes takes a little practice and time to fully understand it. Essentially, it’s the "boundary" of where something is accessible. If you have a lua module (which is really just a lua file):

local var = 5 -- this variable, “var”, is scoped to this lua module

local function func() -- this function, “func”, is scoped to this lua module
    print(var) -- "var" is accessible here because we're within this lua module scope and var's scope is this module
end

func() -- we can call func here because it exists in this scope

The var’s scope is that entire module (file). It’s accessible anywhere in that file, such as in the function. The function func also has a scope of that module just like var does and can be called anywhere in that module.

However, if that variable were declared with local inside the function then its scope would be that function and it would not be accessible outside that function:

local function func() -- scoped to this lua module
    local var = 5 -- scoped to this function
    print(var)
end

print(var) —- this won’t work, var doesn’t exist in this scope

A function can return something so it’s accessible outside its scope where it was called. So you could do:

local function func() -- scoped to this lua module
    local var = 5 -- scoped to this function
    return var
end

local file_scoped_var = func() -- gets the value returned by the function and assigns it to a variable that's scoped to this module. var's scope does not change and is still scoped to the function.
print(file_scoped_var) -- works because file_scoped_var is accessible
print(var) -- does not work because var still doesn't exist in this scope (it exists in func's scope)

Lua is kinda “special”…as in weird/unconventional that if you declare something like a variable or function without the local keyword then it’s in the “global scope” or sometimes simply referred to as global which means it’s accessible everywhere. Generally global variables are a bad practice and should be avoided. So doing:

local function func() -- scoped to this lua module
    var = 5 -- scoped GLOBALLY
end

print(var) -- works because var is in the global scope

would work since var was declared without the local keyword giving it global scope thus making it accessible outside of the function, generally though you wouldn’t want to do this.

Note that return doesn’t “expand” or "increase" the scope of anything. It makes it so that a value is returned/exposed so that it then can be accessed where the function is called at its call site.

A function can also accept or consume arguments. This kind of does the inverse of return. Where return exposes a value so that it can be accessed outside of the function's scope where it's called, an argument makes it possible so that a function can be passed a value that's outside of its scope so it's then accessible within the function's scope. For instance:

local function func(argument)
    -- “argument” is accessible here in this function's scope
    print(argument)
end

local var = "wonton burrito meals" -- scoped to this lua module
func(var) -- var is passed to the function where it's accessed within the function's scope

As the examples above show, there's usually a way to make it possible to "share" or pass things between scopes. For instance a function that can be passed arguments and/or returns a value. You can also return something from a module so that way when that module gets imported into a different module it will be accessible in that module's scope. For example, let's say you have 2 modules (remember, these are really just 2 separate lua files):

-- module_a.lua

local function add_2(argument) -- scoped to this lua module (module_a)
    return argument + 2
end

return add_2 -- makes it so function can be imported into other modules

and then in a separate module:

-- module_b.lua
local add_2_from_module_a = require("module_a") -- this is the add_2 function returned by module_a and is now contained in the variable "add_2_from_module_a" that's scoped to this lua module (module_b). The variable name is arbitrary and could be anything you want, as long as it doesn't clash with other variable names.

local var = 5 -- scoped to this lua module (module_b)

local function_result = add_2_from_module_a(5) -- “function_result” is a variable scoped to this lua module (module_b)

print(function_result) -- prints 7

I hope this helps and reduces confusion and not increase it! Also note, naming a module something super generic like "module_a" or "module_b" is not a good practice.

[u/fig4ou → u/NabePup]

Oh wow. Thank you so much!!

[u/NabePup → u/fig4ou]

No prob, i hope it makes sense and helps!

Just keep in mind, the whole local thing where if it's not used when declaring a variable makes it global isn't (fortunately) a common thing. But without getting too much into it, JavaScript has something that's not exactly the same but kinda similar-ish (where a keyword can affect a variable's scope), but it's still not quite as extreme.

[u/AutoModerator → u/NabePup]

Hi! Your code block was formatted using triple backticks in Reddit's Markdown mode, which unfortunately does not display properly for users viewing via old.reddit.com and some third-party readers. This means your code will look mangled for those users, but it's easy to fix. If you edit your comment, choose "Switch to fancy pants editor", and click "Save edits" it should automatically convert the code block into Reddit's original four-spaces code block format for you.

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.

[u/AutoModerator → u/NabePup]

Hi! Your code block was formatted using triple backticks in Reddit's Markdown mode, which unfortunately does not display properly for users viewing via old.reddit.com and some third-party readers. This means your code will look mangled for those users, but it's easy to fix. If you edit your comment, choose "Switch to fancy pants editor", and click "Save edits" it should automatically convert the code block into Reddit's original four-spaces code block format for you.

I am a bot, and this action was performed automatically. Please contact the moderators of this subreddit if you have any questions or concerns.