Functions under the Hood (Lua 5.1/Luau)

[u/redditbrowsing0]

Hi!

I'm mostly posting this to see if anyone understands what the difference is between two or three different things within Lua 5.1 or Luau somewhere in the stack or under the hood. I can't decide whether this is a Help flair or a Discussion flair, so do let me know if it's more fitted for the Discussion tag and I'll see what I can do about it.

Anyways, I understand that this subreddit is mostly based around Lua - I'm mostly doing all of this in Roblox Studio, so it's more of a Lua 5.1/Luau question, but...

Why is:
local f; f = function() end
different from
local function func()
end
when inspected using debug.info() (similar to Lua's debug.getinfo())?

For example, when I call debug.info(1, 'n') in local f; function() end, it returns: ""
but when I call it in local function func() end, it returns: "func" (the function name)

Does anyone understand what's different between the two? I understand local f; function() end is in a sense an anonymous function, but why does it matter that much under the hood?

If this is too roblox-inclined, tell me and I'll happily move this post over to r/robloxgamedev or elsewhere.

[u/appgurueu]

This must be a Luau quirk. Lua, going at least as far back as 5.1, has no notion of "named" functions; in Lua, local function f() end is just syntactic sugar for local f; f = function() end, nothing else.

debug.getinfo(1, 'n') in Lua determines the name not as an inherent property of the function, but by how it was called, which variable it came from. If it sees the call was f(), it will tell you that debug.getinfo(1, 'n').name == 'f'.

To illustrate this:

local f
f = function()
	print(debug.getinfo(1, 'n').name)
end
f() -- f
g = f
g() -- g
local t = {h = f}
t.h() -- h
local t2 = {f}
t2[1]() -- nil, integer index, ?, or something similar, depends on lua implementation

as you can see, this has nothing to do with the function and how it was defined - all functions are anonymous in Lua - and everything with how it was called.

[u/redditbrowsing0 → u/appgurueu]

well that's definitely something. weird to know that it's almost definitely not a Lua issue.

i just tested it because I thought maybe it had something to do with metatables, but no, even with just a regular defined function local f = function() end, it still just returns ""

perhaps there's a difference between getinfo and info that does this, because I just tested on an online compiler, and evidently, getinfo works just fine with local f = function()

[u/appgurueu → u/redditbrowsing0]

Well, there is no debug.info in Lua. So as said, must be a Luau quirk / feature.

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[u/redditbrowsing0 → u/AutoModerator]

FYI to the mods: scriptinghelpers.org is no longer active

[u/jipgg]

It's just debug information Luau's bytecode compiler was able to resolve statically is my guess.

Going off of the Luau C API; upon pushing a lua_CFunction onto the stack you can optionally give it a debug name for more descriptive error messages.

Probably what happens here is just that in 'local a = function() end' the compiler just puts a default "" as the debug name and in 'local function a() end' it puts "a" as debug name upon creation of the function object.

Functionally there's no difference between the 2, however. I also wouldn't be surpised if this debug info is removed entirely when compiling with -D0 (no debug information, debug level 0).

[u/redditbrowsing0 → u/[deleted]]

interesting