Arguments not going inside of callback as planned

[Violet_sheer (TheOtherSider)]

I’m working on creating modules and using more powerful and intuitive sources from the API. I can’t find out why I can put the variable params into the callback, but not the arguments(…). The 3-dotted arguments just underline when put inside the callback. What is the reason for this?

local TweenService = game:GetService("TweenService")

local Tweens = {}

Tweens.Create = function(instance,info,properties)
	local tween = TweenService:Create(instance,info,properties)
	tween:Play()
	return tween
end

Tweens.Complete = function(tween,callback,...)
	local params = unpack({...})
	tween.Completed:Connect(function()
		callback(params)
	end)
end

return Tweens

[Negativize]

Programming in Lua : 5.2 states the following around using variadic arguments in functions:

Sometimes, a function with a variable number of arguments needs to pass them all to another function. All it has to do is to call the other function using unpack(arg) as argument: unpack will return all values in arg, which will be passed to the other function…

Assigning a variable to the output of unpack(...) rather than using the result of the method itself does not necessarily guarantee that the variable is equal to the return values created by unpack(...). Recall that the unpack(...) function behaves like so:

function unpack(list)
  -- i = 1, j = #list
  return list[i], list[i+1], ..., list[j]
end

There is a subtle difference between passing the results of unpack(...) to a function and passing a variable assigned to whatever unpack(...) returns to a function, illustrated below:

function powers()
  return {1, 2, 4, 8, 16, 32, 64}
end

local first, second, third = table.unpack(powers())

print(first, second, third)
-- 1, 2, 4
print(table.unpack(powers()))
-- 1, 2, 4, 8, 16, 32, 64

As you can see, assigning n variables to the result of unpack(...) will assign the first n values of unpack(...) to those first n variables. If you just assign one value to a call to unpack(...), it’s only going to take on the first value in that list passed to unpack(...). This is why the Lua documentation tells you to just straight up use unpack(...) in the passed arguments of whatever function is accepting it.

A more closely aligned example to your code demonstrating this is below:

function foo(func, ...)
  unpacked = table.unpack(...)
  func(unpacked)
end

function bar(func, ...)
  func(table.unpack(...))
end

foo(print, {"Goodbye", "cruel", "world."})
bar(print, {"Goodbye", "cruel", "world."})

The call to foo only returns "Goodbye" whereas the call to bar returns the entire string, as expected. See this in effect here. I’m sure you want the second, so just use unpack({...}) instead of the unneccesary params assignment.

This is my guess as to why the code is underlined. I could be entirely off the mark, though…

[Violet_sheer (TheOtherSider)]

Woah, thanks for the help. I think its because you can’t put the arguments inside a seperate thread:

callback(...) -- works in Tween.Complete

task.spawn(function() -- or event(.Completed)
	callback(...) - does not work(underlined red)
end)

When I asked the AI assistant if events created a seperate thread, it responded no.