Is there an easy way to unpack two arrays/varargs in Lua? - lua

Question

Lets take a look at this pseudo code example:
-- Wraps a function with given parameters inside a callback
-- Usefull for sending class methods as callbacks.
-- E.g. callback(object.method, object)
local function callback(func, ...)
return function(...)
func(..., ...)
end
end
How would I go about this?
I know there is unpack, but it would get swallowed by the second vararg:
local function callback(func, ...)
local args = { ... }
return function(...)
func(table.unpack(args), ...)
end
end
callback(print, "First", "Second")("Third") --> prints First Third
The only option I found so far, is to concat those together and then unpack it:
local function callback(func, ...)
local args1 = { ... }
return function(...)
local args2 = { ... }
local args = { }
for _, value in ipairs(args1) do
table.insert(args, value)
end
for _, value in ipairs(args2) do
table.insert(args, value)
end
func(table.unpack(args))
end
end
Is this the only solution, or could I do better?
What I would like to have is either a function, that concats two arrays together (which should be faster than those two for loops) and then use table.unpack on it, or make those varargs concat.

Answer 1

From the Lua 5.4 Reference Manual: 3.4 Expressions
If an expression is used as the last (or the only) element of a list
of expressions, then no adjustment is made (unless the expression is
enclosed in parentheses). In all other contexts, Lua adjusts the
result list to one element, either discarding all values except the
first one or adding a single nil if there are no values.
So the only way is to manually combine both lists to a single one befor you use them.
There are several ways to do this.
for i,v in ipairs(list2) do
table.insert(list1, v)
end
for i,v in ipairs(list2) do
list1[#list1+i] = v
end
table.move(list2, 1, #list2, #list1 + 1, list1)
I'm not sure what kind of problem you're actually trying to solve here.
If you want to access your object from its methods use self.
-- Wraps a function with given parameters inside a callback
-- Usefull for sending class methods as callbacks.
-- E.g. callback(object.method, object)
Usually you would do something like this:
local callback = function(params) object:method(params) end
callback(params)
Instead of
callback(print, "First", "Second")("Third")
You could do this
local callback = function (...) print("First", "Second", ...) end
callback("Third")
Edit ( opinonated )
My main goal is to use member functions as callbacks. However, I'd
like to keep it general. The advantage of a callback function is to
omit the function and end keyword. It's shorter and looks closer to
what it would look like, if there would be no need for a self
argument. So this: object.event = function(...) return
self:method(...) end would become to this: object.event =
callback(self.method, self) what would be even nicer, is this (sadly
not possible in lua) object.event = self:method
So instead of
RegisterCallback(function() obj:method() end)
You say it is easier to do this, so you don't have to write function end?
local function callback(func, ...)
local args1 = { ... }
return function(...)
local args2 = { ... }
local args = { }
for _, value in ipairs(args1) do
table.insert(args, value)
end
for _, value in ipairs(args2) do
table.insert(args, value)
end
func(table.unpack(args))
end
end
RegisterCallback(callback(obj.method, obj)())
Your approach is not going to work if any of the arguments is nil to begin with. And there is not a single advantage. You're just typing other words while increasing the chance of running into errors.

Answer 2

You can control the param order to optimize your function.
local function callback(func, ...)
local args = {...}
return function(...)
func(..., table.unpack(args))
end
end
callback(print, "second", "third")("first") -- prints first second third