How do you throw Lua error up? - error-handling

Question

Is it possible to throw a Lua error from a function to be handled by the script calling the function?
For example the following will throw an error at indicated comment
local function aSimpleFunction(...)
string.format(...) -- Error is indicated to be here
end
aSimpleFunction("An example function: %i",nil)
But what I would rather do is catch the error and throw out a custom error by the function caller
local function aSimpleFunction(...)
if pcall(function(...)
string.format(...)
end) == false then
-- I want to throw a custom error to whatever is making the call to this function
end
end
aSimpleFunction("An example function: %i",nil) -- Want the error to start unwinding here
The intention being that in my actual use cases my functions would be more complicated and I would like to provide more meaningful error messages

Answer 1

The stack level of an error can be specified when throwing a new error
error("Error Message") -- Throws at the current stack
error("Error Message",2) -- Throws to the caller
error("Error Message",3) -- Throws to the caller after that
Usually, error adds some information about the error position at the beginning of the message. The level argument specifies how to get the error position. With level 1 (the default), the error position is where the error function was called. Level 2 points the error to where the function that called error was called; and so on. Passing a level 0 avoids the addition of error position information to the message.
Using the example given in the question
local function aSimpleFunction(...)
if pcall(function(...)
string.format(...)
end) == false then
error("Function cannot format text",2)
end
end
aSimpleFunction("An example function: %i",nil) --Error appears here

Answer 2

Use the error function.
error("something went wrong!")

Answer 3

Catching an error is as simple as using pcall
My_Error()
--Error Somehow
end
local success,err = pcall(My_Error)
if not success then
error(err)
end
Undoubtedly you're asking how this works. Well pcall runs a function in a protected thread (protected-call) and returns a bool, if it ran successfully, and a value (what it returned/the error).
Also don't think this means arguments to the function are impossible, simply pass them to pcall as well:
My_Error(x)
print(x)
--Error Somehow
end
local success,err = pcall(My_Error, "hi")
if not success then
error(err)
end
For more error handling control, see http://www.lua.org/manual/5.3/manual.html#2.3 and http://wiki.roblox.com/index.php?title=Function_dump/Basic_functions#xpcall