Lua - How can I grab any return?

Question

I am interested in grabbing any return format of a function. For example

function foo()
  return 1
end

local result = foo() -- foo is numeric 1

function foo()
  return {1,2,3}
end
local result1, result2, result3 = foo()
local result = foo() -- this is bad as result is `1` but `2` and `3` are lost

function foo()
  return 1, 2, 3
end
local result = foo() -- foo is a table with all the numbers, that's ok

I am building a profiler which will overwrite functions with proxy functions but I need to know the data returned, then check type() of it and access accordingly` but from the code it can be seen that I am unable to access all 3 situations with one method. Is there any ?

Answer 1

If the maximum number of returns is known, use something like

v1,v2,v3 = foo()

but you won't be able to tell whether foo returned two values or three, with the last one being nil.

The robust solution is to collect all returns in a table:

v = table.pack(foo())

Then v.n contains the number of returned values, including all nils.

Answer 2

Here's a version that works on any Lua version 5.1+.

local function WrapAndInspect(Inspector, FunctionToCall)
    local function Inspect(...)
        Inspector(...)
        return ...
    end
    local function Wrapper(...)
        return Inspect(FunctionToCall(...))
    end
    return Wrapper
end

What WrapAndInspect does is generate a function that will call the given function, then pass its return values to a second function you provide. That function can do whatever processing you feel is necessary on them. But the framework will ensure that the return values from the original function are passed as they were.

Here's a variation that does something similar, but instead of wrapping the FunctionToCall, it returns a function that takes a function to be called (along with its parameters):

local function CallAndInspect(Inspector)
    local function Inspect(...)
        Inspector(...)
        return ...
    end
    local function Caller(FunctionToCall, ...)
        return Inspect(FunctionToCall(...))
    end
    return Caller
end

You could use this one on any particular function you want to inspect.

Answer 3

Here is a workaround for those that do not have access to table.pack. To me it seems simple and it should work on lua 5.1 and above - and maybe even earlier lua versions.

table_pack should work like table.pack

function table_pack(...)
    return {n=select("#", ...), ...}
end
function foo()
    return 1, 2, 3
end

local v = table_pack(foo())
print(v.n)