Re: Call Lua functions without side effects

["Alex Davies" <alex.mania@...>]

Hey Peter,

Coming from other languages it's confusing, but in Lua function closures are 
only created at runtime. Consider the following to see why this is 
necessary:
eg:

local type = type; -- cache in an upvalue for speedier access
function foo(x)
 print(type(x))
end

Without running the chunk, the upvalue is never set, the closure/function 
never created and the global variable "foo" is not assigned.

So you're kind of asking to be able to assign global variables in a chunk, 
solve for closure upvalues, and create closures, all whilst not causing any 
side effects.
Pretty competing goals there. Furthermore in the above example, if the 
global table has been assigned a metatable which defines __index or 
__newindex virtually any kind of side effect is possible.

As others have said though, you can just run the script in a sandbox and 
allow it to do anything it wants. But that solution is probably not ideal 
either - as I imagine you want the "sub"functions to run with the standard 
environment so that they can access your usual functions. Unfortunately it 
is very challenging to change these environments for non-global functions 
declared in the chunk.

One possible (and unportable between lua versions) solution would be to 
quickly scan the main chunks code to ensure it doesn't perform any calls, 
tailcalls or loops (to ensure it doesn't hang). That would be reasonable 
safe, and allow most main chunks to be run without a problem. To make it 
completely safe you could temporarily remove any metatables assigned to the 
global table and any basic lua types (eg the string library). If this 
solution appeals to you I could provide code, but you're better off finding 
a more portable solution.

- Alex