How to make global variables "immutable" in Lua/LuaJ? - lua

Description
I'm doing a LuaJ program, and here's a lib script like this:
function foo()
print("foo");
end
I want the foo function can be invoked in other scripts directly (no require), but performs immutable in different scripts. ( Even a script overrides it, it performs as the original way in other scripts. )
For example, here's the script 1:
foo = function()
print("bar");
end
and here is the script 2:
foo();
What's done
I have saw these two questions. They do work but not the solution to this problem.
LuaJ How to avoid overriding existing entries in Globals table
Preventing Function Overriding in Lua Table
Making global environment access-only (Lua)
I tried loading lib every time exec a script, or set local _ENV, but because there may be further callbacks from Java to Lua, it doesn't work correctly.
I now handle it by create a Globals and load lib script every time load a script in Java like this:
public static void main(String[] args) {
loadAndCallViaDifferentEnv(libPath, script1);
loadAndCallViaDifferentEnv(libPath, script2);
}
static void loadAndCallViaDifferentEnv(String libPath, String scriptPath) {
Globals globals = JsePlatform.standardGlobals();
globals.loadfile(libPath).call();
globals.loadfile(scriptPath).call();
}
It works well, but costs much. Is there a better way?

I assume you want to protect three functions from overwriting: foo1, foo2 and print
-- define foo1 and foo2 inside protected table instead of as usual globals
local protected = {}
function protected.foo1()
print("foo1");
end
function protected.foo2()
print("foo2");
end
-- if a function to be protected already exists, remove it from globals:
protected.print = print
print = nil
-- Now set the metatable on globals
setmetatable(_G, {
__index = protected,
__newindex =
function(t, k, v)
if not protected[k] then
rawset(t, k, v)
end
end
})
Now you can invoke foo1, foo2 and print from other modules without require, but you can not overwrite them:
-- the script 1:
foo1 = function()
print("bar");
end
foo1(); -- original protected.foo1() is invoked
-- the script 2:
foo1(); -- original protected.foo1() is invoked

Related

How to run a string and get a table that has all the variables that the code created in lua?

So i'm making my own OS for LIKO-12 and I need to run .lk12 files but i need to have a function to run a string as code and return a table that has all of the variables that code created
Example:
function foo()
return "Hello,"
end
function bar()
return " World!"
end
hello = foo()..bar()
should return a table with 2 functions:
A function called foo that returns "Hello,"
A function called bar that returns " World!"
and a variable called hello that has "Hello, World!"
You should be able to call a function from the code using
vars["foo"]() or vars.foo()
Can anyone help me?
When you create or use a global variable it's actually stored in a table called "the environment", or _ENV (which is not global, it's a local that you get automatically)
So your code:
function foo()
return "Hello,"
end
function bar()
return " World!"
end
hello = foo()..bar()
is really doing:
function _ENV.foo()
return "Hello,"
end
function _ENV.bar()
return " World!"
end
_ENV.hello = _ENV.foo().._ENV.bar()
so if we just set _ENV to a new table, and return it, then this is exactly the table you want. You can do this with load (which is how you run strings) by passing it as the 4th argument. This function will do that:
function loadAndReturnEnv(codeString)
local env = {}
load(codeString, nil, nil, env)() -- error checking skipped for demonstration
return env
end
However, note that all the usual global stuff like string.rep, table.sort, load, print don't exist in this new _ENV. If that's what you want, that's great. But I guess you probably do want them. In that case we can use the metatable __index feature, so that if the code looks for something in its _ENV and it's not there, it looks in our (the caller's) _ENV instead.
function loadAndReturnEnvWithGlobals(codeString)
-- note: env is the environment for the code we're about to run
-- (which will be called _ENV in that code), and _ENV is the environment
-- of the loadAndReturnEnvWithGlobals function (i.e. the "real" environment)
local env = {}
setmetatable(env, {__index=_ENV})
load(codeString, nil, nil, env)() -- error checking skipped for demonstration
setmetatable(env, nil) -- set the table back to normal before we return it
return env
end
But wait... the program can now call print when you run it, but if it defines a function, that function can't call print since we deleted the link back to the real environment after the code returned. So I think the best way to fix that is to leave the environment linked, and just copy the functions etc.
function loadAndReturnEnvWithGlobals2(codeString)
local env = {}
setmetatable(env, {__index=_ENV})
load(codeString, nil, nil, env)() -- error checking skipped for demonstration
local result = {}
for k,v in pairs(env) do result[k]=v end
return result
end
although this copies the values, so if you have any functions that change the variables, you won't see the changed variables.
Which version you use is up to you. They each have pros and cons.

Can I use Lua's require to set environment of the calling file?

Is there a way to call require in a Lua file, and have the module set the environment of the file that calls it? For example, if I have a DSL (domain specific language) that defines the functions Root and Sequence defined in a table, can I have something like setfenv(1, dslEnv) in the module that allows me to access those functions like global variables?
The goal I in mind is using this is a behavior tree DSL in a way that makes my definition file look like this (or as close it as possible):
require "behaviortrees"
return Root {
Sequence {
Leaf "leafname",
Leaf "leafname"
}
}
without having to specifically bring Root, Sequence, and Leaf into scope explicitly or having to qualify names like behaviortrees.Sequence.
In short, I'm trying to make the definition file as clean as possible, without any extraneous lines cluttering the tree definition.
Can I have something like setfenv(1, dslEnv) in the module that allows me to access those functions like global variables?
Sure you can. You just have to figure out the correct stack level to use instead of the 1 in your setfenv call. Usually you'd walk up the stack using a loop with debug.getinfo calls until you find the require function on the stack, and then you move some more until you find the next main chunk (just in case someone calls require in a function). This is the stack level you'd have to use with setfenv. But may I suggest a ...
Different Approach
require in Lua is pluggable. You can add a function (called a searcher) to the package.loaders array, and require will call it when it tries to load a module. Let's suppose all your DSL files have a .bt suffix instead of the usual .lua. You'd then use a reimplementation of the normal Lua searcher with the differences that you'd look for .bt files instead of .lua files, and that you'd call setfenv on the function returned by loadfile. Something like this:
local function Root( x ) return x end
local function Sequence( x ) return x end
local function Leaf( x ) return x end
local delim = package.config:match( "^(.-)\n" ):gsub( "%%", "%%%%" )
local function searchpath( name, path )
local pname = name:gsub( "%.", delim ):gsub( "%%", "%%%%" )
local msg = {}
for subpath in path:gmatch( "[^;]+" ) do
local fpath = subpath:gsub( "%?", pname ):gsub("%.lua$", ".bt") -- replace suffix
local f = io.open( fpath, "r" )
if f then
f:close()
return fpath
end
msg[ #msg+1 ] = "\n\tno file '"..fpath.."'"
end
return nil, table.concat( msg )
end
local function bt_searcher( modname )
assert( type( modname ) == "string" )
local filename, msg = searchpath( modname, package.path )
if not filename then
return msg
end
local env = { -- create custom environment
Root = Root,
Sequence = Sequence,
Leaf = Leaf,
}
local mod, msg = loadfile( filename )
if not mod then
error( "error loading module '"..modname.."' from file '"..filename..
"':\n\t"..msg, 0 )
end
setfenv( mod, env ) -- set custom environment
return mod, filename
end
table.insert( package.loaders, bt_searcher )
If you put this in a module and require it once from your main program, you can then require your DSL files with the custom environment from .bt files somewhere where you would put your .lua files as well. And you don't even need the require("behaviortrees") in your DSL files. E.g.:
File xxx.bt:
return Root {
Sequence {
Leaf "leafname",
Leaf "leafname"
}
}
File main.lua:
#!/usr/bin/lua5.1
require( "behaviortrees" ) -- loads the Lua module above and adds to package.loaders
print( require( "xxx" ) ) -- loads xxx.bt (but an xxx Lua module would still take precedence)
At least in Lua 5.2, _ENV is a local that determinates the environment table. You can change the environment of any function, basically, the chunk.
_ENV = behaviortrees;
Another way is to automatically copy each field:
do
_ENV = _ENV or _G;
for k, v in next, behaviortrees do
_ENV[k] = v;
end
end
However it might be more efficient to manually local each field from behaviortrees.
Module "behaviortrees.lua"
local behaviortrees = {
-- insert your code for these functions
Root = function(...) ... end,
Sequence = function(...) ... end,
Leaf = function(...) ... end,
}
-- Now set the environment of the caller. Two ways are available:
-- If you want to make DSL environment isolated from Lua globals
-- (for example, "require" and "print" functions will not be available
-- after executing require "behaviortrees")
setfenv(3, behaviortrees)
-- or
-- If you want to preserve all globals for DSL
setfenv(3, setmetatable(behaviortrees, {__index = getfenv(3)}))
Main Lua program:
require "behaviortrees"
return Root {
Sequence {
Leaf "leafname",
Leaf "leafname"
}
}

Trying to make a lua proxy with metatables

I've read about metatables (here and here) and I asked myself if I could create a function call proxy by just adding a function with the same name to the __index table, so that it calls the index function (mine), and then I can call the normal one.
This is what I've tried:
local test = {}
test.static = function () print("static") end -- The normal function
local old = getmetatable(test) or {} -- Get the orginal meta
old.__index = {
static = function () print("hook") end -- Adding my function
}
test.static() -- call just for test ( prints -> static )
test = setmetatable( test , old ) -- setting the metatable
test.static() -- call just for test ( prints -> static, but should be hook )
Try giving the official source a read, specifically §2.4 – Metatables and Metamethods, and the description of the __index methamethod, which reads:
__index: The indexing access table[key]. This event happens when table is not a table or when key is not present in table. The metamethod is looked up in table.
Despite the name, the metamethod for this event can be either a function or a table. If it is a function, it is called with table and key as arguments, and the result of the call (adjusted to one value) is the result of the operation. If it is a table, the final result is the result of indexing this table with key. (This indexing is regular, not raw, and therefore can trigger another metamethod.)
We can see that your thinking is backwards. Properties on a table referenced by the __index metamethod are only looked up if the original table does not contain the key.
If you want to 'hook' a function you'll need to overwrite it, possibly saving the original to restore it later. If you want to tack functionally on to an existing function, you can write a neat little hooking function, which simply creates a closure around the functions, calling them in turn.
local function hook (original_fn, new_fn)
return function (...)
original_fn(...)
new_fn(...)
end
end
local test = {}
test.foo = function () print('hello') end
test.foo = hook(test.foo, function () print('world!') end)
test.foo() --> prints 'hello' then 'world!'
Alternatively, you can switch between metatables, assuming the original table never overrides the keys of interest, to get different results:
local my_table, default, extra = {}, {}, {}
function default.foo () print('hello') end
function extra.foo() print('world!') end
local function set_mt (t, mt)
mt.__index = mt
return setmetatable(t, mt)
end
set_mt(my_table, default)
my_table.foo() --> prints 'hello'
set_mt(my_table, extra)
my_table.foo() --> prints 'world!'

LuaJ loading two functions with the same name from two different LuaScripts

I have two Lua Scripts containing functions with the same name:
luaScriptA:
function init()
print( 'This function was run from Script A' )
end
luaScriptB:
function init()
print( 'This function was run from Script B' )
end
I would like to load both these functions using LuaJ into the globals environnment, for one script I usually do it as follows:
LuaValue chunk = globals.load(new FileInputStream(luaScriptA), scriptName, "t",
globals);
chunk.call();
This will load the function init() into globals and I can execute this function from java with:
globals.get("init").call();
The problem comes in when I load a second script, this will overwrite all functions with the same name previously declared. Is there any way I can prevent this and easily distinguish between the two functions? For example something like:
globals.get("luaScriptA").get("init").call(); //Access the init function of script A
globals.get("luaScriptB").get("init").call(); //Access the init function of script B
Please note that the script contains other functions as well and my goal is to run individual functions within the script, not the complete script at once.Working on the JME platform.
Put your functions in a table
luaScriptA:
A = {} -- "module"
function A.init()
print( 'This function was run from Script A' )
end
luaScriptB:
B = {} -- "module"
function B.init()
print( 'This function was run from Script B' )
end
Then you would do
globals.get("A").get("init").call();
globals.get("B").get("init").call();
The code below loads scripts in their own environment, which inherits from the global one for reading but not for writing. In other words, you can call print but each defines its own init. You'll probably have to do something to use it in LuaJ, but I don't know what.
local function myload(f)
local t=setmetatable({},{__index=_G})
assert(loadfile(f,nil,t))()
return t
end
local A=myload("luaScriptA") A.init()
local B=myload("luaScriptA") B.init()

Globals are bad, does this increase performance in any way?

I'm working in LuaJIT and have all my libraries and whatnot stored inside "foo", like this:
foo = {}; -- The only global variable
foo.print = {};
foo.print.say = function(msg) print(msg) end;
foo.print.say("test")
Now I was wondering, would using metatables and keeping all libraries local help at all? Or would it not matter. What I thought of is this:
foo = {};
local libraries = {};
setmetatable(foo, {
__index = function(t, key)
return libraries[key];
end
});
-- A function to create a new library.
function foo.NewLibrary(name)
libraries[name] = {};
return libraries[name];
end;
local printLib = foo.NewLibrary("print");
printLib.say = function(msg) print(msg) end;
-- Other file:
foo.print.say("test")
I don't really have the tools to benchmark this right now, but would keeping the actual contents of the libraries in a local table increase performance at all? Even the slightest?
I hope I made mysef clear on this, basically all I want to know is: Performance-wise is the second method better?
If someone could link/give a detailed explaination on how global variables are processed in Lua which could explain this that would be great too.
don't really have the tools to benchmark this right now
Sure you do.
local start = os.clock()
for i=1,100000 do -- adjust iterations to taste
-- the thing you want to test
end
print(os.clock() - start)
With performance, you pretty much always want to benchmark.
would keeping the actual contents of the libraries in a local table increase performance at all?
Compared to the first version of the code? Theoretically no.
Your first example (stripping out the unnecessary cruft):
foo = {}
foo.print = {}
function foo.print.say(msg)
print(msg)
end
To get to your print function requires three table lookups:
index _ENV with "foo"
index foo table with "print"
index foo.print table with "say".
Your second example:
local libraries = {}
libraries.print = {}
function libraries.print.say(msg)
print(msg)
end
foo = {}
setmetatable(foo, {
__index = function(t, key)
return libraries[key];
end
});
To get to your print function now requires five table lookups along with other additional work:
index _ENV with "foo"
index foo table with "print"
Lua finds the result is nil, checks to see if foo has a metatable, finds one
index metatable with "__index"
check to see if the result is is is table or function, Lua find it's a function so it calls it with the key
index libraries with "print"
index the print table with "say"
Some of this extra work is done in the C code, so it's going to be faster than if this was all implemented in Lua, but it's definitely going to take more time.
Benchmarking using the loop I showed above, the first version is roughly twice as fast as the second in vanilla Lua. In LuaJIT, both are exactly the same speed. Obviously the difference gets optimized away at runtime in LuaJIT (which is pretty impressive). Just goes to show how important benchmarking is.
Side note: Lua allows you to supply a table for __index, which will result in a lookup equivalent to your code:
setmetatable(foo, { __index = function(t, key) return libraries[key] end } )
So you could just write:
setmetatable(foo, { __index = libraries })
This also happens to be a lot faster.
Here is how I write my modules:
-- foo.lua
local MyLib = {}
function MyLib.foo()
...
end
return MyLib
-- bar.lua
local MyLib = require("foo.lua")
MyLib.foo()
Note that the return MyLib is not in a function. require captures this return value and uses it as the library. This way, there are no globals.

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