quoting troubles in Lua - lua

I am trying to make a quote function in lua, so i can use the arguments as strings but without quotes or access them in some environment. Much like in the second comment on this question
w = print
function test()
local function _ix( _ , k )
w( " _ix \\ " , _ , k )
local v = rawget( _G , k )
w( " <-- " , k )
return k
end
local _ = setmetatable( {} , { __index = _ix } )
local function q( _ ) return _ end
q = setfenv( q , _ )
return q
end
So, when I run it:
q = test()
w( "q( uno )" , q( uno ) )
It doesn't even call the __index metamethod:
---------- Capture Output ----------
q( uno ) nil
> Terminated with exit code 0.
So, what I'm doing wrong?

If I'm understanding correctly, then what you're trying to do doesn't make much sense. uno will be looked up in the environment that q is called in, not with. In your example it's like calling q(nil). The example from the other question works because they're working in the same, global environment.
You can use write a helper function to intercept your current environments nil-lookups, but it must be called preemptively in any environment you want to use these nil-to-string lookups.
local function intercept (tab)
setfenv(2, setmetatable(tab or {}, {
__index = function (_, key)
return key
end
}))
end
And you'll need an environment cloning function, unless you want to manually create your sandboxes every time, else you'll probably mess up parent environments (e.g., _G). You could move this logic inside of intercept for a cleaner function call, but with less flexibility.
local function clone_current_env ()
local env = {}
for key, value in pairs(getfenv(2)) do
env[key] = value
end
return env
end
Using them together, you can cause nil lookups in whichever environment you're in to become strings.
intercept(clone_current_env())
print(type(foo), type(bar)) --> string string
This is some ugly metaprogramming, and I don't really know why you'd want to write code like this, except as a proof of concept.
A full example.
DEMO
local function clone (tab)
local new = {}
for key, value in pairs(tab) do
new[key] = value
end
return new
end
local function enable_nil_strings ()
setfenv(2, setmetatable(clone(getfenv(2)), {
__index = function (env, key)
return key
end
}))
end
local function disable_nil_strings()
setmetatable(getfenv(2), nil)
end
-----------------------------------------------------
print(type(foo), type(bar)) --> nil nil
enable_nil_strings()
print(type(foo), type(bar)) --> string string
disable_nil_strings()
print(type(foo), type(bar)) --> nil nil
Finally, arguably the best way to implement this would be to simply wrap around an execution context:
local function with_nil_strings (context, ...)
local env = {}
for key, value in pairs(getfenv(2)) do
env[key] = value
end
setfenv(
context,
setmetatable(env, {
__index = function (_, key) return key end
})
)
context(...)
end
print(type(foo)) --> nil
with_nil_strings(function ()
print(type(foo)) --> string
end)
print(type(foo)) --> nil

Related

Lua: define functions from a table

NativeTable = {
["print"] = {},
["LoadResourceFile"] = {}
}
for k, v in pairs(NativeTable) do
k = function(...)
print("test")
end
end
this would result in defining them in the NativeTable and not as a global function
print = function(...)
print("test")
end
LoadResourceFile = function(...)
print("test")
end
So I'm trying to define a global function with a table name
Guess i could do smth like this
But there must be a better way?
NativeTable = {
["test"] = {},
["LoadResourceFile"] = {}
}
local function OverWriteFunction(FuncName, Func)
local Base = [[ = function(...)
print("jaa")
end
]]
local Final = FuncName .. Base
return Final
end
for k, v in pairs(NativeTable) do
load(OverWriteFunction(k))()
end
In your first example you are redefining the variable k, that is local inside the for loop, so this will not be usable outside the one loop where you define it.
Your second example is something you absolutely should avoid if you can, since defining code inside a string and loading it later means, that even only a syntax error will not be shown on "compiling" it, but only when that exact part is executed. And especially, when you are concatenating string that are meant to be code, you just get the risk of glueing something together wrongly, if you try to have that made generically.
If I understood correctly what you are trying to achieve, I would say, it could be something like this:
NativeTable = {
["test"] = {},
["LoadResourceFile"] = {},
[3] = {}
}
for k, v in pairs(NativeTable) do
if type(k) == "string" then
_G[k] = function(...)
print("output")
end
end
end
test()
LoadResourceFile()
-- _G[3]()
Which outputs:
output
output
What I have done here is to use the _G table which is the global environment of lua, all things you define there with a string in the brackets will be globally available (and what is globally available is inside that table, so be careful, since you can override other functions and variables you have defined!), even when writing it as a normal function call. I did also make sure, to only do this, when the type of k was string, otherwise you could start to define functions that can be called like the last commented out call.
This doesn't make too much sense for me:
NativeTable = {
["print"] = {},
["LoadResourceFile"] = {}
}
for k, v in pairs(NativeTable) do
k = function(...)
print("test")
end
end
First you create a table with table elements print and LoadResourceFile.
Then you iterate over that table and replace the table elements with functions.
Why not simply do this:
myTable = {
a = function() print("I'm function a") end,
b = function() print("I'm function b") end,
}
or
myTable = {}
myTable.a = function () print("I'm function a") end
Then you can call the global function like so: myTable.a()
If you insist on having those functions outside of a table you can simply insert them into the global environment table.
for k, v in pairs(myTable) do _G[k] = v end
then you could call a and b globally. But whatever you're trying to accomplish there's probably a better way than this.

Accessing the key in the value (right side) of a table key affectation in Lua

The goal is to match a key of a table with a value depending of the key.
example = { ["dummy"] = this .. " example" }
print example.dummy -- print "dummy example"
Where this is the keyword to refer to the key. Is there any way to do that in Lua?
There is no way to do this directly.
You could do some preprocessing:
example = { ["dummy"] = "{THIS} example" }
for k,v in pairs(example) do
example[k]=v:gsub("{THIS}",k)
end
print(example.dummy)
It's not possible to have as clean an expression as:
t = { foo = this .. ' bar' }
because this would always be expressed without relation to the key or the table. That is, you can't capture an expression as the value of a table entry.
What is possible is implementing some level of indirection using metatables and functions, but it's hardly pretty. Here we do fetch-time evaluation. You could also recompute the results.
local function indirect_table ()
local uptable = {}
return setmetatable({}, {
__index = function (self, key)
local value = uptable[key]
return type(value) == 'function' and uptable[key](key) or value
end,
__newindex = function (self, key, value)
uptable[key] = value
--[[precompute, with no need for an uptable, or __index:
`rawset(self, key, value(key)`]]
end
})
end
local tab = indirect_table()
tab.foo = function (key) return key .. 'bar' end
print(tab.foo) --> 'foobar'
Note: this example uses a closure, but you can implement this kind of pattern using getmetatable as well.
Personally, I'd abstract this into an indirection pattern that allows arbitrary keys and values, and their actions to be specified. I figure this kind of pattern would mostly be used programatically, rather than by hand, where the results of key values are dependent on the inputs received. Again, not pretty, but a little more robust (optional actions).
local function I (_, value) return value end
local K = setmetatable({
__call = function (actor, key)
return actor.action(key, actor.value)
end
}, {
__call = function (K, value, action)
return setmetatable({ value = value, action = action or I }, K)
end
})
local T = setmetatable({
__newindex = function (self, key, value)
if getmetatable(value) == K then
value = value(key)
end
rawset(self, key, value)
end
}, {
__call = function (T, o)
return setmetatable(o or {}, T)
end
})
Simple use:
local function concat (left, right) return left .. right end
local t = T {}
t.foo = K('bar', concat) -- with a common action
t.zar = K({}, unknown_action) -- without action (`nil`)
t.qux = 'qaz' -- standard
print(t.foo, t.zar, t.qux)
This is strange metaprogramming. I'd double-check the reasoning for needing such an approach. Perhaps you're falling into an XY Problem trap? Really feels like a solution to a problem that doesn't need to exist in the first place.

How to implement a trace function for table to functions?

I have a table like this
local ftable = {
getPinId = app.getPinId
}
ftable is passed to another function which exports it as a RPC interface.
This works but now I want to add function call tracing to a log file.
The simple approach is
local ftable = {
getPinId = function(...) print("getPinId") app.getPinId(...) end
}
But, this is not particularly nice.
I'd like to put something like:
local trace = function(func, ...)
return function(...) print(func) func(...) end
end
local ftable = {
getPinId = trace(app.getPinId)
}
But this doesn't produce quite the desired result. The parameters are not being passed through.
One other option is to use a metatable like this:
local ftable = {}
setmetatable(ftable, {
__index = function(_, k)
printf("Call: app.%s\n", k) return app[k] end
})
Which works. But I'd also like to be able to print the parameters that are passed if possible.
Any suggestions?
I'm exclusively using luajit if that makes any difference.
Wrapping a function call is easy in Lua:
local function wrap( f )
local function after( ... )
-- code to execute *after* function call to f
print( "return values:", ... )
return ...
end
return function( ... )
-- code to execute *before* function call to f
print( "arguments:", ... )
return after( f( ... ) )
end
end
local function f( a, b, c )
return a+b, c-a
end
local f_wrapped = wrap( f )
f_wrapped( 1, 2, 3 )
Output is:
arguments: 1 2 3
return values: 3 2
One problem for logging/tracing is that Lua values (including functions) don't have names themselves. The debug library tries to find suitable names for functions by inspecting how they are called or where they are stored, but if you want to make sure, you'll have to supply a name yourself. However, if your functions are stored in (nested) tables (as indicated in a comment), you could write a function that iterates the nested tables, and wraps all functions it finds using the table keys as names:
local function trace( name, value )
local t = type( value )
if t == "function" then -- do the wrapping
local function after( ... )
print( name.." returns:", ... )
return ...
end
return function( ... )
print( "calling "..name..":", ... )
return after( value( ... ) )
end
elseif t == "table" then -- recurse into subtables
local copy = nil
for k,v in pairs( value ) do
local nv = trace( name.."."..tostring( k ), v )
if nv ~= v then
copy = copy or setmetatable( {}, { __index = value } )
copy[ k ] = nv
end
end
return copy or value
else -- other values are ignored (returned as is)
return value
end
end
local ftable = {
getPinId = function( ... ) return "x", ... end,
nested = {
getPinId = function( ... ) return "y", ... end
}
}
local ftableTraced = trace( "ftable", ftable )
ftableTraced.getPinId( 1, 2, 3 )
ftableTraced.nested.getPinId( 2, 3, 4 )
Output is:
calling ftable.getPinId: 1 2 3
ftable.getPinId returns: x 1 2 3
calling ftable.nested.getPinId: 2 3 4
ftable.nested.getPinId returns: y 2 3 4
Some things to be aware of:
Table keys can be arbitrary Lua values, not just short strings entirely consisting of printable characters.
Tables can contain cyclic references. If they do, the naive implementation above will die with a stack overflow.
Use the __call metamethod instead:
M = { __call =
function (t,...) print("calling ",t.name,...) return t.func(...) end
}
trace = function(func,name)
return setmetatable({func=func,name=name},M)
end
function f(...)
print("in f",...)
end
g=trace(f,"f")
g(10,20,30)

Setting __index of current environment in Roblox

In Roblox Studio, I have a ModuleScript object that implements an analogous class to the one shown in chapter 16 of the 1st edition of Programming In Lua, as shown below:
local h4x0r = { }
local function setCurrentEnvironment( t, env )
if ( not getmetatable( t ) ) then
setmetatable( t, { __index = getfenv( 0 ) } )
end
setfenv( 0, t )
end
do
setCurrentEnvironment( h4x0r );
do
h4x0r.Account = { };
setCurrentEnvironment( h4x0r.Account );
__index = h4x0r.Account;
function withdraw( self, v )
self.balance = self.balance - v;
return self.balance;
end
function deposit( self, v )
self.balance = self.balance + v;
return self.balance;
end
function new( )
return setmetatable( { balance = 0 }, h4x0r.Account )
end
setCurrentEnvironment( h4x0r );
end
end
return h4x0r
I then attempted to use the following script to access the Account class, assuming that all of the members of the 2nd do-end block would be assigned to h4x0r.Account:
h4x0r = require( game.Workspace.h4x0r );
Account = h4x0r.Account;
account = Account.new( );
print( account:withdraw( 100 ) );
The above script fails with the error Workspace.Script:5: attempt to call method 'withdraw' (a nil value), so it must be an issue regarding the line where I set the __index field of h4x0r.Account.
Can someone please explain to me where I went wrong?
Try using getfenv(2) and setfenv(2, t) instead of getfenv(0) and setfenv(0, t). You essentially want to change the environment of the encapsulating function, which would be stack level 2.
0 is a special argument that would instead get or set the environment of the thread, which is used as a default environment in some cases, but that does not affect the individual closures that have already been instantiated in the thread, hence it doesn't work in this case.

Unable to figure out lua table inheritence

Hope someone can make sense of what I'm attempting to figure out, Just don't seem to understand Lua enough to achieve this.
--[[
tbl.a.test("moo") returns "Table A moo appears"
tbl.b.test("moo") returns "moo appears"
]]
tbl = {
a = { ID = "Table A" },
b = {
test = function(...) print(... .. " appears") end,
},
}
tbl.a__index = function(self, ...) tbl.b[self](tbl.a.ID .. ...) end
What I'm attempting to do is I could create several tables a, c, d, e and not have to copy test to each one. When tbl.a.test, tbl.c.test, tbl.d.test is used, It'll retrieve the tbl.a.ID var, then call tbl.b.test(ID, "moo")
So far all I'm finding out is it's not able to find .test on anything other than tbl.b
** EDIT **
Thank's to support so far the code is now;
tbl = {
a = { ID = "Table A " },
b = { test = function(...) local id, rest = ... print(id .. ": " .. rest) end },
}
setmetatable(tbl.a, {__index=function(self, k, ...) local rest = ... return tbl.b[k](tbl.a.ID, rest) end})
However, the ... is not being progressed for some odd reason :|
You're missing a period between tbl.a and __index.
__index needs to be on a's metatable, not the table itself.
You don't return anything from your __index function
self in the __index function is the table being indexed, not the key (which is the second argument)
This should work:
setmetatable(tbl.a, {__index=function(self, k) return tbl.b[k](tbl.a.ID) end})
--------------------------------------------------------------------------------
-- [Sub]Class creation
--------------------------------------------------------------------------------
function newclass(new_obj,old_obj)
old_obj = old_obj or {} --use passed-in object (if any)
new_obj = new_obj or {}
assert(type(new_obj) == 'table','New Object/Class is not a table')
assert(type(old_obj) == 'table','Old Object/Class is not a table')
old_obj.__index = old_obj --store __index in parent object (optimization)
return setmetatable(new_obj,old_obj) --create 'new_obj' inheriting 'old_obj'
end
--------------------------------------------------------------------------------
prototype = {
test = function(self,s) print('Table ' .. self.id .. ' ' .. s .. ' appears') end
}
tbl = {}
tbl.a = newclass({id = 'A'},prototype)
tbl.b = newclass({id = 'B'},prototype)
tbl.a:test('moo')
tbl.b:test('moo')
The distinction between class and object in Lua is only theoretical. In practice they are implemented exactly the same way.
Anytime you need to do inheritance, you can use my general-purpose newclass() function to either create a new class/object, or inherit from an existing one.
Any common code & data you would like to have passed on should go into the 'prototype' table (whatever you'd like to call it for each case).
Also, you seem to forget to use the method calling syntax (that uses a colon instead of a dot) when calling methods. Without it, the self parameter is not automatically recognized.

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