How to loadstring()? (1 Viewer)

[Hikooshi]

local a = 1
loadstring("b=a+1 print(b)")

This code doesn't work. Error - a is nil. But if a is global then code works. How to use this function with local a? Lua 5.1

 

[GavinW]

You cannot. In Lua 5.2 loadstring is replaced by load which can take an environment as its fourth argument, but that will not help you with using a local variable.

 

[stetre]

You can, however, use loadstring to define functions that accept arguments, and pass the local variable as argument. For example:

local a = 1
loadstring("return function(a) local b=a+1 print(b) end")( )(a)

 

[Hikooshi]

You can, however, use loadstring to define functions that accept arguments, and pass the local variable as argument. For example:

local a = 1
loadstring("return function(a) local b=a+1 print(b) end")( )(a)

Yes, this works, but what is the third bracets "(a)"?

 

[stetre]

The third bracket is for passing the argument. I'll break it down into individual steps, so to make it (hopefully) clearer:

local a = 1
local f = loadstring("return function(x) local b=x+1 print(b) end") -- step 1
local g = f( ) -- step 2
g(a) -- step 3

At step 1, loadstr returns a function whose body is the string it receives as argument. In this case this is equivalent to writing:

local function f( ) return function(x) local b=x+1 print(b) end

Note that loadstring just returns a function, but it doesn't execute it. This function (f), when executed (step 2), returns another function that accepts an argument. Here I called it 'x' instead of 'a' to make clear that it is the argument, not (yet) the local variable 'a'. Step 2 thus ends up being equivalent to writing:

local function g(x) local b=x+1 print(b) end

Finally, at step 3 we execute the function g passing it 'a' as argument (thus x=a).

The original example is obtained by merging the three steps into one.

BTW, you can do it in another way which is arguably simpler, but I don't know if it works in Lua 5.1:

local a = 1
loadstring("local a=({...})[1] local b=a+1 print(b)")(a)

EDIT: fixed the last code snippet

 

[Hikooshi]

The third bracket is for passing the argument. I'll break it down into individual steps, so to make it (hopefully) clearer:

local a = 1
local f = loadstring("return function(x) local b=x+1 print(b) end") -- step 1
local g = f( ) -- step 2
g(a) -- step 3

At step 1, loadstr returns a function whose body is the string it receives as argument. In this case this is equivalent to writing:

local function f( ) return function(x) local b=x+1 print(b) end

Note that loadstring just returns a function, but it doesn't execute it. This function (f), when executed (step 2), returns another function that accepts an argument. Here I called it 'x' instead of 'a' to make clear that it is the argument, not (yet) the local variable 'a'. Step 2 thus ends up being equivalent to writing:

local function g(x) local b=x+1 print(b) end

Finally, at step 3 we execute the function g passing it 'a' as argument (thus x=a).

The original example is obtained by merging the three steps into one.

BTW, you can do it in another way which is arguably simpler, but I don't know if it works in Lua 5.1:

local a = 1
loadstring("local a=({...})[1] local b=a+1 print(b)")(a)

EDIT: fixed the last code snippet

I didn't answer for a long time, sorry. I understand and thanks for explaining, but i have one more question. For example, if I have document with strings
" function f() b = b + 1 print(b) end",
"function f1() c = c + 1 print(c) end" etc
and I have code:

local b = 1
local c = 2
f()
f1()

So, how can I create functions from strings?
P.S. I know how to read and write from and to document

 

[stetre]

I'm not sure I understand your use case. Is it a given, or are you free to re-consider it?

It doesn't seem very sound (to me, at least) to define in a separate document functions that act on local variables that are defined elsewhere. 'Local' variables are, as the name suggests, 'local' to their scope, and their existence should not be known outside that scope.

If you are free to reconsider the design of your application, I'd suggest you to define in the separate document functions that act on arguments, and do not assume the existence of variables being defined elsewhere.

For example, you could write in the separate document functions that act on the field of a table received as argument:
" function f(t) t.b = t.b + 1 print(t.b) end",
"function f1(t) t.c = t.c + 1 print(t.c) end" etc

and then use it like so:

local tt = {b = 1 , c = 2}
f(tt)
f1(tt)