assign a local variable in the caller’s scope from a function?

[Federico Ferri]

Hello all,

I’m wondering if there’s any way in Lua to assign a local variable in the caller’s scope from within a function.

To clarify, I’d like to write a helper function like this:

    import("foo", "x")

which would behave as if the caller had written:

    local x = require("foo").x

and I’m curious if there’s any metaprogramming trick, debug API feature, or other mechanism that would make this possible. Has anyone tried this before? Is it fundamentally impossible, or just generally discouraged?

Thanks in advance for any insights.

Best regards,
Federico Ferri

[Scott Morgan]

On 06/06/2025 15:24, Federico Ferri wrote:
> Hello all,
>
> I’m wondering if there’s any way in Lua to assign a local variable in
> the caller’s scope from within a function.
>
> To clarify, I’d like to write a helper function like this:
>
>     import("foo", "x")
>
> which would behave as if the caller had written:
>
>     local x = require("foo").x
>
> and I’m curious if there’s any metaprogramming trick, debug API feature,
> or other mechanism that would make this possible. Has anyone tried this
> before? Is it fundamentally impossible, or just generally discouraged?

If I'm reading you right, I don't think you can do that.

Local variables get turned into a call stack entry after the script is
read and compiled. You can't add them after the read & compile stages,
so nothing can be added at runtime.

Try using luac with the -l option to see what's going on after the
compile stage. You'll note that local var names are not present.

However, something like metalua might be of use to you. Although, I
don't think that's being maintained currently.

Scott

[Sainan]

> However, something like metalua might be of use to you. Although, I
> don't think that's being maintained currently.

I guess this is an instance where I can shill for Pluto, which recently added what is essentially macros via $alias:

package.preload["foo"] = function()
return { x = 1 }
end

$alias import(lib, field) = local field = require(lib).field

import("foo", x)
print(x) --> 1

-- Sainan

[Andrey Dobrovolsky]

Hi all,

Federico Ferri wrote:

>To clarify, I’d like to write a helper function like this:
>
> import("foo", "x")
>
>which would behave as if the caller had written:
>
> local x = require("foo").x

Is it possible to use the wonderful _ENV? Something like:

local import = function(module, index)
local m = require(module)
if m[index] then
_ENV[index] = m[index]
end
return m[index]
end

пт, 6 черв. 2025 р. о 18:22 'Sainan' via lua-l <lu...@googlegroups.com> пише:

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[Francisco Olarte]

On Fri, 6 Jun 2025 at 20:40, Andrey Dobrovolsky
<andrey.dobro...@gmail.com> wrote:
> Is it possible to use the wonderful _ENV? Something like:

...

_ENV is for globals, not locals, see 2.2 in the manual, OP is asking for local.

Francisco Olarte.

[Denis Dos Santos Silva]

https://www.whoop.ee/post/understanding-globals-env-and-g.html

[Andrey Dobrovolsky]

Really, locals are not part of the environment and can be reached with
debug.getlocal() (find local with distinct name) followed by
debug.setlocal().
Probably the helper Federico Ferri is curious about may look like

local import(module, ...)
local M = require(module)
local selected = {}
for i, name in ipairs{...} do
selected[i] = M[name]
end
return M, table.unpack(selected, 1, #selected)
end

and called like

local mod, f1, f2, f3 = import("my_mod", "f1name", "f2name", "f3name")

нд, 8 черв. 2025 р. о 21:43 Denis Dos Santos Silva <de...@roo.com.br> пише:

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