set env after into_function?

[brentp]

Hi, I'm trying to do something where this below is the simple case:

use mlua::{Lua, Result};
fn main() -> Result<()> {
    let lua = Lua::new();
    let lua_fn = lua
        .load(r#"
t = 22
return t
    "#).into_function()?;

    eprintln!("before: {}", lua_fn.call::<i32>(())?);

    let env = lua_fn.environment().unwrap();
    env.set("t", 100)?;
    lua_fn.set_environment(env)?;

   // this still prints 22
    eprintln!("after: {}", lua_fn.call::<i32>(())?);

    Ok(())
}

update: note that in the example above, it prints 22 both times, when I want it to print 22 first, then 100.

in my real use-case, I take some user-defined script (here just set t and return it as an example) and then I would like to modify some known variables in it and run it many times. Since I am running it many times (possibly millions) I don't want to eval each time. Is there some way to modify the env in this case?

I get that the obvious way is to .call with t as the argument but given how the user-script is defined/expected, that is difficult.
thanks for any pointers.

[khvzak]

Since I am running it many times (possibly millions) I don't want to eval each time

What do you mean by "eval each time"? The optimal solution would be to load the function once, get envs table, and then re-run the function updating the environment.

[brentp]

that's what I thought. so I tried to .call(), but in the example above, it doesn't update t.

[khvzak]

In the script in the beginning of the call you set t = 22 which overrides any environment variables assigned to t.

This works:

    let lua = Lua::new();
    let lua_fn = lua
        .load(
            r#"
    return t
        "#,
        )
        .into_function()?;

    let env = lua_fn.environment().unwrap();
    env.set("t", 0)?;
    eprintln!("before: {}", lua_fn.call::<i32>(())?);

    for t in 1..3 {
        env.set("t", t)?;
        eprintln!("after: {}", lua_fn.call::<i32>(())?);
    }

prints:

before: 0
after: 1
after: 2

[brentp]

that makes sense. I wanted a way to do it so that the original user-script, which must define the variable (in this case t) would still be valid without any modifications.

[khvzak]

that makes sense. I wanted a way to do it so that the original user-script, which must define the variable (in this case t) would still be valid without any modifications.

Which means you want to modify the original script logic (since the script unconditionally set t = 22 to prevent any environment modification). The only way to do this is use hooks to hijack the execution flow and modify t in the middle.

Sort of:

    let env = lua.create_table()?;

    let env2 = env.clone();
    lua.set_hook(HookTriggers::EVERY_LINE, move |lua, _| {
        lua.globals().set("t", env2.get::<Value>("t")?)?;
        Ok(mlua::VmState::Continue)
    })?;

    for t in 0..2 {
        env.set("t", t)?;
        eprintln!("after: {}", lua_fn.call::<i32>(())?);
    }

[brentp]

thank you!