Question: Is MetaMethod::IPairs missing for lua54?

[jeremychone]

I think I'm missing something. I'm using the lua54 feature, I have user data, and I would like to implement custom pairs and ipairs.

I can do that for pairs with MetaMethod::Pairs, but MetaMethod::IPairs is only enabled for the lua52 and luajit52 features.

Here is an example I would like to create:

methods.add_meta_method(MetaMethod::IPairs, |_, _, ()| -> Result<()> {
    Err(Error::RuntimeError("attempt to iterate over ...".into()))
});

What am I missing?

[khvzak]

Lua 5.4 does not support __ipairs metamethods (it was removed since 5.3).

You can use __index instead:

struct MyUserData(i32);

impl LuaUserData for MyUserData {
    fn add_methods<M: LuaUserDataMethods<Self>>(methods: &mut M) {
        methods.add_meta_method(LuaMetaMethod::Index, |_lua, this, k: i32| {
            if k < this.0 {
                return Ok(Some(k * k));
            }
            Ok(None)
        });
    }
}

lua.load(
    r#"
    my_ud = ...
    for i, x in ipairs(my_ud) do
        print(i, x)
    end
"#,
)
.call::<()>(MyUserData(5))
.unwrap();

or simply provide your own iterator function.

If you want to remove ipairs support, then your __index should return a error for integer keys.

[jeremychone]

@khvzak Perfect. This is what I was missing.

Thank you so much for this precise answer. I have everything I need.

Also, thanks for this robust and complete Lua integration. mlua is amazing.