MLua Extras

[!WARNING] This crate is under active development. All features are currently experimental and are subject to change at any time without notice.

[!NOTE] Feel free to use this crate and start working with ideas and features that could be useful.

Pull requests and contribution is encouraged

If you want to discuss this project, you can do that here

The goal of this project is to add a light convenience layer wrapping the mlua crate. The goal isn't to change the way that mlua is used, but instead to make lua embedded development in Rust more enjoyable.

The biggest part of this library is adding Lua type information and doc comments for your exposed Lua APIs. The type information is a light wrapper around UserData and its related traits UserDataFields and UserDataMethods.

Inspiration

Lua faces a hurdle where when it is embedded, the additional APIs exposed by the host application are not automatically detected. LuaLs handle this very well with support for definition files and addons. Both options give the opportunity to add additional type information to the language server for the exposed API.

Now for the problem. Writing both rust (host application) code and Lua definition files side by side is a lot of extra work, especially when it comes to translating the rust types into the Lua representation. You may want the definition files to be in the workspace, project, itself or in another location and have it be pulled in with an addon. Either way the maintainer would have to write both the rust types and the Lua types.

Now for a potential solution. mlua-extras adds traits that mimic the mlua traits when defining custom types, such as the UserData trait. By using the same methods in the format maintainers are familiar with, the additional work is kept minimal. The impact, however, is fairly large. These traits automagically collect the field, parameter, and return type information leveraging the rust type system. With the add a few extra methods to define doc comments, and now the maintained Lua API types are collected and ready to be transformed.

After collecting the types, a user of mlua-extras can transform the type data into definition files or documentation (or a data format used in documentation). So now a maintainer replaces a couple traits and adds a few derive macros and they have their API's type information ready to be used. Effectively writing their expose lua API once.

Why not use Tealr or Luau?

mlua-extras also doesn't limit the support for a typed syntax of lua. It should work well with any of them and could potentially enhance the users experience with them.

Both are great options and if that is what your application needs/wants then use those options. However, using definition files and luals's LuaCATS (Lua Comment And Type System) annotations allows for potentially more portability and usability.

There is no need to learn a new syntax or API, just write Lua and rust code as expected with mlua.

Also, by using Lua’s officially supported type system, the latest version of Lua can be used without the worry of compatibility.

Notice

Since mlua-extras implements traits for the mlua crate, it requires that it is a dependency. The mlua crate will not compile unless a version of Lua is provided. mlua will fail to compile if your project depends on it with one Lua version feature and this crate depends on it with a different version feature (ex: lua54 vs luajit). With this in mind, this crate re-exposes mlua along with most of its feature flags.

Just make sure you use the exposed mlua crate through this crates API (mlua-extras::mlua).

Features

• Helper Traits

  • Require
    • call require which allows for a lua style call to get data from the lua engine.
    • ex: table.require::<String>("nested.tables.name")? == local name = require('nested.tables').name
  • Module
    • Syntax and API mimics UserData adding fields, functions and methods. Instead of createing a lua userdata type, it adds these values to a lua table.
    • This allows for all the data to be held in lua
    • This allows for a complex lua api without creating a bunch of userdata objects
  • LuaExtras
    • Manipulate the lua path and cpath variables with append, prepend, and set methods for each variant. It also includes the ability to add multiple paths with each variant.
    • Set global variables and functions with set_global("value", "value") and set_global_function("func", |lua, ()| Ok(())) which wold replace lua.globals().set("value", "value) and lua.globals().set("func", lua.create_function(|lua, ()| Ok(()))?) respectively

• Typed Lua Traits

  • Typed
    • Generate a Type and Param for a rust type so it can be used both as a type and as a parameter for a function
  • TypedUserData
    • Typed variant of mlua::UserData with an additional add_documentation method to add doc comments to the UserData type
    • An extra document method is added to the TypedDataFields and TypedDataMethods for add_fields and add_methods. This will queue doc comments to be added to the next field or method that is added.
    • All types from function parameters and and return types are stored for fields, functions, and methods.
    • This trait is mainly used when generating type definitions. If it is called through the UserData derive macro it will ignore all types and documentation
  • TypedDataFields: Implemented on a generator for TypedUserData (add_fields)
  • TypedDataMethods: Implemented on a generator for TypedUserData (add_methods)
  • TypedDataDocumentation: Implemented on a generator for TypedUserData (add_documentation)

• Derive Macros

  • Typed: Auto implement the Typed trait to get type information for both struct and enum
  • UserData: Auto implement the mlua::UserData trait for rust types that also implement TypedUserData. This will pass through the UserData add_methods and add_fields to the TypedUserData's version. This will ignore all documentation and types.

• Macros

  • function: Write lua functions more like Rust's syntax

  Instead of this:

  lua.create_function(|lua, ()| Ok(())) 
  

  You can now write this:

  function!{
      lua fn name(lua) {
          Ok(())
      }
  }
  

  The difference isn't huge, but it could make the syntax more readable.

  [!NOTE] This also helps with assigning functions to nested tables.

  [!WARNING] This requires the LuaExtras trait when adding functions to nested tables with lua as the starting point. This requires the Require trait when starting from any other table.

  lua.require::<Table>("nested.fn").set("name", lua.create_function(|lua, ()| Ok(())));
  
  // vs
  
  function! {
      lua fn lua::nested::fn.name(lua) {
          Ok(())
      }
  }
  

Ideas and Planned Features

• Fully featured definition file generation
• Fully featured documentation generation
• Fully featured addon generator when creating a lua modules with mlua's module feature
• Better and more informative type errors associated with lua type definitions and output generation
• More expresive way of defining exposed lua api types
  • Generic types
  • Doc comments for params and return types

References

• lua
• mlua
• Tealr
• Luau
• Lua Language Server

Example Syntax

Helpers

use mlua::{Lua, Table, Function, Variadic, Value};

fn main() -> mlua::Result<()> {
    let lua = Lua::new();

    // Prepend path to the lua `path`
    let path = lua.globals().get::<_, Table>("package")?.get::<_, String>("path");
    lua.globals().get::<_, Table>("package")?.set("path", format!("?.lua;{path}"))?;

    let temp = lua.create_table()?;
    temp.set("getName", lua.create_function(|lua, ()| Ok("name"))?;

    // Get a nested function: `table.unpack`
    let unpack = lua.globals().get::<_, Table>("table")?.get::<_, Function>("unpack")?;
    // Call the `table.unpack` function
    let _ = unpack.call::<_, Variadic<Value>>(temp)?;
    Ok(())
}

use mlua_extras::{
    mlua::{self, Lua, Table, Variadic, Value}
    extras::{Require, LuaExtras},
    typed::TypedFunction,
    function,
};

fn main() -> mlua::Result<()> {
    let lua = Lua::new();

    // Prepend path to the lua `path`
    lua.prepend_path("?.lua")?;

    let temp = lua.create_table()?;
    function! {
        lua fn temp.name(lua) {
            Ok("name")
        }
    }

    // Get a nested function: `table.unpack`
    let unpack = lua.require::<TypedFunction<Table, Variadic<Value>>>("table.unpack")?;
    // Call the `table.unpack` function
    let _ = unpack.call(temp)?;
    Ok(())
}

Types

use serde::Deserialize;
use mlua_extras::{
    mlua::{self, Lua, Table, Variadic, Value},
    extras::{ Require, LuaExtras },
    typed::{
        generator::{Definition, Definitions, DefinitionFileGenerator},
        TypedFunction, TypedUserData
    },
    Typed, UserData, function,
};

#[derive(Default, Debug, Clone, Copy, Typed, Deserialize)]
enum SystemColor {
    #[default]
    Black,
    Red,
    Green,
    Yellow,
    Blue,
    Cyan,
    Magenta,
    White,
}

#[derive(Debug, Clone, Copy, Typed, UserData, Deserialize)]
#[serde(untagged)]
enum Color {
    System(SystemColor),
    Xterm(u8),
    Rgb(u8, u8, u8),
}
impl Default for Color {
    fn default() -> Self {
        Color::System(SystemColor::default())
    }
}
impl<'lua> FromLua<'lua> for Color {
    fn from_lua(value: Value<'lua>, lua: &'lua Lua) -> mlua::prelude::LuaResult<Self> {
        match value {
            Value::UserData(data) => data.borrow::<Self>().map(|v| *v),
            // Use serde deserialize if not userdata
            other => lua.from_value(other),
        }
    }
}

#[derive(Debug, Clone, Copy, Typed, UserData, Deserialize)]
struct Example {
    color: Color
}
impl TypedUserData for Example {
    fn add_documentation<F: mlua_extras::typed::TypedDataDocumentation<Self>>(docs: &mut F) {
        docs.add("This is a doc comment section for the overall type");
    }

    fn add_fields<'lua, F: TypedDataFields<'lua, Self>>(fields: &mut F) {
        fields
            .document("Example complex type")
            .add_field_method_get_set(
                "color",
                |_lua, this| Ok(this.color),
                |_lua, this, clr: Color| {
                    this.color = clr;
                    Ok(())
                },
            );
    }
}


fn main() -> mlua::Result<()> {
    let definitions = Definitions::generate()
        .define("init", Definition::generate()
            .register_enum::<SystemColor>()?
            .register_enum::<Color>()?
            .register::<Example>()
            .value_with::<Example, _>("example", ["Example module"])
            .function_with::<String, (), _>("greet", (), ["Greet the name that was passed in"])
            .function_with::<Color, (), _>("printColor", (), ["Print a color and it's value"])
        )
        .finish();

    let gen = DefinitionFileGenerator::new(definitions);
    for (name, writer) in gen.iter() {
        // Writes to a new file `init.d.lua`
        writer.write_file(name).unwrap();
    }
    println!();
    Ok(())
}

Produces the following definition file

--- init.d.lua
--- @meta

--- @alias SystemColor "Black"
---  | "Red"
---  | "Green"
---  | "Yellow"
---  | "Blue"
---  | "Cyan"
---  | "Magenta"
---  | "White"

--- @alias Color SystemColor
---  | integer
---  | { [1]: integer, [2]: integer, [3]: integer }

--- This is a doc comment section for the overall type
--- @class Example
--- Example complex type
--- @field color Color

--- Example module
--- @type Example
example = nil

--- Greet the name that was passed in
--- @param param0 string
function greet(param0) end

--- Print a color and it's value
--- @param param0 Color
function printColor(param0) end