Lua

Lua is an ergonomic interface to Luerl, aiming to be the best way to use Luerl from Elixir.

Features

• ~LUA sigil for validating Lua code at compile-time
• deflua macro for exposing Elixir functions to Lua
• Improved error messages and sandboxing
• Deep setting/getting variables and state
• Excellent documentation and guides for working with Luerl

Lua the Elixir library vs Lua the language {: .info}

When referring to this library, Lua will be stylized as a link.

References to Lua the language will be in plaintext and not linked.

Executing Lua

Lua can be run using the eval!/2 function

iex> {[4], _} = Lua.eval!("return 2 + 2")

Compile-time validation

Use the ~LUA sigil to parse and validate your Lua code at compile time

iex> import Lua, only: [sigil_LUA: 2]

#iex> {[4], _} = Lua.eval!(~LUA[return 2 +])
** (Lua.CompilerException) Failed to compile Lua!

Using the c modifier transforms your Lua code into a t:Lua.Chunk.t/0 at compile-time, which will speed up execution at runtime since the Lua no longer needs to be parsed

iex> import Lua, only: [sigil_LUA: 2]
iex> {[4], _} = Lua.eval!(~LUA[return 2 + 2]c)

Exposing Elixir functions to Lua

The simplest way to expose an Elixir function to Lua is using the Lua.set!/3 function

import Lua, only: [sigil_LUA: 2]

lua = 
  Lua.set!(Lua.new(), [:sum], fn args ->
    [Enum.sum(args)]
  end)

{[10], _} = Lua.eval!(lua, ~LUA[return sum(1, 2, 3, 4)]c)

For easily expressing APIs, Lua provides the deflua macro for exposing Elixir functions to Lua

defmodule MyAPI do
  use Lua.API
      
  deflua double(v), do: 2 * v
end

import Lua, only: [sigil_LUA: 2]
    
lua = Lua.new() |> Lua.load_api(MyAPI)

{[10], _} = Lua.eval!(lua, ~LUA[return double(5)])

Calling Lua functions from Elixir

Lua can be used to expose complex functions written in Elixir. In some cases, you may want to call Lua functions from Elixir. This can be achieved with the Lua.call_function!/3 function

defmodule MyAPI do
  use Lua.API, scope: "example"

  deflua foo(value), state do
    Lua.call_function!(state, [:string, :lower], [value])
  end
end

import Lua, only: [sigil_LUA: 2]

lua = Lua.new() |> Lua.load_api(MyAPI)

{["wow"], _} = Lua.eval!(lua, ~LUA[return example.foo("WOW")])

Modify Lua state from Elixir

You can also use Lua to modify the state of the lua environment inside your Elixir code. Imagine you have a queue module that you want to implement in Elixir, with the queue stored in a global variable

defmodule Queue do
  use Lua.API, scope: "q"
  
  deflua push(v), state do
    # Pull out the global variable "my_queue" from lua
    queue = Lua.get!(state, [:my_queue])
    
    # Call the Lua function table.insert(table, value)
    {[], state} = Lua.call_function!(state, [:table, :insert], [queue, v])
    
    # Return the modified lua state with no return values
    {[], state}
  end
end

import Lua, only: [sigil_LUA: 2]

lua = Lua.new() |> Lua.load_api(Queue)

{[queue], _} =
  Lua.eval!(lua, """
  my_queue = {}

  q.push("first")
  q.push("second")

  return my_queue
  """)
  
["first", "second"] = Lua.Table.as_list(queue)

Accessing private state from Elixir

When building applications with Lua, you may find yourself in need of propagating extra context for use in your APIs. For instance, you may want to access information about the current user who executed the Lua script, an API key, or something else that is private and should not be available to the Lua code. For this, we have the Lua.put_private/3, Lua.get_private/2, and Lua.delete_private/2 functions.

For example, imagine you wanted to allow the user to access information about themselves

defmodule User do
  defstruct [:name]
end

defmodule UserAPI do
  use Lua.API, scope: "user"
  
  deflua name(), state do
    user = Lua.get_private!(state, :user) 
    
    {[user.name], state}
  end
end

user = %User{name: "Robert Virding"}

lua = Lua.new() |> Lua.put_private(:user, user) |> Lua.load_api(UserAPI)

{["Hello Robert Virding"], _lua} = Lua.eval!(lua, ~LUA"""
  return "Hello " .. user.name()
""")

This allows you to have simple, expressive APIs that access context that is unavailable to the Lua code.

Encoding and Decoding data

When working with Lua, you may want inject data of various types into the runtime. Some values, such as integers, have the same representation inside of the runtime as they do in Elixir, they do not require encoding. Other values, such as maps, are represented inside of Lua as tables, and must be encoded first. Values not listed are not valid and cannot be encoded by Lua and Luerl, however, they can be passed using a {:userdata, any()} tuple and encoding them.

Values may be encoded with Lua.encode!/2

Elixir type	Luerl type	Requires encoding?
nil	nil	no
boolean()	boolean()	no
number()	number()	no
binary()	binary()	no
atom()	binary()	yes
map()	:luerl.tref()	yes
{:userdata, any()}	:luerl.usdref()	yes
(any()) -> any()	:luerl.erl_func()	yes
(any(), Lua.t()) -> any()	:luerl.erl_func()	yes
{module(), atom(), list()	:luerl.erl_mfa()	yes
list(any())	list(luerl type)	maybe (if any of its values require encoding)

Userdata

There are situations where you want to pass around a reference to some Elixir datastructure, such as a struct. In these situations, you can use a {:userdata, any()} tuple.

defmodule Thing do
  defstruct [:value]
end

{encoded, lua} = Lua.encode!(Lua.new(), {:userdata, %Thing{value: "1234"}})

lua = Lua.set!(lua, [:foo], encoded)

{[{:userdata, %Thing{value: "1234"}}], _} = Lua.eval!(lua, "return foo")

Trying to deference userdata inside a Lua program will result in an exception.

Credits

Lua piggy-backs off of Robert Virding's Luerl project, which implements a Lua lexer, parser, and full-blown Lua virtual machine that runs inside the BEAM.