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)

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.