# Lua

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Lua is an ergonomic interface to [Luerl](https://github.com/rvirding/luerl), aiming to be the best way to use Luerl from Elixir.

## Features

* Ergonomic API for Elixir <> Lua FFI
* Improved error messages
* Deep-setting variables and state
* Excellent documentation and guides for working with Luerl

## Executing Lua

Lua can be run using the `eval!/2` function

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

```

## Exposing Elixir functions to Lua

`Lua` provides the `deflua` macro for exposing Elixir functions to Lua

``` elixir
defmodule MyAPI do
  use Lua.API
      
  deflua double(v), do: 2 * v
end
    
lua = Lua.new() |> Lua.load_api(MyAPI)

{[10], _} =
  Lua.eval!(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

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

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

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

{["wow"], _} = Lua.eval!(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

``` elixir
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])
    
    {[], state} = Lua.call_function!(state, [:table, :insert], [queue, v])
    
    # Return the modified lua state with no return values
    {[], state}
  end
end

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 an Lua lexer, parser, and full-blown lua virtual machine that runs inside the BEAM.