Luerl - an implementation of Lua in Erlang

An experimental implementation of Lua 5.2 written solely in pure Erlang.

Some things which are known not to be implemented or work properly:

• label and goto

• tail-call optimisation in return

• only limited standard libraries

• proper handling of __metatable

• ...

When to use Luerl

Fast Language Switch: Luerl should allow you to switch between Erlang and Lua incredibly fast, introducing a way to use very small bits of logic programmed in Lua, inside an Erlang application, with good performance.

Multicore: Luerl provides a way to transparently utilize multicores. The underlying Erlang VM takes care of the distribution.

Microprocesses: It should give you a Lua environment that allows you to effortlessly run tens of thousands of Lua processes in parallel, leveraging the famed microprocesses implementation of the Erlang VM. The empty Luerl State footprint will be yet smaller than the C Lua State footprint.

Forking Up: Because of the immutable nature of the Luerl VM, it becomes a natural operation to use the same Lua State as a starting point for multiple parallel calculations.

However, Luerl will generally run slower than a reasonable native Lua implementation. This is mainly due the emulation of mutable data on top of an immutable world. There is really no way around this. An alternative would be to implement a special Lua memory outside of the normal Erlang, but this would defeat the purpose of Luerl. It would instead be then more logical to connect to a native Lua.

Some valid use cases for Luerl are:

• Lua code will be run only occasionally and it wouldn't be worth managing an extra language implementation in the application.

• The Lua code chunks are small so the slower speed is weighed up by Luerl's faster interface.

• The Lua code calculates and reads variables more than changing them.

• The same Lua State is repeatedly used to 'fork up' as a basis for massively many parallel calculations, based on the same state.

• It is easy to run multiple instances of Luerl which could better utilise multicores.

There may be others.

Interface functions in luerl.erl

All functions optionally accept a Lua State parameter. The Lua State is the state of a Lua VM instance. It can be carried from one call to the next. If no State is passed in, a new state is initiated for the function call.

eval and do functions differ only in what they return. The do functions return results and a new Lua State, the eval functions return a tuple starting on 'ok' or 'error', then the result, or cause of error.

do --> {Result, State} 

eval --> {ok, Result} | {error, Reason}

luerl:eval(String|Binary|Form[, State]) -> {ok, Result} | {error, Reason}.

Evaluate a Lua expression passed in as a string or binary, and return its result.

luerl:evalfile(Path[, State]) -> {ok, Result} | {error, Reason}.

Load and execute a file, and return the result.

luerl:do(String|Binary|Form[, State]) -> {Result, NewState}.

Evaluate a Lua expression and return its result, and the new Lua State.

luerl:dofile(Path[, State]) -> {Result, NewState}.

Load and execute the Lua code in the file and return its result, and the new Lua State. Equivalent to doing luerl:eval("dofile('FileName')").

luerl:load(String|Binary) -> {ok, Form} | {error, Reason} .

Parse a Lua chunk as string or binary, and return a compiled chunk.

luerl:loadfile(Path) -> {ok,Form}.

Parse a Lua file, and return a compiled chunk.

luerl:init() -> State.

Get a new Lua State = a fresh Lua VM instance.

luerl:call(Form, Args[, State]) -> {Result,State}

Call a compiled chunk or function.

luerl:stop(State) -> GCedState.

Garbage collects the state and (todo:) does away with it.

luerl:gc(State) -> State.

Runs the (experimental) garbage collector on a state and returns the new state.

N.B. This interface is subject to change!

Examples

execute a string

luerl:do("print(\"Hello, Robert(o)!\")"),

execute a file

luerl:dofile("./examples/hello/hello.lua"),

separately parse, then execute

{ok, Chunk} = luerl:load("print(\"Hello, Chunk!\")"),
State = luerl:init(),
{_Ret, _NewState} = luerl:do(Chunk, State),

For more examples see examples/hello/hello2.erl.

./hello.erl is a very brief example while examples/hello/hello2.erl is a comprehensive lists of most ways that come to mind of how to use the individual interface functions.

You can build and run these samples with:

make hello
make hello2

Currently implemented functions in the libraries

• _G
• _VERSION
• assert
• collectgarbage
• dofile
• eprint
• error
• getmetatable
• ipairs
• load
• loadfile
• next
• pairs
• pcall
• print
• rawequal
• rawget
• rawlen
• rawset
• select
• setmetatable
• tonumber
• tostring
• type
• io.flush
• io.write
• math.abs
• math.acos
• math.asin
• math.atan
• math.atan2
• math.ceil
• math.cos
• math.cosh
• math.deg
• math.exp
• math.floor
• math.fmod
• math.frexp
• math.huge
• math.ldexp
• math.log
• math.log10
• math.max
• math.min
• math.modf
• math.pi
• math.pow
• math.rad
• math.random
• math.randomseed
• math.sin
• math.sinh
• math.sqrt
• math.tan
• math.tanh
• os.clock
• os.date
• os.difftime
• os.getenv
• os.time
• string.byte
• string.char
• string.find
• string.format (very limited as yet)
• string.gmatch
• string.gsub
• string.len
• string.lower
• string.match
• string.rep
• string.reverse
• string.sub
• string.upper
• table.concat
• table.insert
• table.pack
• table.remove
• table.sort
• table.unpack

Known Bugs

Functions defined in a loop, while, repeat and for, and when the loop is exited with a break will generate an error when called. For example the functions defined in

for i=1,10 do
  a[i] = {set = function(x) i=x end, get = function () return i end}
  if i == 3 then break end
end

N.B. This only occurs if the loop is actually exited with the break, otherwise there is no problem.