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Erlang-Lua

Erlang C Node to run Lua scripts

This library provides a way to run Lua code from within Erlang. It differs from the other Lua - Erlang integrations available, in that it runs the Lua VM as an external Node (using Erlang's Port and C Node capabilities).

It's early days in making this available to the public, so be aware that some assembly is required.

Building

The Erlang-Lua C C Node code currently compiles and tests successfully on Mac OS X (10.9, using XCode command line utilities) and Ubuntu (Trusty 14.04.1 LTS). It probably works on further Mac OS X versions and Linux distros, but I've not tried it yet.

A Lua installation (5.1 or 5.2) is required to be on the system. It's needed for the header files and to link the C Node. You can get Lua from http://www.lua.org/ . The current 5.1 source package can be downloaded from http://www.lua.org/ftp/lua-5.1.5.tar.gz , and the 5.2 one from http://www.lua.org/ftp/lua-5.2.3.tar.gz . To build on Mac OS X, you run make macosx test, and on Ubuntu run make linux test. You'll need to install it somewhere, run make INSTALL_TOP=Path_to_Lua_installation install.

Building the Erlang-Lua C Node uses rebar (https://github.com/rebar/rebar), and a small Makefile is provided to wrap around the calls to rebar. You need to edit the rebar.config file and edit the setting of the Lua path to point to your Lua installation:

{ port_env,[
    {"LUA", "/Path_to_Lua_installation"},
...
]}.

After that, make compile compiles it all up (expect a warning about missing braces around initializer) and make test runs the Eunit test suite. The latter produces a whole bunch of logging to standard output and, if all is good, ends with All 87 tests passed. A make clean does the obvious.

What It Can Do

Starting a Lua VM through

(rtr@127.0.0.1)1> erlang_lua:start_link(foo).
{ok,<0.40.0>}

brings up a gen_server that provides the interface to running and monitoring the Lua VM. It starts a C program to run the Lua VM via open_port, monitors it and receives logging from it. The C program itself initialises itself as an Erlang C Node and connects to the Erlang Node that launched it.

Running Lua code on the external Lua VM is accomplished by sending messages to the C Node and receiving answers back. The Lua results are converted to Erlang terms.

(rtr@127.0.0.1)2> erlang_lua:lua(foo, <<"return {x=1, y='foo'}">>).
{lua,[[{y,<<"foo">>},{x,1}]]}
(rtr@127.0.0.1)3> erlang_lua:lua(foo, <<" x = 42 + 'fourty two' ">>).
{error,"[string \" x = 42 + 'fourty two' \"]:1: attempt to perform
 arithmetic on a string value"}

Some support for automatically translating Erlang values to Lua is available via the call interface:

(rtr@127.0.0.1)4> erlang_lua:lua(foo, <<"find = string.find">>).
{lua,ok}
(rtr@127.0.0.1)5> erlang_lua:call(foo, find, [<<"foobar">>, <<"(o)b(a)">>]).
{lua,[3,5,<<"o">>,<<"a">>]}

It is also possible to call back into Erlang from Lua:

(rtr@127.0.0.1)6> erlang_lua:lua(foo, <<"return erl_rpc('date')">>).
{lua,[[2014,12,3]]}
(rtr@127.0.0.1)7> erlang_lua:lua(foo, <<"return erl_rpc('erlang', 'date')">>).
{lua,[[2014,12,3]]}
(rtr@127.0.0.1)8> erlang_lua:lua(foo, <<"return erl_rpc('lists', 'seq', 2, 15, 3)">>).
{lua,[[2,5,8,11,14]]}
(rtr@127.0.0.1)9> S = <<"foobar">>.
<<"foobar">>
(rtr@127.0.0.1)10> {lua, [ R ]} = erlang_lua:call(foo, erl_rpc, [base64, encode, S]).
{lua,[<<"Zm9vYmFy">>]}
(rtr@127.0.0.1)11> {lua, [ S ]} = erlang_lua:call(foo, erl_rpc, [base64, decode, R]).
{lua,[<<"foobar">>]}

The Lua VM is stopped using

(rtr@127.0.0.1)12> erlang_lua:stop(foo).
ok

Value Translation From Lua To Erlang

	nil -> 'nil' Atom

	true -> 'true' Atom
	false -> 'false' Atom

	erl_atom"string" -> 'string' Atom

	integer number -> Integer Number
	floating point number -> Float Number

	"string" -> Binary

	erl_string"string" -> "string" String

	erl_tuple{ V1, V2, V3, ..., Vn } -> { V1, V2, V3, ..., Vn }

	{ V1, V2, V3, ..., Vn } -> [ V1, V2, V3, ..., Vn ]

	{ K1=V1, K2=V2, K3=V3, ..., Kn=Vn } -> [ {K1, V1}, {K2, V2}, {K3, V3}, ..., {Kn, Vn} ]
		/ Order of pairs not guaranteed,
		/ If type(K) == "string" and #K < 256 then Erlang K is Atom

	{ V1, V2, ..., Vn, Kn+1=Vn+1, Kn+2=Vn+2, ..., Kn+k=Vn+k  }
			-> [ V1, V2, ..., Vn, {Kn+1, Vn+1}, {Kn+1, Vn+2}, ..., {Kn+1, Vn+n} ]
		/ Order of {K, V} pairs not guaranteed
		/ If type(K) == "string" and #K < 256 then Erlang K is Atom

	Unusable types:
		function -> 'function' Atom
		userdata -> 'userdata' Atom
		thread -> 'thread' Atom
		lightuserdata -> 'lightuserdata' Atom

Value Translation From Erlang To Lua

	'nil' Atom -> nil
	'true' Atom -> true
	'false' Atom -> false

	Atom -> string

	Integer Number -> number
	Float Number -> number

	Binary -> string
	/ Note: Regular Erlang Strings are Lists: "abc" -> { 97, 98, 99 }

	{ V1, V2, V3, ..., Vn } -> { V1, V2, V3, ..., Vn }
	[ V1, V2, V3, ..., Vn ] -> { V1, V2, V3, ..., Vn }

	[ {K1, V1}, {K2, V2}, {K3, V3}, ..., {Kn, Vn} ] -> { K1=V1, K2=V2, K3=V3, ..., Kn=Vn }
		/ If all Erlang K are Atoms

	[ V1, {K2, V2}, V3, {K4, V4}, V5, ..., Vn, {Kn+1, Vn+1}, ... ] -> { V1, V3, V4, ..., Vn, K2=V2, K4=V4, ..., Kn+1=Vn+1 }
		/ If Erlang K is Atom
		/ Note: All elements that are not a 2-tuple with the first element an Atom, become array elements in Lua
		/      Only the ordering of non 2-tuples is preserved! 

	Unusable types:
		Reference, Fun, Port, Pid -> nil