Using Lua in Ruby

Ruby is different in Lua in that there is a single Ruby VM, whereas there can be many Lua states. A Lua virtual machine is wrapped in an instance of the Lua::State class. You can have multiple Lua::States.

First, you need to require the RubyLuaBridge library and create a Lua::State. With this instance, you can run Lua code, load Lua libraries, and access Lua objects.

   require 'rubyluabridge'
   l = Lua::State.new    # we're gonna do a lot with this guy

Once you have a Lua::State, you can run Lua code using the Lua::State.eval. This method will return whatever value is returned by the passed code. It will also raise an exception on errors.

   l.eval "print('hello world')"                  # => nil
   l.eval "function give_me_one() return 1 end"   # => nil
   l.eval("return give_me_one()")                 # => 1

Since Lua can return multiple values, Lua::State.eval_mult will return all values in an Array.

   require 'pp'
   pp l.eval_mult("return")          # => []
   pp l.eval_mult("return 1")        # => [1.0]
   pp l.eval_mult("return 1, 2, 3")  # => [1.0, 2.0, 3.0]

Due to the dynamic natures of Ruby and Lua, you can easily query and set Lua values in a natural way. You can use square brackets [key], which is necessary if your key is not a string. Otherwise, string keys can also be accessed by invoking it like a method (see example below). Querying from the Lua::State will access the Lua global table.

   l.eval "        n = 5                 -- see how Ruby long strings\n        s = \"hello\"           -- make it easy to embed Lua code?\n        a = { 1, 2, 3, 4 }\n        h = { a='x', b='y', }\n   LUA_END\n   l.n       # => 5\n   l.s       # => \"hello\"\n   l.a       # => Lua::Table\n   l.a[1]    # => 1\n   l.h       # => Lua::Table\n   l.h['a']  # => \"x\"\n   l.h.a     # => \"x\"\n   \n"

Similarly, you can use this style to set Lua variables. To make a new table, invoke new_table_at on the object, specifying where you want it. Setting a Ruby Arrays or Hashes to a Lua variable will create and assign a new table with a shallow copy of the Ruby container.

    l.n = 5                 # n = 5
    l.new_table_at 't'      # t = {}
    l.t.u = 4               # t.u = 4
    l.t['u'] = 4            # t.u = 4
    l.a = [1,2]             # t.a = { 1, 2 }
    l.h = { 'a' => 'b' }    # t.h = { a = 'b' }

When RubyLuaBridge has to marshal a table to Ruby, it return a Lua::Table. The Lua::Table implements various iterators that take Ruby Blocks. There are two ways of looking at a Lua table. The first is as a hash, where it has keys of any type mapping to values of any type. The second is as an array, where it has integer keys from 1 to N, where N is the size of the array. The each_i* methods iterate the table as an array, whereas the each_* methods (without the i) iterate the table as a hash. In each case you can iterate of the keys, the values, or the pair [key, value]. Lua::Table#each is aliased to Lua::Table#each_pair and Lua::Table#each_index is aliased to Lua::Table#each_ikey.

    l.eval( "array = { 100, 200, 300, 400 }" )
    l.array.each_ikey   { |k|   print "#{k} " }        # 1 2 3 4
    l.array.each_ivalue { |v|   print "#{v} " }        # 100.0 200.0 300.0 400.0 
    l.array.each_ipair  { |k,v| print "#{k},#{v} " }   # 1,100.0 2,200.0 3,300.0 4,400.0 

    l.eval( "hsh = { a = 100, b = 200, [1] = 300, [5] = 400 }" )
    l.hsh.each_key    { |k|   print "#{k} " }          # a 1.0 5.0 b 
    l.hsh.each_value  { |v|   print "#{v} " }          # 100.0 300.0 400.0 200.0
    l.hsh.each_pair   { |k,v| print "#{k},#{v} " }     # a,100.0 1.0,300.0 5.0,400.0 b,200.0 
    l.hsh.each_ikey   { |k|   print "#{k} " }          # 1
    l.hsh.each_ivalue { |v|   print "#{v} " }          # 300.0
    l.hsh.each_ipair  { |k,v| print "#{k},#{v} " }     # 1,300.0

You can also extract a Ruby Hash or Array that is a shallow-copy of the table.

    l.eval "a = {1,2} ; h = { a=3, b=4 }"
    l.a.to_array     # => [1, 2]
    l.a.to_hash      # => { 1 => 1, 2 => 2 }
    l.h.to_array     # => []
    l.a.to_hash      # => { 'a' => 3, 'b' => 4 }

Note that by default, all the Lua standard libraries are loaded into the Lua::State. You can control this by passing :loadlibs option to Lua::State.new, or invoking Lua::State.__loadlibs:

    l = Lua::State.new( :loadlibs => :none ) 
    l = Lua::State.new( :loadlibs => [:base, :io, :debug] ) 
    l.__loadlibs( :package )
    l.__loadlibs( [:string, :math] )
    

Note that Ruby Array’s start with index 0, whereas Lua array’s start with index 1.

Note for advanced Lua users: all of these accesses use the Lua object’s metamethods.

RubyLuaBridge has a comprehensive test suite (LuaInRuby_Test). Examine it to see various simple uses of this library.

Language Mismatchs

There are two sticky issues when trying to express ideas with RubyLuaBridge.

Lua uses a colon : syntax to define Lua’s “methods”, functions that have an implicit extra parameter self. Ruby methods always have this self parameter implicitly. RubyLuaBridge’s will only pass the self parameter to a Lua function invocation if you end the method name with bang !. The bang was chosen because it vaguely resembles a colon : !. Note that in Lua, the colon is before the method name, but in RubyLuaBridge, the bang is after (but attached to) the method name.

Another issue is that it there is an ambiguity whether an index is meant to be a property or a function call. This happens when there are no arguments, since Ruby’s parentheses are optional. RubyLuaBridge always returns the object when there are no arguments, unless it’s name is suffixed with an underscore _. With the underscore, RubyLuaBridge will invoke the object like a function. The underscore can be used with multiple arguments as well. Awkwardly, it will do this parameter-less behavior even when paretheses are present, so don’t do that!

The one exception to the no argument behavior is if the object is a Lua function, in which case it is dispatched. To get a reference to a function, rather than invoking it, use [].

Examples of this notation, with their semantics shown in the more explicit Lua code:

    l.eval "        obj = {\n            foo = function (...)\n            end,\n            bar = \"a\"\n        }\n    END_OF_LUA\n\n    l.obj.foo       # return obj.foo()\n    l.obj.foo()     # return obj.foo()\n    l.obj.foo_      # return obj.foo()\n    l.obj.foo 2     # return obj.foo(2)\n    l.obj.foo_ 2    # return obj.foo(2)\n    l.obj.foo!      # return obj:foo(obj)\n    l.obj.foo! 2    # return obj:foo(obj, 2)\n    l.obj['foo']    # return obj.foo\n\n    l.obj.bar       # return obj.bar\n    l.obj.bar()     # return obj.bar  -- yuck!\n    l.obj.bar_      # error(\"object not callable\")\n    l.obj.bar!      # error(\"object not callable\")\n    l.obj.bar(2)    # error(\"object not callable\")\n    l.obj['bar']    # return obj.bar\n"