From: Gavin Kistner <gavin.kistner@...>
Date: 2006-10-05T08:18:24+09:00
Subject: Associating data with a function

Because I just had to solve this problem in both JavaScript and Lua, and
because I love Ruby more than either, I thought I'd ask people here how
they'd solve it in Ruby.

The general question is: how would do you associate a few object
instances with methods on a per-method basis?
 
The Situation/Requirements
--------------------------
A class has 2 different methods.
Each method needs a couple 'scratch' objects to perform its
calculations.
The methods call each other; they must not use the same scratch objects.
It is expensive to instantiate a scratch object.
It's not expensive to initialize an existing scratch object with data.
We like OOP coding, and want to call these as methods of a receiver.

Wasteful Example:
class Foo
  def c1
    tmp1 = ExpensiveObject.new
    tmp2 = ExpensiveObject.new
    # stuff involving tmp1/tmp2
    result = tmp1 + tmp2 + c2
  end
  def c2
    tmp1 = ExpensiveObject.new
    tmp2 = ExpensiveObject.new
    # stuff involving tmp1/tmp2
    result = tmp1 + tmp2
  end
end


The Solution We'll Ignore
--------------------------
We *could* create a distinct instance variable for each needed variable.
I personally don't like the weird coupling this creates, though. So I'm
ignoring it.

class Foo
  def initialize
    @tmp1 = ExpensiveObject.new
    @tmp2 = ExpensiveObject.new
    @tmp3 = ExpensiveObject.new
    @tmp4 = ExpensiveObject.new
  end
  def c1
    tmp1, tmp2 = @tmp1, @tmp2
    #...
  end
  def c2
    tmp1, tmp2 = @tmp3, @tmp4
    #...
  end
end


The JavaScript Solution
-----------------------
The solution I used for JavaScript was that functions/methods are
first-class objects that can have arbitrary data assigned, and can get a
reference to themselves during execution. So:

Foo.prototype.c1 = function( ) {
  var tmp1 = arguments.callee.eo1;
  var tmp2 = arguments.callee.eo2;
  //...
}
Foo.prototype.c1.eo1 = new ExpensiveObject;
Foo.prototype.c1.eo2 = new ExpensiveObject;


There were a few potential solutions for Lua. I'll list the two best,
just to help give you ideas:

Lua Solution #1: Closures (would work for JS, too)
--------------------------------------------------
Foo.c1 = ( function( )
  local tmp1 = ExpensiveObject:new( )
  local tmp2 = ExpensiveObject:new( )
  return function( self )
    -- stuff using tmp1 and tmp2
  end
end)( )


Lua Solution #2: Changing the global environment for a function
---------------------------------------------------------------
function associateFunctionData( inF, inData )
  setfenv(inF, setmetatable(inData,{__index=getfenv(inF)}))
end

function Foo:c1( )
  -- stuff using tmp1 and tmp2
  -- as though they were globals
end

associateFunctionData( Foo.c1, {
  tmp1 = ExpensiveObject:new( ),
  tmp2 = ExpensiveObject:new( )
} )