From: Mike Gold <mike.gold.4433@...>
Date: 2008-11-01T04:20:12+09:00
Subject: Re: How to access to local variables in enclosing scopes?

Yuh-Ruey Chen wrote:
> 
>> � �MyClass.class_eval do
>> � � �define_method(m) { puts "Hi from #{m}!" }
>> � �end
>>
>> and similar.
> 
> Yes I'm familiar with them, and I actually would like to use them, but
> blocks can't have default arguments. Ideally, I would like to write
> everything with blocks and avoid all this magical "new scope"
> nonsense. It makes metaprogramming tricky in my experience.

I use often use Class.new and define_method for just the reasons you 
describe: metaprogramming most naturally goes with locals bound to 
closures.  This style also avoids a handful of potential problems with 
the "regular" way of using different scopes, in particular name 
conflicts and lack of true read-only protection for variables.

I was told that I wasn't doing things in a "rubyish" way.  Disregard 
that BS.  Use closures and define_method for metaprogramming.  Your code 
will be better.

Note 1.9 allows default arguments for lambdas as well as a &block 
parameter.  1.8.7 allows a &block parameter where 1.8.6 does not.  For 
1.8.x you can get the same effect of default arguments like this

class << self
    define_method(:foo) { |*args|
        a, b =
        case args.size
        when 0
            ["fred", "barney"]
        when 1
            [args.first, "barney"]
        when 2
            args
        else
            raise ArgumentError,
            "wrong number of arguments (#{args.size} for 2)"
        end

        puts "foo: #{a} #{b}"
    }
end

foo         #=> foo: fred barney
foo :a      #=> foo: a barney
foo :a, :b  #=> foo: a b

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