From: why the lucky stiff <ruby-talk@...>
Date: 2002-07-25T14:25:39+09:00
Subject: Re: A Code Challenge

Tom Sawyer (transami@transami.net) wrote:
> Problem:
> 
>   given a proc p and an object o with method x, such that the arity of x
> and p are the same, dynamically append p into x.
> 

not too elegant but a starting point:

class Object
  # meth is aSymbol, p is aProc ;)
  def append_proc_to_method( meth, &p )

    # check arity
    if method( meth ).arity.abs != p.arity.abs
      raise ArgumentError, "Arity doesn't match #{type}##{meth.id2name} and #{p.inspect}"
    end
  
    # stores procs in this instance var
    @__proc_add__ ||= {}
    unless @__proc_add__.has_key?( meth )
      @__proc_add__[meth] = [p]

      # eval is a code smell but oh oh well
      type.module_eval <<-"end;"
        alias_method :__#{meth.to_i}__, :#{meth.id2name}
        private :__#{meth.to_i}__
        def #{meth.id2name}( *args )
          __#{meth.to_i}__( *args )
          if @__proc_add__.is_a? Hash
            if @__proc_add__.has_key?( :#{meth.id2name} )
              @__proc_add__[:#{meth.id2name}].each { |p|
                p.call( *args )
              }
            end
          end
        end
      end;

    else
      @__proc_add__[meth] << p
    end

  end
end

# Simple test
class O                                                                                               
  def x(a)
    puts "do something with #{a}"
  end
end

# Add two procs to this instance's x meth
o = O.new
p = Proc.new { |a| puts "do something else with #{a}" }
o.append_proc_to_method( :x, &p )

o.append_proc_to_method( :x ) { |a| puts "do another something else with #{a}" }
o.x(2)

# Make sure this instance's x is still just running the original
o2 = O.new
o2.x(4)

_why