From: ahoward@... (ara howard)
Date: 2002-09-20T02:01:00+09:00
Subject: alias/method redfinition

it seems alias/method redefing can be a slipery slope.  i wanted to
make the Array class indexable by field name for some postgres work i
was doing.  i use the following method of redefining the [] and []=
methods of the Array class, can any one comment on the effectiveness
of this technique?

USAGE :

tuple = [0,1,2]
tuple.fields = ['zero','one','two]

puts tuple['zero'] -> 0
puts tuple['one']  -> 1
puts tuple['two']  -> 2

IMPLEMENTATION :

class Array
  def fields=(*fields)
    @fields = fields.flatten 
    @fieldpos = Hash.new
    @fields.each_with_index {|f,i| @fieldpos[f] = i}

    # redef only once!
    if not @oldcalls
      @oldcalls = {
		    '[]'   => method('[]'),
		    '[]='  => method('[]='),
		  };

      m = Module.new
      m.module_eval %q{
	def [] idx 
	  if @fields and idx.type == String
	    pos = @fieldpos[idx]
	    return nil unless pos
	    self[pos]
	  else
	    @oldcalls['[]'].call idx
	  end
	end
	def []= idx, value 
	  if @fields and idx.type == String
	    pos = @fieldpos[idx]
	    if not pos
	      pos = size
	      @fieldpos[idx] = pos
	    end
	    self[pos] = value
	  else
	    @oldcalls['[]='].call idx, value 
	  end
	end
      }
      self.extend m
    end
  end
  attr :fields
  attr :fieldpos
end



implementation :