underscore.rb

module Maybe
  def wrap_block(block)
    lambda do |a|
      a==nil ? nil : block.call(a)
    end
  end
end

module MaybeNumeric
  def wrap_block(block)
    lambda do |a|
      block.call(a.is_a?(Numeric) ? a : 0)
    end
  end
end

module Underscore
  def method_missing(sym,*args,&block)
    if sym.to_s =~ /^_(.+)/ && respond_to?($1) && respond_to?(:wrap_block)
      send($1.to_sym,*args,&wrap_block(block))
    else
      super
    end
  end
end

Object.send :include, Underscore

[1,2,nil].map{|x| x * 2} #=> NoMethodError

src = [1,2,nil]
src.extend Maybe
src._map{|x| x * 2} #=> [2, 4, nil]

src = [1,2,nil]
src.extend MaybeNumeric
src._map{|x| x * 2} #=> [2, 4, 0]
src << "abc"
src._map{|x| x * 2} #=> [2, 4, 0, 0]