bind `it' to block, as default.

array-iter-with-it.rb

class Array
  class It
    def initialize(it)
      define_singleton_method(:it) { it }
    end
  end

  def self.iter_with_it(*methods)
    methods.each { |m|
      old = "_#{m.to_s}".to_sym
      alias_method old, m

      define_method(m) {|&block|
        if block and block.arity.zero?
          send(old) {|val| It.new(val).instance_eval &block }
        else
          send(old, &block)
        end
      }
    }
  end

  iter_with_it :each, :map, :map!, :keep_if, :take_while, :drop_while
end

# [1,2,3].map { it + 1 }