f0r.rb

def f0r(init, condition, step, &block)
  var, step = *step

  while condition[var]
    eval "i = #{var}", block.binding
    yield
    var = step[var]
  end
end

class Fixnum
  alias :smaller :<
  alias :add :+

  def <(other)
    ->(i){ i.smaller(other) }
  end

  def +(other)
    [self, ->(i){ i.add(other) }]
  end
end

f0r i = 1, i < 5, i += 1 do
  puts "in block #{i}"
end

Don't try this at home!