partially_apply.rb

class SliceApplicator
  def initialize(target, *slice)
    @target = target
    @slice  = slice
  end

  def respond_to_missing?(name)
    @target.respond_to?(name)
  end

  def method_missing(name, *args, &block)
    slice  = @target[*@slice]
    result = slice.public_send(name, *args, &block)
    @target[*@slice] = slice

    result.equal?(slice) ? @target : result
  end
end

class Array
  def partially_apply(*slice)
    SliceApplicator.new(self, *slice)
  end
end

ary = [1,2,3,4,5,6,7,8]
ary.partially_apply(0, 4).reject! do |x| x.odd? end
ary # => [2, 4, 5, 6, 7, 8]