protocol.rb

# an object representing a duck type that can be used in a case expression
class Protocol
  def initialize(*selectors)
    @selectors = selectors
  end

  def ===(object)
    @selectors.all? {|selector| object.respond_to?(selector)}
  end
end

rest = Protocol.new(:get, :post, :put, :delete)
sql =  Protocol.new(:select, :insert, :update, :delete)

case obj
when rest
  obj.get(things)
when sql
  obj.select(things)
end