On Mon, Jun 24, 2013 at 8:49 AM, Larry Evans <cppljevans@suddenlink.net>wrote:
Could you explain exactly how Expected is better?
either<ipv4, ipv6> get_ipaddr(); // X either<ipv6, addr_result> get_ipaddr(); // Y expected<ipv6, addr_result> get_ipaddr(); // Z 'either' _means_ the two result are equally valid. Or maybe doesn't imply any relationship. 'expected' _means_ one value is expected (success) the other is failure. So the _meaning_ - the semantics - are different. 'expected' has a more specific meaning, and should be chosen when that specific meaning is what you are trying to convey. So Z is better than Y above - the intent is more clear, more specific. So *one* of the uses of 'either' is better handled with 'expected'. That leaves the other use. how is 'either' better than 'variant': variant<ipv4, ipv6> get_ipaddr(); either<ipv4, ipv6> get_ipaddr(); I suspect 'either' has benefits here, but is it enough? And of course, what happens when someone adds another option: variant<ipv4, ipv6, ipv8> get_ipaddr(); (of course what will really happen is: variant<ipv4, ipv6, error> get_ipaddr(); instead of expected<either<ipv4, ipv6>, error> get_ipaddr(); and we've lost some meaning. Doesn't mean we shouldn't try!) Also, why is 'either' only 2 types? In English, it isn't: "I am either going on Monday, Tuesday, or Wednesday". How common is 2 choices over 3 choices (outside of value + error)? Tony