Even when the interface and class you've extended them to are related by inheritance? I thought the most-derived implementation was chosen, and only when there's a "tie" like a class that implements two unrelated interfaces, both of which have had the protocol extended to them. If it were completely undefined, all sorts of useful idioms would become untenable. For example, you couldn't extend a protocol to Object and also to various other classes. Dave's scenario is like this, in which TUnion is more-derived than TBase, and so as far as I know should always "win" in the polymorphic dispatch. Reading through core_deftype's functions 'pref and 'find-protocol-impl seem to support this assertion; am I missing something?
On Friday, May 10, 2013 2:20:31 PM UTC-7, Stuart Sierra wrote: > > When you extend a protocol to multiple Java interfaces / abstract classes, > then call the methods on an instance which implements/extends more than one > of those, the result is *undefined*. > > The problem is that this permits multiple inheritance of concrete > behavior, the reason Java doesn't allow multiple base classes. > > Protocols do not (currently) have any mechanism to "prefer" one method > implementation over another. > > In general, I recommend extending protocols only to concrete types, not > interfaces. > > -S > > > On Friday, May 10, 2013 4:45:57 AM UTC+10, Dave Kincaid wrote: >> >> I've not worked with protocols much, but saw a good fit recently. >> However, I'm a little bit unsure about this situation. I have some Thrift >> objects that I'd like to be able to easily unpack into maps, so I created a >> protocol >> >> (defprotocol Unpackable >> (unpack [x])) >> >> Thrift has two main data types - structs and unions that need to be >> handled differently. Structs always implement the interface TBase. Unions >> extend the abstract class TUnion which in turn implements the interface >> TBase. So I'm extending my protocol to these types like this: >> >> (extend-protocol IUnpackable >> TUnion >> (unpack [x] (unpack-union x)) >> >> TBase >> (unpack [x] (unpack-struct x (get-meta-data (class x))))) >> >> It all seems to work correctly, but I'm a little unsure that it is >> guaranteed to work especially in the case of a union. Since a union extends >> TUnion, but also implements TBase will a call to (unpack the-union) do the >> right thing (go to unpack-union instead of unpack-struct) every time? >> >> Thanks. >> > -- -- You received this message because you are subscribed to the Google Groups "Clojure" group. To post to this group, send email to clojure@googlegroups.com Note that posts from new members are moderated - please be patient with your first post. To unsubscribe from this group, send email to clojure+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/clojure?hl=en --- You received this message because you are subscribed to the Google Groups "Clojure" group. To unsubscribe from this group and stop receiving emails from it, send an email to clojure+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
