A couple of things on this subject: 1) protocols were invented primarily for their speed. If one was going to write Clojure in Clojure it would be a little hard to implement PersistentHashMaps using pure data. You have to start with something. Thus deftype and defprotocol were born.
2) Unlike in most OOP, the protocol methods are not tied to the object. Instead they are tied to the function. You can see that with the function clojure.core/extend. Extend says "when you call function A with a type T as the first argument, dispatch to function B". This is quite a bit different from normal polymorphism (especially in languages like Python/Ruby, etc). 3) Thirdly I'd say that using protocols/types for data is often considered a bit un-idiomatic. For example, see the ClojureScript compiler: https://github.com/clojure/clojurescript/blob/master/src/clj/cljs/compiler.clj#L377 The AST of ClojureScript is stored in nested hashmaps, and dispatching is done on the :op value in each map. No records, no protocols, no types. Just hashmaps and multimethods. I would argue that this is the "Clojure way". Use data, except when performance requirements dictate otherwise. Almost everytime I've built a large codebase using protocols instead of multimethods, I've regretted it. "Data all the things"...as the saying goes. Timothy Baldridge On Sun, Jan 19, 2014 at 2:55 PM, James Reeves <ja...@booleanknot.com> wrote: > On 19 January 2014 19:55, Brian Craft <craft.br...@gmail.com> wrote: > >> http://www.youtube.com/watch?v=ROor6_NGIWU >> >> Around 56:28 Rich is talking about whether components pass a data >> structure, or "an object that has all these verbs and knows how to do stuff >> ...". >> >> Clojure data types also have verbs, in protocols, and if one component >> passes a record to another component, the receiver will need to use those >> verbs to make use of the data. How is that different than the OOP example? >> There's the syntactic difference that in OOP you write it object, verb, >> arguments, and in clojure you write it verb, object, arguments. But that's >> trivial. How is it architecturally different to pass an object with methods >> vs passing a record with protocol? >> > > Protocols should be considered a tool for polymorphism, rather than a tool > for encapsulation. You shouldn't consider protocols to be tightly coupled > to a record or type. > > To give a more concrete example, consider a edn data structure like: > > #inst "2014-01-19T21:44:46.471-00:00" > > By default Clojure reads this as a java.util.Date object, but we could > equally read this as a org.joda.time.DateTime object, or > a hirondelle.date4j.DateTime object. > > All these implementations describe the same data, but will have subtly > different methods and interfaces. The data structure serialised in edn > doesn't mandate a particular class, or a particular set of methods. > > Similarly, if we imagine a data structure like: > > #myapp/Person {:first "Fred" :last "Mertz"} > > You should think of this as a map tagged with "#myapp/Person", not a > representation of a specific Clojure record that has a specific set of > functions. It's up to the receiver to interpret this data, and the > functions that operate on this data might differ between components. For > instance, a component that displays the data to the user might have a > function to derive a person's full name, but a component that archives the > data wouldn't need this functionality. > > - James > > -- > -- > 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. > -- “One of the main causes of the fall of the Roman Empire was that–lacking zero–they had no way to indicate successful termination of their C programs.” (Robert Firth) -- -- 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.
