How to share protocols between Clojure & ClojureScript

Question

I have a protocol in a crossover namespace:

(ns xxx.shared.interfaces)

(defprotocol ITimer
  (seconds [timer] "Return time in seconds since timer began"))

I have an implementation for Clojure:

(ns xxx.time
  (:require [xxx.shared.interfaces :refer [ITimer]]))

(defrecord Timer [start-nanos]
  ITimer
  (seconds [timer] (double (/ (- (System/nanoTime) (:start-nanos timer))
                              1000000000))))

The problem is that when I use this code in some Clojure code, having required the xxx.time namespace with :refer :all it complains it can't find seconds:

Unable to resolve symbol: seconds in this context

First of all, is it possible to share protocols in this way?

Secondly, any idea how I can make this work?

Thirdly, is this actually a good way to go about this kind of code sharing? Ideally I'd like to share the record too, but it relies on Java code, so I'd need to break that out into a function. Would that be a better way to go about this?

Thanks!

Michał Marczyk · Accepted Answer · 2013-08-30 20:23:52Z

6

seconds is defined in xxx.shared.interfaces, so that's what you need to :require / :use to be able to call it:

(ns foo
  (:require [xxx.shared.interfaces :refer [seconds]]))

;; (seconds ...) will now work

To elaborate, a defprotocol expression creates a Var holding the protocol, an underlying JVM interfaces on the JVM side (no counterpart to this on the ClojureScript side) and a Var for each of the protocol methods. The protocol methods can then be called through those Vars just like any other Clojure function.

When you implement a protocol for a record / type, as you do in your xxx.time namespace, you don't even need to pull in those Vars if you don't actually call the methods (a separate issue from providing implementations). Similarly, a namespace which needs to call the protocol methods, but doesn't much care about the particular types of objects it calls them on, would only need to pull in the protocol-defining namespace and use the relevant Vars and would not need to :require any implementing namespaces.

answered Aug 30, 2013 at 20:23

Michał Marczyk

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1 Comment

Michał Marczyk Over a year ago

As for the code sharing questions, I'd say it's fine to share whatever works as shared code and factoring out platform-specific implementations of protocol methods is certainly a valid approach to making more code shareable. Whether it's worth it would probably depend on how much repetition is actually avoided. In any case, as explained above, the original problem is unrelated to sharing.

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