Support dynamic runtime extension of protocols to types


  • Type: Enhancement Enhancement
  • Status: Closed Closed
  • Priority: Minor Minor
  • Resolution: Completed
  • Affects Version/s: None
  • Fix Version/s: None
  • Component/s: None
  • Labels:
  • Patch:
    Code and Test


Here is a transliteration of a semi-common pattern used with Clojure protocols to dynamically extend protocols to concrete types implementing other protocols (or interfaces, on the JVM):

(defprotocol P (m [this]))

(extend-protocol P
  (m [this]
    (if (seq? this)
        (extend-type (type this) P
          (m [this] (count this)))
        (m this))
      (throw (ex-info "Cannot extend m to type" {:type (type this)})))))

(I think dnolen was the first to talk about this outside of irc.) Unfortunately, this does not work in ClojureScript; extend-type currently requires that the type be specified as a symbol:

clojure.lang.ExceptionInfo: clojure.lang.PersistentList cannot be cast to clojure.lang.Named at line 4  {:tag :cljs/analysis-error, :file nil, :line 4, :column 5}

I can (hackily?) make this work by simply not attempting to resolve tsym here. However, that leaves lists in as values for :tag metadata (which might be used by the analyzer and/or other tools that depend upon it?), which I presume is not OK.

If someone can provide guidance on a sane path from here, I'll do what I can to produce a plausible patch.


Chas Emerick made changes -
Field Original Value New Value
Attachment CLJS-527.diff [ 12039 ]
Patch Code and Test [ 10002 ]
David Nolen made changes -
Assignee Chas Emerick [ cemerick ]
Chas Emerick made changes -
Assignee Chas Emerick [ cemerick ] David Nolen [ dnolen ]
David Nolen made changes -
Priority Major [ 3 ] Minor [ 4 ]
David Nolen made changes -
Resolution Completed [ 1 ]
Status Open [ 1 ] Resolved [ 5 ]
David Nolen made changes -
Status Resolved [ 5 ] Closed [ 6 ]


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