Why can't I extend Clojure's IFn using extend-type?

Question

I saw David Nolen's talk on ClojureScript where he makes extends IFn to Regexp and so that we can call a regex literal as a fuinction on a string to check for matches.

I tried something similar with Strings in regular Clojure, but I get this exception. I see that IFn is an interface written in Java but how does the implementation work in CLojrue front.

(extend-type java.lang.String
  #_=>    clojure.lang.IFn
  #_=>   (-invoke
  #_=>    ([this index]
  #_=>     (get (seq this) index))))
IllegalArgumentException interface clojure.lang.IFn is not a protocol  clojure.core/extend (core_deftype.clj:742)

Answer 1

The exception should give you a hint. IFn is not a protocol in Clojure. It is in ClojureScript. You can extend protocols, implement Java interfaces, and subclass classes. In this particular case, you cannot even subclass java.lang.String because it's declared final.