fickle

Dynamically redefine Clojure functions!

The goal, simply put is to redefine functions on a per-thread basis similar to how binding works, but in the context of with-redefs. For example, the following does not work with with-redefs because it changes the root of the var being redefined by another thread.

We create two futures, both trying to redefine the same function, and the redefinition is not isolated to the future that is redefining it:

(defn funk
  [& args]
  :default-value)

(def f1
  (future
    (with-redefs [funk (constantly :something-new)]
      (Thread/sleep 1000)
      (funk 1))))

(def f2
  (future
    (with-redefs [funk (constantly :something-else)]
      (Thread/sleep 2000)
      (funk 1))))

[(funk 1) @f1 @f2]

;; After two seconds, we expect:
;; [:default-value :something-new :something-else]
;;
;; But we can get any thing else, like:
;; [:something-else :something-else :default-value]
;;
;; Or any of the following:
;; [:something-new :something-else :something-new]
;; [:something-new :something-new :something-new]

This expectation of thread-level isolation of redefinition can be achieved using fickle.core/with-dynamic-redefs

Usage

Following is an example of the usage of per-thread redefinitions.

(require '[fickle.core :as fc])

(defn funk
  [& args]
  :default-value)

(def f1
  (future
    (fc/with-dynamic-redefs [funk (constantly :something-new)]
      (Thread/sleep 1000)
      (= :something-new (funk 1)))))

(def f2
  (future
    (fc/with-dynamic-redefs [funk (constantly :something-else)]
      (Thread/sleep 2000)
      (= :something-else (funk 1)))))


;; the following should return true after 2 sec
(and @f1 @f2 (= :default-value (funk 1)))

Known caveats

• This per-thread rebinding will only work with Clojure concurrency primitives which copy per-thread bindings to newly spawned threads, eg, using Clojure futures. But will not work for, say a java.lang.Thread.
• As of now this only supports functions being bound and not other vars which store values, say (def x 19) for example.

Pull requests welcome!

License

This program and the accompanying materials are made available under the terms of the Eclipse Public License 2.0 which is available at http://www.eclipse.org/legal/epl-2.0.

This Source Code may also be made available under the following Secondary Licenses when the conditions for such availability set forth in the Eclipse Public License, v. 2.0 are satisfied: GNU General Public License as published by the Free Software Foundation, either version 2 of the License, or (at your option) any later version, with the GNU Classpath Exception which is available at https://www.gnu.org/software/classpath/license.html.