clojure.core/alter-var-root

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Type: function Added: Clojure 1.0 Examples: 5 Runnable: 2

([v f & args])

Atomically alters the root binding of var v by applying f to its current value plus any args

Examples

by fogus on 2010-09-08 . May have evaluation errors. ▶

(defn sqr [n] "Squares a number" (* n n)) (sqr 5) 25 (alter-var-root (var sqr) ; var to alter (fn [f] ; fn to apply to the var's value (fn [n] ; returns a new fn wrapping old fn (println "Squaring" n) (f n)))) (sqr 5) Squaring 5 25

by manishkumarmdb on 2016-08-27 . May have evaluation errors. ▶

;;change the value of a var, instead of (def varName value) (def string "abcd") #'user/string string "abcd" (alter-var-root #'string (constantly "wxyz")) "wxyz" string "wxyz"

by reborg on 2018-12-28 ▶

;; alter-var-root does not play well with inlined vars (definline timespi [x] `(* ~x 3.14)) (alter-var-root #'timespi (fn [_] (constantly 1))) (timespi 10) ;; 31.400000000000002 ;; Remove the inlining to pass through the var indirection (credits @bronsa) (alter-meta! #'timespi dissoc :inline-arities :inline) (timespi 10) ;; 1

by MicahElliott on 2022-03-29 ▶

(defn instrument-ns "Add some basic instrumentation to each var in a given namespace `sut`. A poor man's profiler, this simply prints out the name of each fn (var) when run." [sut] (let [fs (vals (ns-interns sut))] (println "Instrumenting fns to be printed as run for NS:" sut) (doseq [curf fs] (when (some? (:arglists (meta curf))) ; ensure a fn (alter-var-root curf (fn [f] (fn [& args] ;; Just print out the name of the function evaluated (println (:name (meta curf))) ;; (System/currentTimeMillis) ; could add timing info before/after (apply f args))))))))

by Crowbrammer on 2022-05-25 . May have evaluation errors. ▶

;; `alter-var-root` uses the same signature as `swap!` and `update`. ;; It has uncomfortable, persistent effects if you're not careful. (def x "Hi") #'user/x (alter-var-root #'x #(str % ", world!")) "Hi, world!" (alter-var-root #'x #(str % ", world!")) "Hi, world!, world!" (alter-var-root #'x #(str % ", world!")) "Hi, world!, world!, world!"

See also

• clojure.core/with-redefs (1)
• clojure.core/with-redefs-fn (0)
• clojure.core/intern (0)
• clojure.core/var (0)
• clojure.core/set! (1)

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Content from the matching ClojureDocs page, with authors credited on each contribution.