Wrapping functions in Clojure

wrap_fn.clj

(defn wrap-fn
  "Wrap or replace some function with your own function"
  [qualifier wrapper]
  (alter-var-root
   qualifier
   (fn [original-fn]
     (fn [& caller-arguments]
       (wrapper caller-arguments original-fn)))))

;; Replace your function implementation:

;; Arity 0:

(defn wrap-fn-original
  []
  \"wrap-fn-original-return-value\")

(wrap-fn #'wrap-fn-original
         (fn [caller-arguments original-fn]
           \"wrap-fn-replaced-return-value\"))

(wrap-fn-original) ;; => \"wrap-fn-replaced-return-value\"

;; Arity 1:

(defn wrap-fn-original-arity-1
  [a]
  (format "original-value-is-%s" a))

(wrap-fn #'wrap-fn-original-arity-1
         (fn [caller-arguments original-fn]
           (format "changed-value-is-%s" (first caller-arguments))))

(wrap-fn-original-arity-1-and-optional-keys "aaa" :b 1 :c 2 :d 3) ;; => "changed-values-for-a-aaa-b-1-c-2-d-3"

;; More complicated case:

(defn wrap-fn-original-arity-1-and-optional-keys
  [a &{:keys [b c d]}]
  (format "original-values-for-a-%s-b-%d-c-%d-d-%d" a b c d))

(wrap-fn-original-arity-1-and-optional-keys "aaa" :b 1 :c 2 :d 3) ;; => "changed-values-for-a-aaa-b-1-c-2-d-3"

;; Chain your function implementation:

(defn wrap-fn-original
  []
  "wrap-fn-original-return-value")

(wrap-fn #'wrap-fn-original
         (fn [caller-arguments original-fn]
           (let [original-result (apply original-fn caller-arguments)]
             (str original-result " " "wrap-fn-chained-return-value"))))

(wrap-fn-original) ;; => "wrap-fn-original-return-value wrap-fn-chained-return-value"

;; Chain function (arity 1) with a different implementation:

(defn wrap-fn-original-arity-1
  [a]
  (format "original-value-is-%s" a))

(wrap-fn #'wrap-fn-original-arity-1
         (fn [caller-arguments original-fn]
           (let [original-result (apply original-fn caller-arguments)]
             (str original-result " " (format "chained-value-is-%s" (first caller-arguments))))))

(wrap-fn-original-arity-1 "aaa") ;; => "original-value-is-aaa chained-value-is-aaa"

;; Chain function (arity 1 and optional keys) with a different implementation

(defn wrap-fn-original-arity-1-and-optional-keys
  [a &{:keys [b c d]}]
  (format "original-values-for-a-%s-b-%d-c-%d-d-%d" a b c d))

(wrap-fn #'wrap-fn-original-arity-1
         (fn [caller-arguments original-fn]
           (let [original-result (apply original-fn caller-arguments)]
             (str original-result " " (format "chained-value-is-%s" (first caller-arguments))))))

(wrap-fn-original-arity-1-and-optional-keys "aaa" :b 1 :c 2 :d 3) ;; => "original-values-for-a-aaa-b-1-c-2-d-3 chained-values-for-a-aaa-b-1-c-2-d-3"