add helper function to check if a twig function has already been defined

src/clj_jtwig/core.clj

@@ -2,6 +2,7 @@
   (:require [clojure.walk :refer [stringify-keys]]
             [clojure.java.io :as io])
   (:import (com.lyncode.jtwig JtwigTemplate JtwigContext)
+           (com.lyncode.jtwig.functions.exceptions FunctionNotFoundException)
            (com.lyncode.jtwig.functions.repository DefaultFunctionRepository)
            (com.lyncode.jtwig.functions JtwigFunction)
            (java.io File FileNotFoundException)))
@@ -10,6 +11,13 @@
 ; any custom functions added will be added to this instance
 (defonce functions (atom (new DefaultFunctionRepository (make-array JtwigFunction 0))))
 
+(defn- twig-fn-exists? [name]
+  (try
+    (.retrieve @functions name)
+    true
+    (catch FunctionNotFoundException ex
+      false)))
+
 (defn- get-resource-path [filename]
   (-> (Thread/currentThread)
       (.getContextClassLoader)