functionsince v0.0-2985 clojure.repl/source-fnEdit
(source-fn env x)

Source docstring:
Returns a string of the source code for the given symbol, if it can
find it.  This requires that the symbol resolve to a Var defined in
a namespace for which the .clj is in the classpath.  Returns nil if
it can't find the source.  For most REPL usage, 'source' is more

Example: (source-fn 'filter)
Source code @ clojurescript:src/main/clojure/cljs/repl.cljc
(defn source-fn
  [env x]
  (when-let [v (ana-api/resolve env x)]
    (when-let [filepath (:file v)]
      (let [f (io/file filepath)
            f (if (.exists f)
                (io/resource filepath))]
        (when f
          (with-open [pbr (PushbackReader. (io/reader f))]
            (let [rdr (readers/source-logging-push-back-reader pbr)]
              (dotimes [_ (dec (:line v))] (readers/read-line rdr))
              (binding [reader/*alias-map*    identity
                        reader/*data-readers* (merge tags/*cljs-data-readers*
                (-> (reader/read {:read-cond :allow :features #{:cljs}} rdr)
                  meta :source)))))))))