[Enchancement proposal] Support for optional inclusions using #opt-include reader tag

[titonbarua]

It is a common pattern to include a default configuration with your application and then override those defaults from other sources(if found) during deployment. Aero'r #include tag seems particularly useful in such situations. The problem is, it does not handle missing files gracefully. Also, the course of action taken for a missing file varies greatly depending on the resolver in use.

After some discussion in clojure slack, someone suggested that a custom reader tag would do the job and it would be trivial to implement. As such, here's an implementation of the #opt-include tag,
which takes a filename along with a default value(which is used if the file is missing), passed as a 2-element vector:

(require 'clojure.java.io)
(require 'clojure.string)
(require 'aero.core)
(import [java.io])

(defn missing?
  "Missing file detection, compatible with relative-resolver."
  [file]
  (or (nil? file)
      (and (= (type file) java.io.StringReader)
           (clojure.string/includes? (slurp file) "missing-include"))))

(defmethod aero.core/reader 'opt-include
  [{:keys [resolver source] :as opts} tag [filename default-val]]
  (try
    (let [f (if (map? resolver)
              (get resolver filename)
              (resolver source filename))]
      (if (missing? f)
        default-val
        (aero.core/read-config f)))
    (catch java.io.FileNotFoundException e
      default-val)))

Example configuration file using #opt-include:

#merge [
   {:host "localhost"}
   #opt-include ["opt-config.edn" {:port 8080}]
]

I plan to submit a pull request later.

[malcolmsparks]

I think this is better to be done in user-code and not Aero. As you say, Aero:

it does not handle missing files gracefully

which is sort of the point. Aero sticks to the principle of zero overrides - the absence of a config file shouldn't cause a change to the config. Instead, it should cause the config loading to fail.

I realise current resolvers' behaviours vary, and that isn't ideal. However, I think we should still preserve the philosophy of Aero in this regards.