README.md

A little Clojure configuration reader

This is a handy bit of code I've written more than once. I thought I'd throw it in here so I can refer back to it later. Basically, it lets you read a config file to produce a Clojure map. The config files themselves can contain one or more forms, each of which can be either a map or a list. Maps are simply merged. Lists correspond to invocations of extension points, which in turn produces a map, which is merged along with everything else.

An Example

Consider the following files:

names.edn

{:first-name "Craig"
 :middle-name "Bert"}

;; Comments are ignored - cool!
{:last-name "Andera"
 :middle-name "Alan"} ; Later entries override earlier ones

addresses.edn

{:real-world {:city "Fairfax" :state "VA"}
 :email "craig@craigandera.org"}

craig.edn

(include "names.edn")
(include "addresses.edn")

{:favorites {:colors [:red :black]}}

Calling (read-config "/path/to/craig.edn") would produce the following map:

{:first-name "Craig"
 :last-name "Andera"
 :middle-name "Alan"}
 :real-world {:city "Fairfax" :state "VA"}
 :email "craig@craigandera.org"
 :favorites {:colors [:red :black]}}

Note that you can use relative or absolute pathnames in the (include ... calls, and they'll be resolved relative to the file they appear in

config.clj

(defn form-seq
  "Lazy seq of forms read from a reader"
  [reader]
  (let [form (read reader false reader)]
    (when-not (= form reader)
      (cons form (lazy-seq (form-seq reader))))))

(defmulti invoke-extension
  "An extension point for operations in config files."
  (fn [_ operation & _] operation))

(defn mapify
  "Turns a form read from a config file at `origin` into a map."
  [form origin]
  (cond
   (map? form) form
   (list? form) (let [[operation & args] form]
                  (invoke-extension origin operation args))
   :else (throw (ex-info (str "No support for mapifying form " (pr-str form))
                         {:reason :cant-mapify
                          :form form
                          :origin origin}))))

(defn read-config
  "Returns a map read from `path`. Map will be generated by merging all
  forms found in file. Lists are interpreted as invocations of the
  `invoke-extension` multimethod, dispatched on the symbol in the
  first position.

  Example:

  (include \"something.edn\")
  ;; Comments are ignored
  {:foo :bar
   :bar 1234}

  Might yield:

  {:foo :bar
   :from-something 4321
   :bar 1234}"
  [path]
  (let [source (-> path
                   io/file
                   java.io.FileReader.
                   java.io.PushbackReader.)]
    (reduce merge (map #(mapify % path) (form-seq source)))))

(defn path-relative-to
  "Given two paths `p1` and `p2`, returns a path that is the
  combination of them. If `p2` is absolute, `p2` is returned.
  Otherwise p1/p2 is returned."
  [p1 p2]
  (let [f1 (java.io.File. p1)
        f2 (java.io.File. p2)]
    (if (.isAbsolute f2)
      p2
      (.getPath (java.io.File. p1 p2)))))

(defn parent
  "Returns the parent directory of `path`"
  [path]
  (.getParent (io/file path)))

;; E.g. (include "foo.edn") => {:some :map}
;; TODO: globbing support
(defmethod invoke-extension 'include
  [origin operation [path]]
  (read-config (path-relative-to (parent origin) path)))