Using memoization to change a creation function into get-or-create.

N.B. This should work in any language with first class functions, memoization, and immutable values.

tl;dr:  In the past I'd always thought of memoization as a way to save the computer work. It hadn't occurred to me that it could also save me work.

I was re-reading "The Joy of Clojure" and came across a gem I'd missed the first time. Listing 14.12 is described as "A function to create or retrieve a unique Agent for a given player name".

(def agent-for-player
  (memoize
    (fn [player-name]
      (-> (agent [])
          (set-error-handler! #(println "ERROR: " %1 %2))
          (set-error-mode! :fail)))))

;; The above doesn't quite work for me, set-error-handler!
;; doesn't seem to return the agent. Doesn't make the pattern
;; less compelling, though.

The authors comment that this allows you to maintain player-name as an index into a table of agents without having to explicitly manager and lookup agents.

There are two caveats to this approach: 1) player-name must be immutable, and 2) You really need to understand the memoization mechanism.  clojure.core/memoize, for instance, will keep a internal map of args/response until the end of time. You could use http://clojure.github.io/core.memoize/ to modify the strategy if you so choose.

The place where I'd try this first is in what I call "micro-logs". Frequently as I'm working, I want to log some data to a side channel, and this pattern saves having to manage this manually and cluttering up my code.

(def get-or-create-micro-log
  (memoize
   (fn [file]
     (io/make-parents file)
     (.createNewFile file)
     (-> (io/writer file :append true)
         (agent
          :error-mode :fail
          :error-handler #(println "ERROR: " %1 %2))))))

(defn microlog
  "Useful micro-pattern to send off a write to various
  files via agents without having to maintain a lookup
  table. Symlinks can get you into trouble; at a 
  minimum they will duplicate the agent."
  [lg line]
  (let [a (-> (io/file lg)
              (.getAbsoluteFile)
              (get-or-create-micro-log))
        output (str (str/trim-newline line) "\n")]
    (send-off a
          (fn [writer]
            (doto writer
              (.write output)
              (.flush))))))

(defn microlog-all
  [lg all-data]
  (doseq [d all-data]
    (microlog lg d)))

Edits: Used io/make-parents instead of File calls. Changed send to send-off, since this is I/O. Cleaned up creation of agent.