Clojurians Log v2

Not lucky enough so far to use Clojure in a professional setting but I've been messing around with it lately (watched a lot of talks, reading SICP & Joy of Clojure), came across this in reddit: https://www.reddit.com/r/Clojure/comments/2i1dem/help_me_make_this_fn_betterprettier/ One of my pastimes is to spend yak-shaving levels of time on a piece of code trying to make it as elegant as possible.

I know it's a bit long but any criticism is greatly appreciated 🙂

My first thought is that it's not idiomatic to map println over a sequence. Instead of (dorun (map println coll)) instead do (run! println coll)

is there a standard way to run transducers for side effects though?

(you can do it with transduce, but that means setting up the accumulation machinery with nils)

I don't find seen/`append` to be intuitive or readable. I think they over-complicate things

My feeling is that something like this is clearer:

(defn say-hello
  [names]
  (->> names
       (mapv (greeter))
       (conj "Goodbye")
       (run! println)))

(defn greeter
  []
  (let [seen (atom #{})]
    (fn [a-name]
      ...)))

I do see the appeal of using chained transducers for chained operations though, instead of just nesting calls

@seancorfield That's nice! Today I Learned of mapv.

Didn't know there was a map without the laziness

Given this is all about side-effects (printing), I would have just replaced the original recursive solution with a doseq, followed by an imperative (println "Goodbye") with no conditionals (except on the "seen" part, which could be a local atom in the say-hello function...

I keep forgetting map can take multiple collections, then trying to find a zip function then rediscover this every once in a while. Variadic arities are beautiful

(defn say-hello
  [names]
  (let [seen (atom #{})]
    (doseq [n names]
      (if (@seen n)
        (println "Welcome back" n "!")
        (do
          (swap! seen conj n)
          (println "Hello" n "!"))))
    (println "Goodbye")))

(untested but, hey)

Works. Unceremonious and crisp

Huh... worked without the deref

It didn't work for me without the deref. Works with it.

(if (seen n) ...) would fail since an atom is not a fn

if you use a ref instead, it does work without a deref though

but then you need to transact etc.

Ah. I have never used ref -- never needed to coordinate multiple stateful vars.

user=> ((ref #{:a}) :a)
:a

(Iike vars)