Clojure program works fine when debugged, fails in repl

I'm learning core.async and have written a simple producer consumer code:

(ns webcrawler.parallel
  (:require [clojure.core.async :as async
             :refer [>! <! >!! <!! go chan buffer close! thread alts! alts!! timeout]]))

(defn consumer
  [in out f]
  (go (loop [request (<! in)]
        (if (nil? request)
          (close! out)
          (do (print f)
            (let [result (f request)]
              (>! out result))
              (recur (<! in)))))))

(defn make-consumer [in f]
  (let [out (chan)]
    (consumer in out f)
    out))

(defn process
  [f s no-of-consumers]
  (let [in (chan (count s))
        consumers (repeatedly no-of-consumers #(make-consumer in f))
        out (async/merge consumers)]
    (map #(>!! in %1) s)
    (close! in)
    (loop [result (<!! out)
           results '()]
      (if (nil? result)
        results
        (recur (<!! out)
               (conj results result))))))

This code works fine when I step in through the process function in debugger supplied with Emacs' cider.

(process (partial + 1) '(1 2 3 4) 1)
(5 4 3 2)

However, if I run it by itself (or hit continue in the debugger) I get an empty result.

(process (partial + 1) '(1 2 3 4) 1)
()

My guess is that in the second case for some reason producer doesn't wait for consumers before exiting, but I'm not sure why. Thanks for help!

The problem is that your call to map is lazy, and will not run until something asks for the results. Nothing does this in your code.

There are 2 solutions:

(1) Use the eager function mapv:

(mapv #(>!! in %1) items)

(2) Use the doseq, which is intended for side-effecting operations (like putting values on a channel):

(doseq [item items]
  (>!! in item))

Both will work and produce output:

(process (partial + 1) [1 2 3 4] 1) => (5 4 3 2)

---

P.S. You have a debug statement in (defn consumer ...)

(print f)

that produces a lot of noise in the output:

<#clojure.core$partial$fn__5561 #object[clojure.core$partial$fn__5561 0x31cced7
"clojure.core$partial$fn__5561@31cced7"]>

That is repeated 5 times back to back. You probably want to avoid that, as printing function "refs" is pretty useless to a human reader.

Also, debug printouts in general should normally use println so you can see where each one begins and ends.