Question

I’m learning Clojure and as an exercise I wanted to write something like the unix “comm” command.

To do this, I read the contents of each file into a set, then use difference/intersection to show exclusive/common files.

After a lot of repl-time I came up with something like this for the set creation part:

(def contents (ref #{}))
(doseq [line (read-lines "/tmp/a.txt")]
  (dosync (ref-set contents (conj @contents line))))

(I’m using duck-streams/read-lines to seq the contents of the file).

This is my first stab at any kind of functional programming or lisp/Clojure. For instance, I couldn’t understand why, when I did a conj on the set, the set was still empty. This lead me to learning about refs.

1. Is there a better Clojure/functional way to do this? By using ref-set, am I just twisting the code to a non-functional mindset or is my code along the lines of how it should be done?
2. Is there a a library that already does this? This seems like a relatively ordinary thing to want to do but I couldn’t find anything like it.

Answer 1

Clojure 1.3:

user> (require '[clojure.java [io :as io]])
nil
user> (line-seq (io/reader "foo.txt"))
("foo" "bar" "baz")
user> (into #{} (line-seq (io/reader "foo.txt")))
#{"foo" "bar" "baz"}

line-seq gives you a lazy sequence where each item in the sequence is a line in the file.

into dumps it all into a set. To do what you were trying to do (add each item one by one into a set), rather than doseq and refs, you could do:

user> (reduce conj #{} (line-seq (io/reader "foo.txt")))
#{"foo" "bar" "baz"}

Note that the Unix comm compares two sorted files, which is likely a more efficient way to compare files than doing set intersection.

Edit: Dave Ray is right, to avoid leaking open file handles it’s better to do this:

user> (with-open [f (io/reader "foo.txt")]
        (into #{} (line-seq f)))
#{"foo" "bar" "baz"}