clojure.core/map isn't working - clojure

Question

I am trying to figure out why one of my map calls isn't working. I am building a crawler with the purpose of learning Clojure.
(use '[clojure.java.io])
(defn md5
"Generate a md5 checksum for the given string"
[token]
(let [hash-bytes
(doto (java.security.MessageDigest/getInstance "MD5")
(.reset)
(.update (.getBytes token)))]
(.toString
(new java.math.BigInteger 1 (.digest hash-bytes)) ; Positive and the size of the number
16)))
(defn full-url [url base]
(if (re-find #"^http[s]{0,1}://" url)
url
(apply str "http://" base (if (= \/ (first url))
url
(apply str "/" url)))))
(defn get-domain-from-url [url]
(let [matcher (re-matcher #"http[s]{0,1}://([^/]*)/{0,1}" url)
domain-match (re-find matcher)]
(nth domain-match 1)))
(defn crawl [url]
(do
(println "-----------------------------------\n")
(if (.exists (clojure.java.io/as-file (apply str "theinternet/page" (md5 url))))
(println (apply str url " already crawled ... skiping \n"))
(let [domain (get-domain-from-url url)
text (slurp url)
matcher (re-matcher #"<a[^>]*href\s*=\s*[\"\']([^\"\']*)[\"\'][^>]*>(.*)</a\s*>" text)]
(do
(spit (apply str "theinternet/page" (md5 url)) text)
(loop [urls []
a-tag (re-find matcher)]
(if a-tag
(let [u (nth a-tag 1)]
(recur (conj urls (full-url u domain)) (re-find matcher)))
(do
(println (apply str "parsed: " url))
(println (apply str (map (fn [u]
(apply str "-----> " u "\n")) urls)))
(map crawl urls)))))))))
(defn -main
"I don't do a whole lot ... yet."
[& args]
(crawl "http://www.example.com/"))
First call to map works:
(println (apply str (map (fn [u]
(apply str "-----> " u "\n")) urls)))
But the second call (map crawl urls) seems to be ignored.
The crawl function is working as intended, slurping the url, parsing with the regex for a tags for fetching the href and the accumulation in the loop works as intended, but when i call map with crawl and the urls that have been found on the page, the call to map is ignored.
Also if I try to call (map crawl ["http://www.example.com"]) this call is, again, ignored.
I have started my Clojure adventure a couple of weeks ago so any suggestions/criticisms are most welcomed.
Thank you

Answer 1

In Clojure, map is lazy. From the docs, map:
Returns a lazy sequence consisting of the result of applying f to the
set of first items of each coll, followed by applying f to the set
of second items in each coll, until any one of the colls is
exhausted.
Your crawl function is a function with side effects - you're spit-ing some results to a file, and println-ing to report on progress. But, because map returns a lazy sequence, none of these things will happen - the result sequence is never explicitly realized so it can stay lazy.
There are a number of ways of realizing a lazy sequence (that has been created e.g. using map), but in this case, as you want to iterate over a sequence using a function that has side-effects, it's probably best to use doseq:
Repeatedly executes body (presumably for side-effects) with
bindings and filtering as provided by "for". Does not retain
the head of the sequence. Returns nil.
If you replace the call to (map crawl urls) with (doseq [u urls] (crawl u)), you should get the desired result.
Note: your first call to map works as expected because you are realizing the results using (apply str). There is no way to (apply str) without evaluating the sequence.