How to parse URL parameters in Clojure? - clojure

Question

If I have the request "size=3&mean=1&sd=3&type=pdf&distr=normal" what's the idiomatic way of writing the function (defn request->map [request] ...) that takes this request and
returns a map {:size 3, :mean 1, :sd 3, :type pdf, :distr normal}
Here is my attempt (using clojure.walk and clojure.string):
(defn request-to-map
[request]
(keywordize-keys
(apply hash-map
(split request #"(&|=)"))))
I am interested in how others would solve this problem.

Answer 1

Using form-decode and keywordize-keys:
(use 'ring.util.codec)
(use 'clojure.walk)
(keywordize-keys (form-decode "hello=world&foo=bar"))
{:foo "bar", :hello "world"}

Answer 2

Assuming you want to parse HTTP request query parameters, why not use ring? ring.middleware.params contains what you want.
The function for parameter extraction goes like this:
(defn- parse-params
"Parse parameters from a string into a map."
[^String param-string encoding]
(reduce
(fn [param-map encoded-param]
(if-let [[_ key val] (re-matches #"([^=]+)=(.*)" encoded-param)]
(assoc-param param-map
(codec/url-decode key encoding)
(codec/url-decode (or val "") encoding))
param-map))
{}
(string/split param-string #"&")))

Answer 3

You can do this easily with a number of Java libraries. I'd be hesitant to try to roll my own parser unless I read the URI specs carefully and made sure I wasn't missing any edge cases (e.g. params appearing in the query twice with different values). This uses jetty-util:
(import '[org.eclipse.jetty.util UrlEncoded MultiMap])
(defn parse-query-string [query]
(let [params (MultiMap.)]
(UrlEncoded/decodeTo query params "UTF-8")
(into {} params)))
user> (parse-query-string "size=3&mean=1&sd=3&type=pdf&distr=normal")
{"sd" "3", "mean" "1", "distr" "normal", "type" "pdf", "size" "3"}

Answer 4

Can also use this library for both clojure and clojurescript: https://github.com/cemerick/url
user=> (-> "a=1&b=2&c=3" cemerick.url/query->map clojure.walk/keywordize-keys)
{:a "1", :b "2", :c "3"}

Answer 5

Yours looks fine. I tend to overuse regexes, so I would have solved it as
(defn request-to-keywords [req]
(into {} (for [[_ k v] (re-seq #"([^&=]+)=([^&]+)" req)]
[(keyword k) v])))
(request-to-keywords "size=1&test=3NA=G")
{:size "1", :test "3NA=G"}
Edit: try to stay away from clojure.walk though. I don't think it's officially deprecated, but it's not very well maintained. (I use it plenty too, though, so don't feel too bad).

Answer 6

I came across this question when constructing my own site and the answer can be a bit different, and easier, if you are passing parameters internally.
Using Secretary to handle routing: https://github.com/gf3/secretary
Parameters are automatically extracted to a map in :query-params when a route match is found. The example given in the documentation:
(defroute "/users/:id" [id query-params]
(js/console.log (str "User: " id))
(js/console.log (pr-str query-params)))
(defroute #"/users/(\d+)" [id {:keys [query-params]}]
(js/console.log (str "User: " id))
(js/console.log (pr-str query-params)))
;; In both instances...
(secretary/dispach! "/users/10?action=delete")
;; ... will log
;; User: 10
;; "{:action \"delete\"}"

Answer 7

You can use ring.middleware.params. Here's an example with aleph:
user=> (require '[aleph.http :as http])
user=> (defn my-handler [req] (println "params:" (:params req)))
user=> (def server (http/start-server (wrap-params my-handler)))
wrap-params creates an entry in the request object called :params. If you want the query parameters as keywords, you can use ring.middleware.keyword-params. Be sure to wrap with wrap-params first:
user=> (require '[ring.middleware.params :refer [wrap-params]])
user=> (require '[ring.middleware.keyword-params :refer [wrap-keyword-params])
user=> (def server
(http/start-server (wrap-keyword-params (wrap-params my-handler))))
However, be mindful that this includes a dependency on ring.