router.clj

(ns router.core
  (:gen-class)
  (:use [clojure.string
         :only [replace]
         :rename {replace str-replace}]))

(defn- build-route [[url-spec handler]]
  (let [regex (re-pattern (str-replace url-spec #":.+?\/" "(.+?)/"))]
    `(fn [url#]
       (if-let [match# (re-matches ~regex url#)]
         (if (coll? match#)
           (fn [] (apply ~handler (drop 1 match#)))
           (fn [] (~handler)))))))

(defn not-found [] (println "404 Not Found"))

(defmacro defroutes [& route-forms]
  (let [handlers (map build-route (partition 2 route-forms))]
    `(def routes [~@handlers (fn [url#] (fn [] (not-found)))])))

(defroutes
  "/router/both/"            (fn [] (println "router both"))
  "/only/controller/"        (fn [] (println "only controller"))
  "/router/params/:x/:y/:z/" (fn [x y z] (println x y z)))

(defmacro with-handler [[var url] & body]
  `(let [url# ~url
         handler# (some #(% url#) routes)
         ~var handler#]
     (do ~@body)))

(defn route [url]
  (with-handler [handler url]
    (handler)))

(defn -main [& args]
  (route "/router/both/")
  (route "/only/controller/")
  (route "/router/params/1/2/3/")
  (route "NOPE"))

This is supposed to be the routing component of a web framework - so think Compojure. The user api is only this: defroutes takes strings and functions, interleaved; route calls the handler associated with that url, or not-found if none exists. Still no support for http methods or actual request/response "objects." Here's what lein run should output:

router  [master] $ lein run
router both
only controller
1 2 3
404 Not Found

I'd like the opinion of someone who knows Clojure better than I do, cause this is probably not very idiomatic. I'm mostly bothered by the very deep indentation in build-route, which may be solvable by some magic Clojure incantation that I don't know.

:10s/if-let/when-let/g