Let's get some clojure.spec:

(require '[clojure.spec.alpha :as s]
         '[clojure.spec.gen.alpha :as sgen])

Spec has built-in specs for a number of types, including URLs:

(s/valid? uri? "http://conan.is")
=> true

Many of these also include generators:

  (sgen/sample (s/gen uri?) 3)
  (#object[java.net.URI 0x2c33ebfb "http://c4833483-df0c-42b5-9256-3a13db2639d2.com"]
   #object[java.net.URI 0x41621c4e "http://41527909-73d2-4b95-9b5d-b6507c83898b.com"]
   #object[java.net.URI 0x1e98c9ff "http://d585f324-7f35-4f3b-a39e-d48ea5489697.com"])

This generator isn't very good - the URLs it generates are valid, but they're all pretty similar. We need a more comprehensive generator that will produce a wide variety of URLs; after all, variety is the spice of generative testing.

URLs consist of many parts:

  • protocol
  • username
  • password
  • host
  • port
  • path
  • query
  • anchor

Your average URL won't contain them all, but we want our tests to be as comprehensive as possible.

Component generators

Let's write our own generator for URLs, and use it in an improved URL spec. A generator is a no-arg function that returns a clojure.test.check.generators.Generator.

To construct URLs we can use Chas Emerick's handy url library:

(require '[cemerick.url :as url])

It features a handy constructor for URLs, ->URL, which takes a vector of the above components and gives us back a java.net.URI object (Types? Constructors? Objects? OOPs!). We'll need a generator for each of them, but first let's set ourselves up with a generator that makes non-empty alphanumeric strings - we'll be needing a lot of these later:

(defn non-empty-string-alphanumeric
  (sgen/such-that #(not= "" %) 

(sgen/generate (non-empty-string-alphanumeric))
=> "j1cf5jDg6toLyP"

The fmap in the first generator here lets us apply a function to the result of a generator, in this case to url-encode the string.

The such-that in the second generator takes a predicate and another generator, and filters that generator's output using the predicate. This does mean that some of the generated values are thrown away, which is wasted work, but we just want to filter out the empty strings, and they're rare compared to all possible outputs of string-alphanumeric.


This is easy, we just want to allow http and https for now, so we can use elements to select randomly from a set:

(sgen/elements #{"http" "https"})

Username, password, host

These are just strings, and surprisingly they can be empty. We can use string-alphanumeric.


Ports can be any integer from 1 to 65535, and the choose function lets us create generators which select from a range of numbers:

(sgen/choose 1 65535)


This is where things get interesting. The path section of a URL is a sequence of non-empty strings separated by forward slashes. We can make more complex generators like this by combining simpler ones.

Now that we have a generator that can create non-empty strings, we can easily create non-empty vectors of non-empty url-encoded strings, and separate them with slashes:

(sgen/fmap #(->> %
                (interleave (repeat "/"))
                (apply str))

Starting from the bottom, we're using our non-empty-url-encoded-string-alphanumeric generator and passing it to vector to get a vector of non-empty strings. We're ensuring the vector itself is not-empty. Finally we're applying a function to the generated vector using fmap which interleaves the strings with forward slashes, and gives us back the whole lot as a single string.


To generate query params, we need a map of random keys and values, which we can create using map. IT takes two generators, one for the keys and one for the values, and a map of options:

  {:max-elements 3}) 


Anchors can be empty, so we use string-alphanumeric.

URL generator

Here's how our full URL generator looks:

(defn url-gen
  "Generator for generating URLs; note that it may generate 
  http URLs on port 443 and https URLs on port 80, and only 
  uses alphanumerics"
    (partial apply (comp str url/->URL))
      ;; protocol
      (sgen/elements #{"http" "https"})
      ;; username
      ;; password
      ;; host
      ;; port
      (sgen/choose 1 65535)
      ;; path
      (sgen/fmap #(->> %
                       (interleave (repeat "/"))
                       (apply str))
      ;; query
        {:max-elements 2})
      ;; anchor

We've used tuple to generate the vector of elements we need using each of our generators, and passed that vector to our URL constructor. As the last step we turn it into a string.

We can now generate random URLs:

(sgen/generate (url-gen))
=> "https://S5xusj6zS:Up9S786kGF@1QK4956802NQuGZE4vgq7Q5w689v:61603/a4jWg250687kTTS9iA3FCXLKbxT1/5aa2Pzlg0Xg9Z5gFd22v09r3/507Q838m1513t339sXeCYuhSU2RV/63HP3s0Lw9BeTgDL7?u0X1VI4hPy7P392yY8Jn4e9L394lg4=LRiDy9zLyi6MBb2J#"

URL Spec

We can now create a spec using our URL generator:

(s/def ::url (s/with-gen
               (s/and string?
                         (url/url %)
                         (catch Throwable t false)))

with-gen takes a spec and a no-args function that returns a generator (for why?). Our spec has a short circuit to check we're dealing with a string, and then tries to construct a URL from it, failing if an exception is thrown.

We can now validate URLs:

(s/valid? ::url "http://conan.is")
=> true

You can find the code here.