Howto write test.check custom generators

[Timothy Washington]

Hi there, 

I'm trying to get my head wrapped around test.check. My current stumbling block is custom generators. Of course I've combed through the docs and source. And I'm pretty sure I need to use gen-bind to get this all working. But it's just not clicking yet. 

Let's say I want to create a user and a group, where a user can belong to many groups. So let's say I have a generic create-user function. 

  (defn create-user [opts]

    (merge {:id (mu/generate-uuid)

            :username ""

            :password (crypto/base64 12)

            :first-name ""

            :last-name ""

            :email ""

            :country {}}

           opts))

And a generic create-group function. 

  (defn create [opts]

    (merge {:id (mu/generate-uuid)

            :name ""

            :users []}

           opts))

1. Now, if I try `(gen/sample create-user)`, or `(gen/sample (gen/vector du/create))`it will fail. 
   
2. I also want to do the same thing with groups (ie (gen/sample create-user)). 
   
3. Additionally however, I want to assert that a created group will always have at least 1 user in the `:users []` k/v field. 
   

How can I make test.check generators for these data structures? 

Tim Washington 

Interruptsoftware.com 

[Sam Ritchie]

You can to use gen/fmap to build up generators. Here's an example:

;; generates a millisecond using gen/choose, then maps that to a time instance using my u/ms-to-time function.
(def result-gen
  (->> (gen/choose 0 u/max-time)
       (gen/fmap u/ms-to-time)))

;; Generates EITHER a result or nil, then maps that into an event entry data structure:
(def event-entry-gen
  (->> (gen/one-of [(gen/return nil) result-gen])
       (gen/fmap (fn [result]
                   (merge {:_id "id"
                           :_rev "rev"
                           :type "event-entry"
                           :division {:age-group "U17"
                                      :boat-type "K1"
                                      :gender "male"}
                           :event-id "event-id"
                           :regatta-id "regatta-id"
                           :athletes []
                           :racer-number 10}
                          (when result
                            {:result {:division-size 10
                                      :rank {:overall 1
                                             :division 1}
                                      :time result}}))))))

Once you have a generator instance you can copy the rest of the test.check examples, run gen/sample on the generator directly... all the good stuff.

Timothy Washington

August 10, 2014 at 12:38 PM

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[Timothy Washington]

Oook, this is starting to sink in. 

I've gotten a little farther, in that I can run quick-check with my custom generator. However, after a bunch of values had been generated, it fails on a NullPointerException. How can I know where the breakdown is happening? 

=> (tc/quick-check 100 u/has-agroup)

{:email , :last-name , :first-name , :username }

{:email , :last-name , :first-name , :username 0}

{:email , :last-name , :first-name Rq, :username 0}

{:email 5D2, :last-name R4F, :first-name x, :username G0H}

...

{:email 8KQgSr497EPw14X80oEbWC0, :last-name cdkLb8D5ol, :first-name 8DCOXz1U4S3JM641u5I7yPwQ, :username DADOk}

{:email x, :last-name Qq3h07d6Cbs4, :first-name 6H6Enrwu, :username yf40SClLjJHp1ptbnx4I9xFbwI3}

NullPointerException   clojure.test.check.generators/gen-bind/fn--18990 (generators.clj:77)

bkell.bkell> (pst *e)

NullPointerException

        clojure.test.check.generators/gen-bind/fn--18990 (generators.clj:77)

        clojure.test.check.generators/gen-bind/fn--18990 (generators.clj:79)

        clojure.test.check.generators/gen-bind/fn--18990 (generators.clj:77)

        clojure.test.check.generators/gen-fmap/fn--18985 (generators.clj:70)

        clojure.test.check.generators/call-gen (generators.clj:56)

        clojure.test.check/quick-check (check.clj:57)

        bkell.bkell/eval19267 (form-init7280611083923741467.clj:1)

        clojure.lang.Compiler.eval (Compiler.java:6703)

        clojure.lang.Compiler.eval (Compiler.java:6666)

        clojure.core/eval (core.clj:2927)

        clojure.main/repl/read-eval-print--6625/fn--6628 (main.clj:239)

        clojure.main/repl/read-eval-print--6625 (main.clj:239)

This is my domain function: 

  (defn create-user 

    ([] (create {}))

    ([opts]

       (println opts)

       (let [sans-nils (apply merge (for [[k v]

                                          opts

                                          :when (not (nil? v))]

                                      {k v}))]

         (merge {:id (mu/generate-uuid)

                 :username ""

                 :password (crypto/base64 12)

                 :first-name ""

                 :last-name ""

                 :email ""

                 :country {}}

                sans-nils))))

And these are my test functions. 

  (def gend-user

    (gen/fmap du/create-user

              (gen/hash-map :username gen/string-alpha-numeric

                            :first-name gen/string-alpha-numeric

                            :last-name gen/string-alpha-numeric

                            :email gen/string-alpha-numeric)))

  (def has-agroup

    (prop/for-all [v (gen/sample gend-user)]

                  (-> v nil? not)))

Tim Washington 

Interruptsoftware.com 

[Reid Draper]

Hey Tim,

When you write a property, like your `has-agroup`, there's no need to call `gen/sample`. You can simply write: (prop/for-all [v my-generator] ...)

-Reid