how is it possible to prevent a generator from doing a retry without re-calling a generator function? Here's an ugly phone number generator

(defn phone-number-generator []
  (gen/fmap (fn [[cc phone-num]]
              (str "+" cc " (0) " phone-num))
            (gen/tuple (gen/return (rand-nth ["1" "37" "39" "44" "49"]))
                       (gen/return (-> (* Integer/MAX_VALUE (Math/random))
                                       int
                                       str)))))

"VALID?" "+1 (0) 265957916" false

Execution error (ExceptionInfo) at clojure.test.check.generators/such-that-helper (generators.cljc:320).
Couldn't satisfy such-that predicate after 100 tries.

@hlolli generally you design the generator so that it has no need to retry

I can't see the spec so I don't know what that would be

I see, it's this here

(defn-spec phone-number-valid? boolean?
  [phone-number string?]
  (let [instance (PhoneNumberUtil/getInstance)
        phone-number-parsed (.parse instance phone-number "")]
    (.isValidNumber instance phone-number-parsed)))

(s/def :user/phone-number ;  format
  (s/with-gen phone-number-valid?
    generators/phone-number-generator))

okay well there's some stuff buried in the jvm method but for example, is the last portion supposed to be a particular number of digits?

yes, it depends I think which country code is used, so I just multiply some random number to sizeof(int), should give me a more likely way of number that purely random number generator, I think?

Or I could choose 1 counry code and substring to a specific length

(* Integer/MAX_VALUE (Math/random)) is going to be pretty skewed towards certain lengths

if it's easy to get a length associated with each country code, then you could do it much better

yes, these rules are difficuly, despite all lengths being right, sometimes the first digit must fit some standard

I can read more about this ofc

but I thought the generator could try again and again until it's valid

like with regex specs

(gen/for [[country-code length] (gen/elements country-codes-and-lengths)]
          number (apply gen/tuple (repeat length (gen/choose 0 9)))]
  (str "+" country-code " (0) " number))

cool, instersting 🙂

relying on the generator trying again is not a good plan unless you're sure it's highly likely to succeed

because you can easily be in a situation where it only succeeds e.g. 0.001% of the time

and then you'll be burning a lot of CPU

yes true, I thought the idea would be to give the generator a bit of help. The odds of alpha-numberic random gen hitting a valid phone number are probably astronomical, I'd assume

yeah, you can definitely do worse than what you have 🙂

@hlolli incidentally, here's a generator that's evenly distributed between digit lengths

(gen/for [digit-count (gen/choose 5/10)
          digits (apply gen/tuple (repeat digit-count (gen/choose 0 9)))]
  digits)

that might work better for you

ok, but I didn't get gen/for, in which namespace is it?

I'll try soon and let you know, I'm generating fake firebase users for testing

clojure.test.check.generators

it's just syntax sugar for gen/fmap and gen/bind mostly

you can do the above just using gen/bind

ah I see, I was searching for it in clojure.spec.gen.alpha

thanks 🙂