Let's create our own 78 digits long string type:

(def hex-digit (set "0123456789"))

(defn hex-digit? [x] (contains? hex-digit x))

(defn hex-str? [s] (every? hex-digit? (seq s)))

(s/def ::hash (s/and hex-str? #(= (count %) 78)))

::hash is now registered, now we go to the REPL:

(s/valid? ::hash "23179372")

;;=> false

(s/valid? ::hash "nope93721907914920047210715459933122004671648400678953445710500236944435987060")

;;=> false

(s/valid? ::hash "231793721907914920047210715459933122004671648400678953445710500236944435987060")

;;=> true

That's cool, but we want to generate too:

(defn hex-str-gen [n] (let [digit (g/elements hex-digit)] (g/fmap clojure.string/join (g/vector digit n))))

(s/def ::hash (s/with-gen (s/and hex-str? #(= (count %) 78)) #(hex-str-gen 78)))

We extended our ::hash spec with a generator function, so we can do this now:

(g/generate (s/gen ::hash))

;;=> "310584715385467847758653938894742415543975445356609397864862925839413265904779"

(+ Pro tip: you could spec functions too)

Learn more about spec here: https://clojure.org/guides/spec