Clojure spec - s/or single branch generator

Question

Is it possible to override a default spec generator so that data is always generated only for a single branch of the s/or composite spec?

(s/def ::x
  (s/or :x-a nat-int?
        :x-b string?))

(gen/sample (s/gen ::x))
;; generate strings only

Answer 1

You can use s/with-gen to supply a custom generator:

(s/def ::x
  (s/with-gen
    (s/or :x-a nat-int?
          :x-b string?)
    #(s/gen string?)))

(gen/sample (s/gen ::x))
=> ("" "j" "e" "Jmi" "" "d" "bc" "ul" "H65P0ni" "OEDK")

You could also use it only where you're sampling, without modifying the base ::x spec:

(gen/sample (s/gen (s/with-gen ::x #(s/gen string?))))

There are other spec functions that accept a map of overrides for the same purpose, e.g. s/exercise:

(s/exercise ::x 10 {::x #(s/gen string?)})
=>
(["" [:x-b ""]]
 ["" [:x-b ""]]
 ["" [:x-b ""]]
 ["" [:x-b ""]]
 ["13R0" [:x-b "13R0"]]
 ["7cT30" [:x-b "7cT30"]]
 ["uia0b" [:x-b "uia0b"]]
 ["" [:x-b ""]]
 ["bP" [:x-b "bP"]]
 ["4k2t6bW" [:x-b "4k2t6bW"]])