Observe what happens in Clojure if you attempt to use {{s/with-gen}} on an undefined spec:
Clojure 1.10.0
user=> (require '[clojure.spec.alpha :as s])
nil
user=> (s/def ::foo (s/with-gen ::bar #(s/gen #{:a})))
Syntax error (IllegalArgumentException) compiling at (REPL:1:14).
No implementation of method: :specize* of protocol: #'clojure.spec.alpha/Specize found for class: nil
In ClojureScript, on the other hand, things don't fail fast, and when they fail later the error can be somewhat cryptic to developers:
$ clj -Sdeps '{:deps {org.clojure/clojurescript {:mvn/version "1.10.439"}}}' -m cljs.main
ClojureScript 1.10.439
cljs.user=> (require '[clojure.spec.alpha :as s])
nil
cljs.user=> (s/def ::foo (s/with-gen ::bar #(s/gen #{:a})))
:cljs.user/foo
cljs.user=> (s/valid? ::foo 10)
TypeError: null is not an object (evaluating 'self__.pred.call')
cljs$spec$alpha$Spec$conform_STAR_$arity$2 (cljs/spec/alpha.cljs:526:35)
cljs.spec.alpha/conform* (cljs/spec/alpha.cljs:40:14)
cljs$core$IFn$_invoke$arity$2 (cljs/spec/alpha.cljs:375:22)
cljs$spec$alpha$valid_QMARK_ (cljs/spec/alpha.cljs:371:1)
cljs.user=>
