cljs.spec.alpha/?

macropreviously cljs.spec/?Edit
(? pred-form)

Source docstring:
Returns a regex op that matches zero or one value matching
pred. Produces a single value (not a collection) if matched.
Source code @ clojurescript:src/main/cljs/cljs/spec/alpha.cljc
(defmacro ?
  [pred-form]
  `(maybe-impl ~pred-form '~(res &env pred-form)))