cljs.spec.alpha/alt

macropreviously cljs.spec/altEdit
(alt & key-pred-forms)

Source docstring:
Takes key+pred pairs, e.g.

(s/alt :even even? :small #(< % 42))

Returns a regex op that returns a map entry containing the key of the
first matching pred and the corresponding value. Thus the
'key' and 'val' functions can be used to refer generically to the
components of the tagged return.
Source code @ clojurescript:src/main/cljs/cljs/spec/alpha.cljc
(defmacro alt
  [& key-pred-forms]
  (let [pairs (partition 2 key-pred-forms)
        keys (mapv first pairs)
        pred-forms (mapv second pairs)
        pf (mapv #(res &env %) pred-forms)]
    (clojure.core/assert (clojure.core/and (even? (count key-pred-forms)) (every? keyword? keys)) "alt expects k1 p1 k2 p2..., where ks are keywords")
    `(alt-impl ~keys ~pred-forms '~pf)))