macropreviously cljs.spec/fspecEdit
(fspec & {:keys [args ret fn gen]})

Source docstring:
takes :args :ret and (optional) :fn kwargs whose values are preds
and returns a spec whose conform/explain take a fn and validates it
using generative testing. The conformed value is always the fn itself.

See 'fdef' for a single operation that creates an fspec and
registers it, as well as a full description of :args, :ret and :fn

fspecs can generate functions that validate the arguments and
fabricate a return value compliant with the :ret spec, ignoring
the :fn spec if present.

Optionally takes :gen generator-fn, which must be a fn of no args
that returns a test.check generator.
Source code @ clojurescript:src/main/cljs/cljs/spec/alpha.cljc
(defmacro fspec
  [& {:keys [args ret fn gen]}]
  (let [env &env]
    `(fspec-impl (spec ~args) '~(res env args)
                           (spec ~ret) '~(res env ret)
                           (spec ~fn) '~(res env fn) ~gen)))