first steps with clojure.spec

repl.clj

user=> (require '[clojure.spec :as s])
nil
user=> (s/def ::big #(< 10 %))
:user/big
user=> (s/valid? ::big 10)
false
user=> (s/valid? ::big 100)
true

user=> (s/def ::big-or-even (s/or ::big even?))
:user/big-or-even
user=> (s/valid? ::big-or-even 100)
true
user=> (s/valid? ::big-or-even 101)
false
;; why ?
user=> (s/explain ::big-or-even 101)
val: 101 fails spec: :user/big-or-even at: [:user/big] predicate: even?
;; why o why ?

(s/def ::big-or-even (s/or :big ::big :even even?))
;; is the correct way to describe a s/or (specs must be tagged) 
;; thanks to @mamuninfo2 and @puredanger