(ns spectest
  (:require [clojure.spec.alpha :as s]))

(s/def ::fixed-length (s/& (s/+ int?) #(= 2 (count %))))

(defn variable-length?
  [{:keys [length data]}]
  (= length (count data)))

(s/def ::variable-length (s/and (s/cat :length int?
                                       :data (s/* int?))
                                variable-length?))

(s/conform ::fixed-length [1 2]) ;; => [1 2]

(s/conform ::variable-length [2 1 1]) ;; => {:length 2, :data [1 1]}

(s/conform (s/cat :fixed ::fixed-length
                  :variable ::variable-length)
           [1 2 2 1 1]) ;; => :clojure.spec.alpha/invalid