Overriding generator function spec in `stest/instrument`

core.clj

(ns spec-ex.core
  (:require
   [clojure.spec.alpha :as s]
   [clojure.spec.gen.alpha :as gen]
   [clojure.spec.test.alpha :as stest]))

;; [org.clojure/spec.alpha "0.1.123"]

;; The goal is to stub functions which require some kind external
;; dependency, such as a service or other I/O.

(defprotocol Y
  (-do-y [r]))

(def y? (partial satisfies? Y))
(s/def ::y y?)

;; Protocol methods can't be spec'd, so wrap them in a function.

(defn do-y [r]
  (-do-y r))

(s/fdef do-y :args (s/cat :y-er ::y))

;; Example of the protocol implementation that we're going to stub.

(defrecord BadYer []
  Y
  (-do-y [_] (throw (Exception. "can't make me!"))))


;; Confirm BadYer instances are valid with respect to the protol spec.

(s/valid? ::y (->BadYer))
;; => true

;; And confirm BadYer instances will throw when called.

(try
  (do-y (->BadYer))
  (catch Exception e
    (.getMessage e)))
;; => "can't make me!"


(def y-gen (gen/return (->BadYer)))

;; Confirm generator works as expected:

(gen/sample y-gen 1)
;; => (#spec_ex.core.BadYer{})

;; We want to stub `do-y`, providing y-gen as a generator for `::y`

(try
  (stest/instrument `do-y {:stub #{`do-y}
                           :gen {::y (fn [] y-gen)}})
  (catch Exception e
    (ex-data e)))
;; => #:clojure.spec.alpha{:path [:y-er], :form :spec-ex.core/y, :failure :no-gen}

;; However, we *can* stub `do-y` if we replace its spec.

(stest/instrument `do-y
                  {:stub #{`do-y}
                   :spec {`do-y (s/fspec
                                  :args (s/cat :y-er (s/with-gen ::y
                                                       (fn [] y-gen))))}})
;; => [spec-ex.core/do-y]