Conform :ret into :fn

[cjohansen]

Fixes #26

Some notes:

The spec docs says to ignore :fn in the case where either :args or :ret is missing, but Orchestra has opted to treat a missing :args as a failure. I initially intended to treat a missing :ret the same way, but was unable to write a failing test for it... It seems that even without specifying a :ret spec, the :ret spec is set to something anyway. I tried:

(defn func-no-ret-spec [meow]
  (Math/abs meow))
(s/fdef func-no-ret-spec
  :args (s/cat :meow number?)
  :fn (constantly true))

(deftest func-no-ret-spec-test
  (is (thrown? #?(:clj RuntimeException :cljs :default)
               (func-no-ret-spec -42))))

But this did not fail as expected. Inspecting the code (:ret spec) was set even though I didn't provide one. I don't know the Orchestra implementation well enough to deduct why.

[jeaye]

I believe this logic has an issue. In the case where no :fn is provided (and one is certainly not required), the :ret value will no longer be taken from the fn application. Instead, it will be nil because of the when-let.

We probably want something like this:

(let [; ...
      ret (.applyTo ^clojure.lang.IFn f args)
      cret (if-some [spec (:ret fn-spec)]
             (conform! v :ret spec ret ::s/ret)
             ret)]
  ; ...
  )

[cjohansen]

Yeah, my bad. Like I said, I initially treated the missing ret spec as an error, but was unable to verify it with a test. Added a safe-guard to ensure there is always a value there.

[borkdude]

NOTE: this bug in Orchestra became apparent by instrumenting map, filter, keep etc. of https://github.com/borkdude/speculative. E.g. the map spec is:

(s/fdef clojure.core/map
  :args (s/alt :transducer (s/cat :f ::ss/ifn)
               :seqable (s/cat :f ::ss/ifn :colls (s/+ ::ss/seqable)))
  :ret ::ss/seqable-or-transducer
  :fn (fn [{:keys [args ret]}]
        (= (key args) (key ret))))

I'm interested in a fix for this bug, so I can use Orchestra to validate the ret and fn specs of speculative by instrumenting larger codebases.