clojure.spec :fn equivalent?

[knubie]

In clojure.spec you can do something like this:

(s/fdef ranged-rand
  :args (s/and (s/cat :start int? :end int?)
               #(< (:start %) (:end %)))
  :ret int?
  :fn (s/and #(>= (:ret %) (-> % :args :start))
             #(< (:ret %) (-> % :args :end))))

Where the :fn can be used to verify the relationship between args and return value.

Is there any way to do this in malli?

[ikitommi]

No, there is not yet such a thing, but it would be easy to add. Interested in doing a PR for that?

[knubie]

Well, if it's easy, then yeah I can give it a shot. 😎 I'll admit I haven't looked at the source code. Can you give me any pointers on where to start?

[ikitommi]

You could start from malli.core/-instrument. It's the (pure) functional core behind the functions schemas, everything else builds on top of that.

[ikitommi]

https://github.com/metosin/malli/blob/master/docs/function-schemas.md#instrumentation

[ikitommi]

fixed in #1000