How can I get Clojure :pre & :post to report their failing value?

Question

(defn string-to-string [s1] 
  {:pre  [(string? s1)]
   :post [(string? %)]}
  s1)

I like :pre and :post conditions, they allow me to figure out wh

Answer 1

You could wrap your predicate with the is macro from clojure.test

(defn string-to-string [s1] 
  {:pre  [(is (string? s1))]
   :post [(is (string? %))]}
 s1)

Then you get:

(string-to-string 10)
;FAIL in clojure.lang.PersistentList$EmptyList@1 (scratch.clj:5)
;expected: (string? s1)
;actual: (not (string? 10))