Clojure :pre report failing value when destructuring - error-handling

Question

Following this SO post, I would like to print the value of the preconditions in my function. However it fails for me in the following case (probably destructuring) :
I have a dir? helper function (feel free to skip this one) :
(defn dir? [s]
"returns true if the string passed is is an existing directory"
(->> (clojure.java.io/file s)
((juxt #(.exists %) #(.isDirectory %)))
(every? true?)))
It works just fine, and using the is macro, I get some nice error messages where I can see both the test and the parameters that were passed :
(is (dir? (io/file "resources/static"))) ;; => true
(is (dir? (io/file "resources/statice"))) ;; typo, error below
FAIL in clojure.lang.PersistentList$EmptyList#1
(boot.user4515592986834245937.clj:86) expected: (dir? (io/file
"resources/statice")) actual: (not (dir? #object[java.io.File
0x6730a420 "resources/statice"]))
However, when trying to use it in the precondition :pre, I get an ugly error :
(defn make-something
[&{:keys [dir]
:or {dir "default-dir"}}]
{:pre [(is (dir? (clojure.java.io/file dir)))]}
;;... do something with these
)
(make-something :dir "resources/statices") ;; doesn't exist
clojure.lang.Compiler$CompilerException: java.lang.AssertionError:
Assert failed: (is (dir? (io/file dir))),
compiling:(boot.user4515592986834245937.clj:80:12)
java.lang.AssertionError: Assert failed: (is (dir? (io/file dir)))
How can I get a nice error message in my function just like the one above ?
In case it matters, I am using Clojure 1.7.

Answer 1

You need to check your code (dir? function). Following snippet works for me:
(require '[clojure.java.io :as io])
(defn dir? [s]
(let [dir (io/file s)]
(and (.exists dir)
(.isDirectory dir))))
(defn make-something
[& {:keys [dir] :or {dir "default-dir"}}]
{:pre [(is (dir? dir))]}
(println dir))
(make-something :dir "/tmp")
out => /tmp
ret => nil
(make-something :dir "/xxxx")
FAIL in clojure.lang.PersistentList$EmptyList#1 (form-init3332271312167175068.clj:1)
expected: (dir? dir)
actual: (not (dir? "/xxxx"))
AssertionError Assert failed: (is (dir? dir)) user/make-sth (form-init3332271312167175068.clj:1)