(defn string-to-string [s1]
{:pre [(string? s1)]
:post [(string? %)]}
s1)
I like :pre and :post conditions, they allow me to figure out when I have put "square pegs in round holes" more quickly. Perhaps it is wrong, but I like using them as a sort of poor mans type checker. This isn't philosophy though, this is a simple question.
It seems in the above code that I should easily be able to determine that s1 is a function argument in the :pre condition. Similarily, % in the :post condition is always the function return value.
What I would like is to print the value of s1 or % when either of these respective conditions fail within the AssertionError. So I get something like
(string-to-string 23)
AssertionError Assert failed: (string? s1)
(pr-str s1) => 23
With the AssertionError containing a single line for every variable that was identified as being from the function argument list and that was referenced in the failing test. I would also like something similar when the return value of the function fails the :post condition.
This would make it trivial to quickly spot how I misused a function when trying to diagnose from the AssertionError. It would at least let me know if the value is nil or an actual value (which is the most common error I make).
I have some ideas that this could be done with a macro, but I was wondering if there was any safe and global way to basically just redefine what (defn and (fn and friends do so that :pre and :post would also print the value(s) that lead to the test failing.
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))