Clojure’s :pre and :post

by fogus

One of the more exciting features of the upcoming Clojure 1.1 release is the inclusion of pre and post assertion functions. Some preliminary details can be found in the Clojure documentation, but I thought I’d quickly cover a simple example with potentially far-reaching implications; the case of making :post assertions relative to the input argument(s).

Let’s re-write the simple function constrained-sqr as found in the Clojure documentation linked above and re-cast it as constrained-fn:

(defn constrainedfn [f x]
  {:pre  [(pos? x)]
   :post [(= % (* 2 x))]}
  (f x))

(constrainedfn #(* 2 %) 2)
;=> 4
(constrainedfn #(float (* 2 %)) 2)
;=> 4.0
(constrainedfn #(* 3 %) 2)
;=> java.lang.Exception: Assert failed: (= % (* 2 x))

So what have I done? By specifying a :post assertion of (= % (* 2 x)) I’ve constrained the passed function f to be (effectively) a doubling function. I’ve done this by constraining the result of constrained-fn relative to its input. There is nothing entirely ground-breaking about pre and post conditions, but they are surprisingly powerful. By stripping out the assertions into a wrapper function I’ve detached some hairy logic from a potentially globally useful function and isolated it in an aspect (if you will) named constrained-fn. Now we could wrap the function f using a different constraining function depending on the context in which it is intended to be used; for example jimmys-fn, logged-result-fn, only-ints-fn, payroll-dept-fn, etc… In fact, I like that word aspect; it has a nice ring to it. I wonder if anyone has thought of that before?