Thanks for the great comment. There is much to ponder and there no longer and excuse for me to avoid Eifel.

Thanks for reading.

-m

In Clojure, :pre and :post should have been defined on generic functions where they shine.

You don’t want simple :pre and :post, you want abstractions such as constrained-fn in the example above.

In design by contract, constrained-fn should be a multi-method. The contract for constrained-fn should be that:

Precondition: The argument should be positive, BUT CAN BE RELAXED.

Postcondition: The result should be doubled, BUT CAN BE STRENGTHENED.

Thus, preconditions should be combined by disjunction. Postconditions should be combined by conjunction.

This should be done by the multi-method dispatch system. All preconditions for applicable functions should be ORed together and evaluated. The postconditions for all applicable functions should be ANDed together and evaluated.

Why is this essential feature missed by everyone?

One advantage that :pre and :post have over assert is that the former allows the “assertions” to potentially come from a different source than the body of the function. This is what my example implies, even if I didn’t elucidate that exact point. :pre and :post also make it relatively easy to do some fancy automatic test generate and as a bonus provide a form of documentation for the function. Chris Houser added another interesting point that I was not aware of, “using assert instead of :pre is usually pretty straightforward, if you had to do it. Using assert instead of :post is a pain.” Sounds reasonable. Along that point, Chas Emerick’s assessment that :pre and :post cover 99.5% of assert uses, plus provide simple, and consistent hooks.

Hope that helps. Thanks for reading.

-m

(defn constrained-fn [f x] (assert (pos? x)) (let [r (f x)] (assert (= r (* 2 x))) r))

One can easily write a simple macro to handle the boilerplate.

Have I ever mentioned that PLT Scheme never ceases to amaze me? Well, I am now.

-m