Customized conditional assertion error message in Clojure?

Question

Suppose I want alert the user the input type is wrong, for example

(defn my-sqrt [x] {:pre [(not (neg? x))]}        (Math/sqrt x))

Is it possible to issue a message "Positive number only.", instead of something like assert failed?

Answer 1

You can do this using dire . You should look at its support for preconditions .

Answer 2

If you don't want to rely on external libraries, clojure's (assert) supports an optional message.

The drawbacks are, you'll lose the readability of a :pre and will have to save the result in an intermediate variable for a :post check (or wrap it up in some macro magic).

Simple example:

(defn mysqrt [x]
  ;; preconditions
  (assert (pos? x) "Positive numbers only")
  (let [y (Math/sqrt x)]
    ;; postconditions
    (assert (number? y) "Some strange error happened")
    y))